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COMMUNAUTE FRANÇAISE DE BELGIQUE ACADEMIE UNIVERSITAIRE WALLONIE-EUROPE
UNIVERSITE DE LIÈGE GEMBLOUX AGRO-BIO TECH
CONTRIBUTION À L’ÉTUDE DE LA CONSERVATION DES GRAINES DE GRENADE
(PUNICA GRANATUM L.) PAR DÉSHYDRATATION OSMOTIQUE
Brahim BCHIR
Dissertation originale présentée en vue de l’obtention du grade de docteur en sciences agronomiques et ingénierie biologique
Promoteur : Prof. Christophe BLECKER 2011
Copyright. Aux termes de la loi belge du 30 juin 1994, sur le droit d’auteur et les droits voisins, seul l’auteur a le droit de reproduire partiellement ou complètement cet ouvrage de quelque façon et forme que ce soit d’en autoriser la reproduction partielle ou complète de quelque manière et sous quelque forme que se soit. Toute photocopie ou reproduction sous autre forme est donc faite en violation de la dite loi et des modifications ultérieures.
BRAHIM BCHIR. (2011). Contribution à l’étude de la conservation des graines de grenade par déshydratation osmotique. Gembloux, Belgique, Université de Liège-Gembloux Agro-Bio Tech. 198 p., 19 tabl., 16 fig. Résumé :
L’objectif des travaux entrepris au cours de cette thèse visait à mettre en place un procédé global de conservation des graines de grenade (Punica granatum L.). Ce procédé se base essentiellement sur une déshydratation osmotique (DO), associée à un pré-traitement de congélation et un post-traitement de séchage par entrainement. Dans ce contexte, plusieurs paramètres d'optimisation du transfert de masse ont été étudiés, tels que la nature de la solution d’immersion (saccharose, glucose, glucose/saccharose et jus de datte « sous-produit » enrichi en saccharose), la température (30, 40, et 50°C) et l’état du fruit (frais, congelé). En outre, nous avons mis en relation ces conditions avec certaines propriétés des graines : leur texture, leur structure, et leur couleur.
L’étude des paramètres de déshydratation (perte en eau (WL), gain en solides (SG), et réduction en poids (WR)) a montré qu’en utilisant des graines congelées et indépendamment de la température et de la solution utilisée, la majorité du transfert de masse s’effectue pendant les vingt premières minutes de traitement. A l’issue de cette période, la perte en eau est estimée à 46%, 41%, 39%, et 37% respectivement dans les solutions de saccharose, glucose/saccharose, de jus de datte et du glucose. La DO des graines fraîches est caractérisée par une cinétique plus lente, mais une perte finale en eau plus importante. Comme le montrent les analyses en microscopie électronique, cela s’explique par une déstructuration cellulaire survenant à la suite de la congélation des graines, ce qui vient conforter les résultats des observations microscopiques. Les mêmes techniques ont également indiqué une modification de texture/structure induite par le processus de DO. D’autre part, l’utilisation d’une solution de saccharose (55°Brix) et d’une température de 50°C favorise un meilleur transfert de masse.
La détermination des différentes fractions d’eau dans la graine par calorimétrie différentielle (DSC) a montré une augmentation d’un facteur ~2,5 fois de la fraction d’eau non congelable (eau liée) et une réduction de ~3,5 fois de la fraction d’eau congelable (eau libre) favorisant ainsi une meilleure conservation du fruit. Le suivi de la qualité intrinsèque des graines au cours de la DO a montré une perte d’une quantité non négligeable de certains composés (protéines, cendres) de la graine vers la solution, ce qui pourrait avoir une influence majeure sur la qualité organoleptique et nutritionnelle du fruit.
La DO seule ne pourrait pas maintenir une stabilité du produit au cours de la conservation. En effet, l’activité d’eau du produit fini après DO est proche de 0,90. Ainsi, dans un but plus appliqué, un traitement complémentaire de séchage par entrainement (2 m/s durant 4 heures) a été mis en place, à différentes températures (40, 50, et 60°C), afin de réduire l’activité d’eau à une valeur inférieure à 0,65. Afin d’optimiser le traitement de séchage, nous avons étudié en premier lieu l’effet de la température sur l’évolution de la matière sèche, de l’activité d’eau et du pourcentage de séchage des graines. D’autre part, plusieurs paramètres de qualité des graines de grenade (l’activité antioxydante, la teneur en composés phénoliques, les anthocyanines, la couleur, et la texture) ont été étudiés à différentes températures.
Ce travail est une contribution à l’étude des propriétés physico-chimiques des graines de grenade (Punica granatum L.) au cours des procédés de congélation, de déshydratation et de séchage. Les caractéristiques du produit fini peuvent justifier de nouvelles voies de transformation et d’exploitation des graines de grenade. Mots clés : Grenade, Déshydratation osmotique, Congélation, Séchage, Texture, Structure, Analyse calorimétrique différentielle, Perte en eau, Activité antioxydante, Composés phénoliques.
BRAHIM BCHIR. (2011). Contribution to pomegranate seeds conservation by osmotic dehydration. Gembloux, Belgium, University of Liege-Gembloux Agro-Bio Tech, 198 p., 19 tabl., 16 fig. Abstract:
The aim of this work was to create a complete conservation process of pomegranate seeds (Punica granatum L.). This process is essentially based on osmotic dehydration (OD), which was associated to freezing and air-drying process. Several parameters were studied to optimize the process such as osmotic solution (sucrose, glucose, and sucrose/glucose and date juice with sucrose added), temperature (30, 40, and 50°C) and state of the fruit (fresh and frozen). All these conditions were linked to seed proprieties (texture, structure, and colour).
The study of osmotic dehydration parameters (water loss (WL), solids gain (SG) and weight reduction (WR)) showed that most significant changes of mass transfer took place during the first 20 min of dewatering using frozen seeds, independently of temperature and sugar type. During this period, seeds water loss was estimated at 46% in sucrose, 41% in sucrose/glucose mix, 39% in date juice, and 37% in glucose. Mass transfer was slower starting from fresh fruit but led to a higher rate of WL at the end of the process. This fact can be explained by scanning electron microscopy, which showed damage of seed cell structure after freezing. This has practical consequences in terms of the modification of seeds texture. The same process also revealed a modification of seed texture and cell structure after osmotic dehydration. Using a sucrose solution and a temperature of 50°C favoured the best mass transfer. The determination of different water fractions of seed by differential scanning calorimetry (DSC) showed that the % of frozen water decreased 3.5 times contrary the % of unfreezable water that increased 2.5 times. This favours a better seeds conservation. During osmotic dehydration, there was a non negligible leaching of natural solutes from seeds into the solution, which might have an important impact on the sensorial and nutritional value of seeds.
Using only osmotic dehydration could not maintain the stability of seeds during conservation. In fact, after the osmotic process, water activity of seeds was found to be higher than 0.9, allowing to the development of microorganisms and some undesirable reactions. As a consequence, a drying of the pomegranate seeds (during four hours) was investigated at three different temperatures (40, 50, and 60 °C) with air flow rate of 2 ms-1. Prior to the drying process, seeds were osmodehydrated in a sucrose solution (55°Brix) during 20 min at 50°C. The drying kinetics and the effects of OD and air-drying temperature on antioxidant capacity, total phenolic, colour, and texture were determined.
This work is a contribution to the study of physico-chemical properties of pomegranate seeds (Punica granatum L.) during freezing, osmotic dehydration and drying. After the global process, the pomegranate seed characteristics lead to new industrial developments. Keywords: Pomegranate; Osmotic dehydration; Freezing, Drying; Texture; Structure; Differential scanning calorimetry analysis; Water loss; Antioxidant activity; Phenolic compound.
Dédicaces
Je dédie ce travail :
Aux deux êtres les plus chers, mon père Mohamed Naceur BCHIR et ma mère
Radhia HIZEM, pour leur amour, leur soutien et leurs sacrifices, en témoignage
de ma grande estime et mon amour pour eux.
A mes frères Aymen, Habib et Rached pour la motivation et les encouragements
incessants qu’ils m’ont fournies en élaborant ce travail.
A mon cousin Samir pour la confiance qu’il m’a toujours accordée.
Remerciements
Au terme de ce travail qui a été réalisé dans le cadre d’une collaboration entre le
laboratoire de Biophysique et Ingénierie des Formulations de Gembloux Agro-Bio Tech
(Université de Liège, Belgique) et le laboratoire d’Analyse Alimentaire de l’ENIS (Université
de Sfax, Tunisie) je voudrais remercier les personnes qui, de près ou de loin, ont participé à
son aboutissement.
Tout d’abord, je remercie Monsieur Claude DEROANNE, ancien responsable de l’Unité
de Technologie des Industries Agroalimentaires, pour m’avoir accueilli au sein de son Unité
et pour la gentillesse qu’il m’a témoignée.
Mes plus chaleureux remerciements s’adressent à mon promoteur de doctorat, Monsieur
Christophe BLECKER (Chef du service de Technologie des Industries Agroalimentaires)
qui était un grand soutien moral pour moi et m’a toujours encouragé pendant les moments
difficiles. De plus, les conseils qu’il m’a prodigués ont toujours été clairs et précis, me
facilitant l’accomplissement de ce projet.
Je remercie Monsieur Hamadi ATTIA, Responsable de l’Unité Analyses Alimentaires de
l’Ecole Nationale d’ingénieurs de Sfax, de m’avoir offert l’occasion d’y travailler et pour
l’intérêt qu’il a accordé à mes travaux tout au long de leur réalisation.
Je remercie également Monsieur Souhail BESBES (Maître de conférences à Université de
Sfax, Tunisie), qui nous a donné l’idée de travailler sur ce projet et m’a suivi tout au long de
ce travail et dispensé ses conseils avisés.
Mesieurs Benoît HAUT et Frank DELVIGNE, ont accepté de juger ce travail et d’en
être des rapporteurs. Je leur dois une nette amélioration de la qualité de ce document. Qu’ils
trouvent ici toute ma reconnaissance.
Monsieur Michel PAQUOT, Monsieur Georges LOGNAY, Monsieur François BERA,
Madame Sabine DANTHINE ont été membres de mon comité de thèse. Ils m’ont assuré un
accompagnement scientifique de qualité.
Je tiens également à remercier Madame Lynn Doran pour son aide au laboratoire et sa
profonde gentillesse.
Nicole Rucquoy, Claire Ndayisenga, Thomas Bertrang, Fabian Rouffiange, Vanessa
Ardito, Marjorie Servais, Guy Delimme, Stéphane Guillaume, Sandrino Filocco, Alain
Someville, Dominique Cortese, Maguy Pétré, Isabelle Van de Vreken etc. ont tous
contribué au bon fonctionnement de ce travail car la bonne ambiance qui régnait dans le labo
grâce à eux est un élément indispensable pour mener à terme de longues manipulations.
Je remercie mes collègues Mazen Ibrahim, Romdhane Karoui, Jean-Michel Giet,
Gaoussou Karamoko, Prudent Anihouvi, Caroline Vanderghem, Gilles Olive, Christine
Anceau, Emilie Arnould, Paul Callewaert et Olivier Roiseux. Faire la recherche à leur côté
a été un réel plaisir.
Tous mes remerciements vont également aux membres du personnel du laboratoire
d’Analyse Alimentaire de l’ENIS (Sfax, Tunisie) qui m’ont chaleureusement accueilli et
m’ont permis de réaliser ce travail dans une ambiance excellente.
LEXIQUE
aw water activity ;
ANOVA analyse de la variance ;
AA antioxidant activity (%) ;
Abs absorbance;
°Brix degré brix (%) ;
C* chroma ;
C° degré celcius ;
Cp chaleur spécifique (J·kg-1·K-1) ;
Deff coefficient de diffusion (m2.s-1) ;
DII déshydratation-imprégnation par immersion ;
DM dry matter ;
DO déshydratation osmotique ;
DPPH 2,2,-diphenyl-2-picryl-hydrazyl ;
DR drying rate (g water/g dry matter min-1);
DSC differential scanning calorimetry ;
FM fresh matter;
FS fresh seed;
GAE gallic acid equivalents;
h° angle de teinte ;
Hfus enthalpie de fusion (j.g-1);
HMF hydroxy-méthyl-furfural;
HPLC high performance liquid chromatography;
K1, K2 Peleg’s parameters;
L* luminosité;
min minute;
PPO polyphenol oxidase;
R2 coefficient de corrélation (%);
RA: relative activity (%);
RMN résonance magnétique nucléaire;
SG solids gain (g/100 g fresh seeds);
SEM scanning electron microscopy;
TEAC trolox-equivalent antioxidant capacity ;
TAA total antioxidant activity (%);
t time;
Tf melting point (°C);
Tg’ glass transition temperature (°C);
UFW unfreezable water (g.g-1 dry matter);
WL water loss (g/100 g fresh seeds);
WR weight reduction (g/100 g fresh seeds);
% pourcent.
TABLE DES MATIÈRES
Introduction générale ...................................................................................... 1
Chapitre 1 : Synthèse bibliographique............................................................. 7 Publication I : Synthèse des connaissances sur la déshydratation osmotique ...................... 7
Résumé ................................................................................................................................... 8
1. Introduction ...................................................................................................................... 11
2. Généralités sur la déshydratation osmotique.................................................................... 13
2.1. Cinétique de la déshydratation osmotique .................................................................. 13
2.2. Principaux facteurs influençant les performances de la DO ....................................... 17
2.2.1. Propriétés des tissus biologiques............................................................................ 18
2.2.2. Concentration et composition de la solution osmotique ........................................ 18
2.2.3. Température de la solution osmotique ................................................................... 20
2.2.4. Durée du traitement ................................................................................................ 21
2.2.5. Mode de mise en contact des phases, effet de l’agitation et du rapport
solide/solution ........................................................................................................... 21
2.2.6. Mise en œuvre de la déshydratation osmotique ..................................................... 22
2.3. Application de la déhydratation osmotique................................................................. 24
2.3.1. Pré-traitement thermique........................................................................................ 24
2.3.1.1. Blanchiment ...................................................................................................... 24
2.3.1.2. Congélation ....................................................................................................... 25
2.3.2. Méthodes combinées de la DO............................................................................... 25
2.3.2.1. Imprégnation sous vide ..................................................................................... 25
2.3.2.2. Haute préssion hydrostatique ............................................................................ 26
2.3.2.3. Ultrasons............................................................................................................ 27
2.3.2.4. Irradiation .......................................................................................................... 27
2.3.2.5. Chlorure de sodium ........................................................................................... 28
2.3.2.6. Centrifugation.................................................................................................... 29
2.3.2.7. Traitement par champ éléctrique pulsé ............................................................ 29
2.3.3. Stabilisation des produits déshydratés osmotiquement par des traitements
physiques................................................................................................................... 29
2.3.3.1. Séchage.............................................................................................................. 30
2.3.3.2. Congélation ....................................................................................................... 31
2.4. Equipements pour la DO ............................................................................................. 31
2.5. Qualité des produits végétaux traités par DO.............................................................. 32
2.5.1. Saveur..................................................................................................................... 33
2.5.2. Couleur ................................................................................................................... 33
2.5.3. Texture ................................................................................................................... 34
2.5.4. Réhydratation ......................................................................................................... 34
3. Valorisation des fruits conservés en solutions sucrées..................................................... 35
4. Conclusion........................................................................................................................ 35
5. Références bibliographiques ............................................................................................ 37
Chapitre 2 : Cinétique de transfert de masse durant la déshydratation
osmotique des graines de grenade ........................................................ 44
Résumé ................................................................................................................................ 45
1. Objectif et stratégie expérimentale........................................................................... 45
2. Principaux résultats .................................................................................................. 47
Publication II : Osmotic dehydration of pomegranate seeds: mass transfer kinetics and
differential scanning calorimetry characterization.................................................... 48
1. Introduction ............................................................................................................. 50
2. Material and methods .............................................................................................. 51
3. Results and discussion............................................................................................. 56
4. Conclusions ............................................................................................................. 71
5. References ............................................................................................................... 73
Chapitre 3 : Effet de la congélation sur la cinétique de transfert de masse
durant la déshydratation osmotique des graines de grenade .............. 77
Résumé ................................................................................................................................ 78
1. Objectif et stratégie expérimentale........................................................................ 78
2. Principaux résultats ............................................................................................... 80
Publication III : Osmotic dehydration of pomegranate seeds (Punica granatum L.): Effect
of freezing pre-treatment .......................................................................................... 81
1. Introduction ........................................................................................................... 83
2. Material and methods ............................................................................................ 84
3. Results and discussion........................................................................................... 90
4. Conclusions ......................................................................................................... 100
5. References ........................................................................................................... 102
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion pour la
déshydratation osmotique des graines de grenade ............................ 107
Résumé .............................................................................................................................. 108
1. Objectif et stratégie expérimentale........................................................................ 108
2. Principaux résultats ............................................................................................... 110
Publication VI : Osmotic dehydration kinetics of pomegranate seeds using date juice as an
immersion solution base ........................................................................................ 111
1. Introduction ............................................................................................................ 113
2. Material and methods ............................................................................................. 114
3. Results and discussion............................................................................................ 120
4. Conclusions ............................................................................................................ 131
5. References .............................................................................................................. 133
Chapitre 5 : Effet des conditions de séchage sur les propriétés
physicochimiques des graines de grenade déshydratées osmotiquement
.............................................................................................................. 139
Résumé .............................................................................................................................. 140
1. Objectif et stratégie expérimentale.............................................................................. 140
2. Principaux résultats ..................................................................................................... 142
Publication V : Effect of air-drying conditions on physico-chemical properties of
osmotically pre-treated pomegranate seeds ............................................................ 143
1. Introduction ............................................................................................................... 145
2. Material and methods ................................................................................................ 147
3. Results and discussion............................................................................................... 154
4. Conclusions ............................................................................................................... 169
5. References ................................................................................................................. 170
Chapitre 6 : Discussion générale, conclusions et perspectives ................... 178
6.1. Discussion générale................................................................................................ 179
6.2. Conclusion générale et perspectives ...................................................................... 187
6.3. Références bibliographiques .................................................................................. 190
Listes des figures.......................................................................................................... 194
Listes des tableaux ...................................................................................................... 196
Productions scientifiques .......................................................................................... 198
Introduction générale
Introduction générale ___________________________________________________________________________
Introduction générale
Ce sujet entre dans le cadre d’une collaboration entre le laboratoire de Biophysique et
Ingénierie des Formulations de Gembloux Agro-Bio Tech (Université de Liège, Belgique) et
le laboratoire d’Analyse Alimentaire de l’ENIS (Université de Sfax, Tunisie), qui s’articule
autour de la valorisation d’agrofournitures, dont principalement les produits et sous-produits
du palmier dattier Phoenix dactylifera L. Ainsi, plusieurs projets antérieurs ont été menés
dans une perceptive de caractérisation et de valorisation de la pulpe ou du noyau de datte, de
la sève du palmier dattier, du citron et des graines de pin d’alep et de cumin (Masmoudi et al.,
2007 ; Ben-Thabet et al., 2007 ; Cheikh-Rouhou et al., 2008 ; Besbes et al., 2009).
Le présent travail de thèse est consacré à l’étude d’un autre fruit important en Tunisie: la
grenade.
Selon les données de l’organisation des Nations Unies pour l’alimentation et l’agriculture
(FAO), la production mondiale de grenade était de 1,5 millions de tonnes en 2009, l’Iran étant
le premier producteur mondial avec une production annuelle de 700 000 tonnes, suivi de
l'Inde, l’Etats-Unis, la Turquie, l'Espagne, Israel, la Tunisie, la Grèce, Chypre et l'Egypte.
La filière grenade a connu un essor remarquable en Tunisie. En effet, la moyenne de
production par an (la saison de récolte dure de 3 à 4 mois) des grenades, qui n’était que de
51 000 tonnes au cours de la période 1992-1997, est passée à 62 000 et 70 000 tonnes
respectivement entre 1997-2001 et 2001-2008, soit une augmentation de 22% et 38%,
respectivement. En 2009, la production a atteint 75 000 tonnes. La variété El-Gabsi de qualité
organoleptique très appréciable, et donc de haute valeur marchande, représente environ 35%
de ce tonnage (Emna, 2010).
1
Introduction générale ___________________________________________________________________________
La grenade est le fruit d’un arbuste appelé grenadier, de nom latin Punica granatum L.
appartenant à la famille des punicacées. Il s’agit d’une baie, de taille et de poids variable
(diamètre entre 7 et 12 cm ; poids compris entre 50 et 800 g selon les variétés), à écorce dure
et coriace, de couleur brune à rouge ou jaune-beige, qui renferme dans des « loges »
délimitées par des cloisons épaisses appelées membranes intercarpellaires, de nombreuses
graines (de 200 jusqu’à 800 graines par fruit). Ces graines sont composées par un pépin, peu
agréable mais comestible, enrobé d'une pulpe translucide juteuse de couleur rose ou rouge, à
saveur aigrelette, plus ou moins sucrée et acide selon les variétés. Les graines constituent la
partie comestible de la grenade (Espiard, 2002).
Les graines de grenade sont riches en éléments nobles qui leur confèrent des
caractéristiques organoleptiques et nutritionnelles très intéressantes. Elles constituent une
excellente source d’hydrates de carbone (glucose, fructose), de minéraux (le magnésium, le
potassium, le calcium), de vitamines (vitamine C, B3...), d’acides organiques, et de
polyphénols. Ces derniers (flavonoïdes, tanins etc.) confèrent aux graines de grenade
d’importantes propriétés anti-oxydantes (Hernandez et al., 1999; Jaiswal et al., 2010). Edas,
(2009) a montré que le jus de grenade présente une activité antioxydante (18 – 20 Trolox-
Equivalent Antioxidant Capacity ‘TEAC’) trois fois supérieur à celle du vin rouge et du thé
vert (6 – 8 TEAC).
Les caractéristiques chimiques des graines de grenade ont suscité l’intérêt des
scientifiques et des industriels. En effet, nommée « ingrédient de l’année » en 2004 par la
société d’études de marché Mintel, la grenade fait l’objet d’un nombre croissant d’études, et
son incorporation dans des formulations alimentaires et cosmétiques est devenue courante.
Depuis 2005, plus de 475 produits à base de grenade ont vu le jour en Amérique. Cette
croissance s'explique principalement par l'attrait des propriétés bénéfiques de la grenade sur la
santé humaine, en relation avec sa composition (Storey, 2007). En effet, la majorité des études
2
Introduction générale ___________________________________________________________________________
publiées sur la grenade sont focalisées sur ses caractéristiques nutritionnelles et leurs impacts
sur la santé humaine, en relation avec sa composition (Adsule et Patill, 1995; Garca et al.,
2004 ; Aslam et al., 2006). D’un point de vue pratique, la plupart de ces travaux de
recherches se sont vus concrétisés dans des projets industriels. Plusieurs marques de produits
cosmétiques (Archipelago, Ushuaïa, Tocophea etc.) incorporent les extraits de grenade dans
leur gamme des produits, pour ses propriétés antioxydantes, anti-inflammatoires et
antimicrobiennes. D’autre part, plusieurs médicaments à base de grenade sont utilisés pour
lutter contre les états inflammatoires, ainsi que l’athérosclérose et les maladies
cardiovasculaires (Curtay et al., 2008). Dans le domaine alimentaire, divers produits à base de
graines de grenade ont été présentés récemment sur le marché mondial : le jus, le vin, la
crème-glacée, etc. (Storey, 2007). Malgré cette diversification de produits, des solutions
doivent être apportées, notamment au niveau de la conservation des graines de grenade, en
particulier dans un pays comme la Tunisie.
La consommation actuelle des grenades en Tunisie est cantonnée à l’état frais durant la
saison de récolte, en raison d’un manque de valorisation industrielle et essentiellement dû à
des problèmes de conservation. En effet, Le potentiel de stockage des grenades est limité par
l’apparition d’un brunissement de l’écorce, et des pourritures provoquées par des
champignons, affectant ainsi la qualité organoleptique intrinsèque du fruit. Ces grenades non
consommées en l’état sont écartées au niveau des stations de conditionnement et de
transformation.
Afin d’échelonner la période de consommation de la grenade en Tunisie, de mieux
exploiter les excellentes propriétés nutritionnelles, biologiques et thérapeutiques de ce fruit, et
de developper un nouveau mode de transformation. La déshydratation osmotique (DO) est
une technique qui permet de répondre à toutes ces attentes, selon diverses études menées sur
plusieurs fruits (abricot, ananas, banane, etc.) et légumes (carotte, haricot, oignon, etc.)
3
Introduction générale ___________________________________________________________________________
(Uddin et al., 2004; Shi et al., 1998, Jena et Das, 2004; Rastogi et Raghavarao, 2004;
Dermesonlouoglou et al., 2008 ; Garcia-Segovia et al., 2010 ; Kowalska et al., 2008). En effet,
cette technique de conservation, économe en énergie, est susceptible de prolonger la période
de disponibilité des produits alimentaires, et leur confère des propriétés sensorielles nouvelles
et appréciées. Elle permet ainsi aux acteurs de la filière agroalimentaire d’écouler leur
production à de meilleurs prix et aux consommateurs d’en disposer tout au long de l’année.
Cette technique est un outil facile à mettre en place, surtout dans les pays en voie de
développement, en raison de son faible coût.
Cependant, la DO est un procédé relativement lent. Il est donc impératif d’utiliser des
procédés complémentaires afin d’augmenter la perméabilité des membranes cellulaires pour
faciliter la libération de l’eau. Un pré-traitement thermique (blanchiment, congélation) est très
souvent utilisé (Kowalska et al., 2008). Le procédé de congélation reste un excellent pré-
traitement pour la déshydratation osmotique, permettant d’améliorer significativement le
transfert de masse (Torreggiani et Bertolo, 2001). Cependant, les changements structuraux
dans la paroi cellulaire peuvent mener à une diminution de la fermeté du produit après
congélation (Torreggiani et Bertolo, 2001).
Les produits issus du procédé de déshydratation osmotique sont classés parmi les produits
à taux d’humidité élevé (Garcia-Martinez et al., 2002). Aussi le produit n’est pas encore
microbiologiquement stabilisé, et l’activité de l’eau peut y être élevée (supérieur à 0,9). Pour
éviter l’altération du produit au cours de l’entreposage, plusieurs post-traitements ont été
proposés tels que le séchage, la friture, etc. (Fernandes et al., 2006).
Le séchage est le procédé le plus communément utilisé par les scientifiques et les
industriels (Ade-Omowaye et al., 2003). L’utilisation du séchage dans les industries
agroalimentaires a de multiples objectifs : accroître la durée de conservation des produits;
stabiliser les produits agricoles et transformer les produits par des réactions biochimiques ou
4
Introduction générale ___________________________________________________________________________
biologiques. Cependant, cette technique est coûteuse en énergie. En effet, le séchage des
produits végétaux nécessite environ 5000 kJ/kg d’eau évaporée (Mujumdar, 2006). Afin de
réduire le coût énergétique global de l’élimination de l’eau, plusieurs auteurs proposent la
combinaison entre la déshydratation osmotique et le séchage (Wang et Sastry, 2000; Ade-
Omowaye et al., 2003 ; Fernandes et al., 2006). En effet, la pré-déshydratation diminue la
teneur en eau initiale dans le produit induisant une réduction du temps de séchage et du besoin
énergétique pour le séchage complémentaire (Fernandes et al., 2006).
L’étude du procédé de conservation des graines de grenade par déshydratation osmotique
semble avoir échappé, tant à l'investigation scientifique qu'à l'exploitation l'industrielle. C'est
cette approche originale que nous nous sommes proposés d'étudier.
Dans ce contexte, le présent travail de thèse s’est attaché à délivrer les bases scientifiques
et techniques pour l’étude des possibilités de conservation des graines de grenade (variété
Tunisienne El-Gabsi) par déshydratation osmotique.
Pour y parvenir, nous avons tout d’abord optimisé la déshydratation osmotique des graines
de grenade en analysant les effets de l’influence de la température (30, 40, 50°C) et de la
composition des solutions osmotiques (saccharose, glucose, saccharose/glucose) sur la
cinétique de transfert de masse, ainsi que sur les caractéristiques physico-chimiques des
graines de grenade. Afin d’explorer l’effet du pré-traitement de congélation sur le procédé de
DO, nous avons comparé, dans un deuxième temps, la cinétique de transfert de masse, la
structure des cellules et la texture des graines fraiches et congelées avant et après DO. Le
troisième volet a porté sur la valorisation d’une deuxième agrofourniture tunisienne, le jus de
datte, dont l’utilisation permet une réduction du coût économique global du procédé. En
Tunisie, le saccharose est un produit importé alors que le jus de datte est issu des dattes
déclassées (à bas prix) pour des raisons de texture et de forme. Ainsi ce volet avait comme
objectif la détermination de l’influence du milieu d’immersion (solution à base de jus de datte
5
Introduction générale ___________________________________________________________________________
6
en substitution du saccharose) sur la cinétique de DO, et la caractérisation des changements
structuraux et texturaux des graines de grenade avant et après DO. Pour une meilleure
stabilisation des graines de grenade au cours de la conservation, nous avons abordé l’effet des
conditions du post-traitement de séchage par entrainement sur les caractéristiques physico-
chimiques et rhéologiques des graines déshydratées osmotiquement.
Afin de mieux élucider l’influence du pré-traitement de congélation, du traitement de DO
et du post-traitement de séchage sur la qualité des graines de grenade, nous avons mis en
œuvre des techniques fines tels que la calorimétrie différentielle à balayage («differential
scanning calorimetry », DSC), la microscopie électronique à balayage, et la texturomètrie.
Avant d’aborder l’ensemble des résultats et leurs discussions, un passage en revue de la
littérature présente les principales connaissances sur le procédé de déshydratation osmotique
des fruits et légumes. (*)
___________________________________________________________________________
* Les références bibliographiques sont reportées en fin du document.
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Chapitre 1:
Synthèse bibliographique :
Ce travail a fait l’objet de la publication suivante :
Bchir, B., Besbes, S., Giet, J., Attia, H., & Blecker, C. (2010). Synthèse des
connaissances sur la déshydratation osmotique. Biotechnologie Agronomie
Société et Environnement, (in press).
7
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Titre : Synthèse des connaissances sur la déshydratation osmotique
Résumé :
Parmi les procédés de conservation des produits végétaux, la déshydratation
osmotique présente un intérêt économique et nutritionnel certain. Cette technique, économe
en énergie, est susceptible de prolonger la période de disponibilité des produits alimentaires,
et leur confère des propriétés sensorielles nouvelles et appréciées. Elle permet ainsi aux
acteurs de la filière agroalimentaire d’écouler leurs productions à de meilleurs prix et aux
consommateurs d’en disposer tout au long de l’année. Cette technique est un outil facile à
mettre en place, surtout dans les pays en voie de développement, en raison de son faible coût.
Le présent article a pour objectif de présenter une synthèse de la littérature concernant la
technique de déshydratation osmotique, afin d’en rappeler les bases théoriques et pratiques,
mais aussi d’en préciser les nouvelles tendances et voies de recherches récentes.
8
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Synthèse des connaissances sur la déshydratation osmotique
*Brahim Bchir1, Souhail Besbes2, Jean-Michel Giet1, Hamadi Attia2, Christophe
Blecker1
1 : Unité de Technologie des industries agro-alimentaires, Université de Liège Agro-Bio
Tech, Passage des Déportés, 2-B-5030 Gembloux, Belgique.
2 : Unité Analyses Alimentaires, Ecole Nationale D’ingénieurs de Sfax, Route de Soukra,
3038 Sfax, Tunisia
* Corresponding author: Tel: +32/ 8162273, Fax: +32/81614222
E-mail address: [email protected]
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Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Résumé :
Parmi les procédés de conservation des produits végétaux, la déshydratation
osmotique présente un intérêt économique et nutritionnel certain. Cette technique, économe
en énergie, est susceptible de prolonger la période de disponibilité des produits alimentaires,
et leur confère des propriétés sensorielles nouvelles et appréciées. Elle permet ainsi aux
acteurs de la filière agroalimentaire d’écouler leurs productions à de meilleurs prix et aux
consommateurs d’en disposer tout au long de l’année. Cette technique est un outil facile à
mettre en place, surtout dans les pays en voie de développement, en raison de son faible coût.
Le présent article a pour objectif de présenter une synthèse de la littérature concernant la
technique de déshydratation osmotique, afin d’en rappeler les bases théoriques et pratiques,
mais aussi d’en préciser les nouvelles tendances et voies de recherches récentes.
Summary:
Among the preservation processes of vegetal products, osmotic dehydration presents
an economic and a nutritional interest. This technique consumes a low quantity of energy,
prolongs the period of availability of foodstuffs, and gives new and appreciated sensory
properties to products. Therefore, the producers can sell their productions with better prices
and the consumers are able to consume fruits and vegetables throughout the year. This
technique is very easy to set up, especially in the developing countries due to its low cost. The
aim of this article is to present a synthesis of the literature concerning the osmotic dehydration
technique, and also to specify the new tendencies and directions of recent research.
Mot-clés : Déshydratation osmotique, conservation, nouvelles tendances
Keywords: Osmotic dehydration, preservation, new tendencies
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1. INTRODUCTION
Les industriels du secteur agro-alimentaire sont aujourd’hui confrontés à deux
problèmes majeurs : d’un part l’attente croissante des consommateurs pour des produits de
hautes qualités nutritionnelles et organoleptiques et, d’autre part, l’augmentation des coûts
énergétiques. En réponse à ces défis, les techniques de stabilisation et de conservation des
aliments, tels que le séchage ou les techniques du froid, connaissent des améliorations
constantes, et sont de mieux en mieux intégrées aux filières industrielles.
Une catégorie de méthodes de séchage fait intervenir une ou plusieurs étapes de mise
en contact des denrées avec une solution aqueuse concentrée en sels (ex. saumurage des
légumes, viandes, poissons ou fromages), en acide (ex. marinage des produits carnés) ou en
sucres (confisage et semi-confisage des fruits). Ce traitement vise à réduire, à moindre coût, le
risque d’altération de la qualité nutritionnelle et organoleptique du produit traité (Ade-
Omowaye et al., 2003). Le confisage est une des techniques traditionnelles dont les
développements récents ont donné naissance aux procédés dits de « déshydratation
osmotique » (DO) ou de « déshydratation-imprégnation par immersion » (DII) (Albagnac et
al., 2002).
La déshydratation osmotique présente un certain nombre d’atouts par rapport aux
techniques traditionnelles de séchage. En particulier, l’aliment est traité à plus basse
température (entre 5 et 85°C), et à l’abri de l’oxygène (puisqu’il est immergé), ce qui est
particulièrement favorable pour les produits sensibles aux réactions de dégradation oxydatives
et thermiques (Lerica et al., 1985; Garcia-Segovia et al., 2010).
De plus, la DO permet de reduire la charge microbienne, et ainsi de prolonger la
période de conservation des produits (Castello et al., 2009).
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La déshydratation osmotique est attribuée au phénomène d’osmose, qui se manifeste à
travers les membranes cellulaires « semi-perméables » (perméables à l’eau, mais moins aux
solutés) des tissus. Raoult-Wack, (1994) a montré que le moteur de ce transfert est une
différence de concentration entre la solution et le matériau à traiter. Il se traduit par deux
écoulements simultanés à contre-courant : une diffusion de l’eau des cellules du produit (la
solution la moins concentrée) vers la solution hypertonique où l’aliment est plongé
(déshydratation) et une entrée de soluté de la solution vers l’aliment (imprégnation).
L’aliment peut ainsi perdre jusqu’à 50% de la teneur initiale en eau en moins de 3 heures
(Lerica et al., 1985). La sortie d’eau s’accompagne généralement d’une perte de solutés
propres au produit alimentaire. Ce transfert, quantitativement négligeable par rapport aux
deux premiers, soulève des critiques quant à son impact sur les qualités organoleptiques et
nutritionnelles du produit transformé (Albagnac et al., 2002).
Afin d’améliorer l’efficacité du processus de déshydratation osmotique, divers
traitements peuvent être appliqués pour faciliter la diffusion de l’eau: ultrasons, irradiation,
champ électrique pulsé, etc. Pour une conservation de très longue durée, le produit obtenu
après déshydratation osmotique peut encore subir un traitement complémentaire, tel qu’un
séchage à l’air ou une congélation (Garcia-Segovia et al., 2010).
Employée industriellement depuis les années 60, la DO fait récemment l’objet d’un
certain regain d’intérêt. Au cours de la dernière décennie, la progression constante du nombre
d’articles scientifiques publiés annuellement sur le sujet en témoigne. Le présent travail se
propose de rappeler les bases de la déshydratation osmotique, d’exposer les différents facteurs
influençant ce processus et de présenter les dernières avancées scientifiques en termes
d’amélioration des performances de la technique.
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2. GENERALITES SUR LA DESHYDRATATION OSMOTIQUE
2.1. Cinétique de la déshydratation osmotique
Les cinétiques de transfert de matière dans les produits végétaux (Tableau 1a-1b)
peuvent se décomposer en deux phases : une première phase, responsable de l’essentiel des
transferts d’eau et de solutés, suivie d’une seconde phase, pendant laquelle la perte en eau
ralentit fortement tandis que les débits d’entrée en solutés continuent d'augmenter
régulièrement (Kowalska et al., 2008). Il semble probable que les membranes cellulaires
soient victimes d'une perte de leur caractère semi-perméables, permettant progressivement
aux solutés de pénétrer dans la cellule (Raoult-Wack, 1994). La durée de la première phase est
très variable suivant le produit traité, d’une demi-heure à deux heures dans les conditions les
plus courantes (morceaux de petites tailles, de l’ordre du cm3). Ces transferts se déroulent à
travers les parois et membranes cellulaires du produit. À l’intérieur de ces derniers, les
espaces intercellulaires servent de lieux d’accumulation ou de passage pour les substances
échangées (Raoult-Wack, 1994; Lenart, 1996; Kowalska et al., 2008).
Deux approches sont employées afin d’étudier la cinétique de la déshydratation
osmotique :
L’approche classique qui se base sur la détermination de deux paramètres. En effet, des
travaux antérieurs ont prouvé que deux paramètres peuvent quantitativement représenter le
processus osmotique. Ces paramètres sont la perte d’eau (« Water loss », WL), indiquant l’eau
qui sort du matériel cellulaire vers la solution, et le gain en solides (« Solids Gain », SG). Ces
paramètres sont habituellement déterminés par la mesure des solides totaux, ou par analyse
chimique (Krokida et al., 2000 ; Riva et al., 2005; Garcia-Segovia et al., 2010).
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Tableau 1 : Application de la déshydratation osmotique sur des fruits (1a) et légumes (1b).
Fruits Conclusions majeures Auteurs
Abricot L’utilisation d’une température de 50°C, une concentration de saccharose de 60°Brix et un rapport solution/produit de 10:1 offrent une meilleure déshydratation.
Khoyi et Hesari, 2007
Ananas La perte en eau maximale (0.64-0.77 kg d’eau/Kg de tranche) a été observée à une température comprise entre 30°C et 60°C.
Jena et Das, 2004
Banane L’utilisation de deux bains successifs aboutit à une réduction de poids de 60% et un gain de soluté de 2%.
Jiokap Nono et al., 2001
Cerise Le temps, le rapport solution/produit, et le sucre utilisé ont un effet significatif sur le transfert de masse.
Sunjka et Raghavan, 2004
Châtaigne La meilleure déshydratation a été obtenue à haute concentration en soluté et basse température.
Chenlo et al., 2007
Fraise Les cellules ne sont pas affectées par la nature du soluté. Le coefficient de diffusion de l’eau est très affecté par le Pré-traitement de blanchiment.
Ferrando et Spiess, 2001
Grenade L’augmentation de la température favorise la perte en eau et le gain en solutés de la graine. L’analyse par DSC montre une réduction de la mobilité de l’eau au cours du procédé. D’autre part, Tg’ dépend du type de sucre et de la teneur en eau dans la graine.
Bchir et al., 2009
Mangue La congélation a un effet positif sur la perte en eau, et un faible effet sur le gain en solutés. L’utilisation d’une solution de saccharose de 45°Brix à 30°C offre une meilleure perte en eau et un gain en soluté des mangues congelées.
Floury et al., 2008
Melon L’utilisation d’une solution de maltose (40-60°Brix) entraine une augmentation de la perte en eau et une diminution du gain en soluté, contrairement au saccharose.
Ferrari et Hubinger, 2008
Kiwi Toute augmentation de température et de la concentration en sucre se traduit principalement par une augmentation des vitesses de transfert d’eau, les transferts de soluté n’étant que peu affectés. L’utilisation de solutés de déshydratation différents (saccharose, saccharose/sucre inverti, sirop de glucose) n’induit que de faible variation au niveau des transferts de matière.
Vial et al., 1990
Papaye L’augmentation de la température (de 30 à 70°C), de la concentration en saccharose dans la solution de déshydratation (de 45 à 72°Brix), la présence de calcium (0.05 M), l’absence de blanchiment préalable et/ou le remplacement du saccharose par un sirop de glucose de faible dextrose équivalent favorisent la perte en eau et freinent la pénétration du sucre.
Heng et al., 1990
Pastèque L'augmentation de la température et de la concentration de la solution osmotique provoque une augmentation du transfert de masse.
Falade et al., 2007
Pomme L’utilisation de sirops vieillis n’a pas d’influence significative sur les cinétiques de transfert de masses.
Jiokap Nono et al., 2001
(1a)
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Légumes Conclusions majeures Auteurs Carotte L’augmentation de la masse moléculaire du soluté entraine une
augmentation du coefficient de diffusion de l’eau. Le traitement par DO avant séchage améliore efficacité du séchage par entrainement.
Uddin et al., 2004
Citrouille La DO entraine une modification de la structure cellulaire (45°Brix, 25°C, 20 :1, 9h).
Mayor et al., 2008
Concombre Le traitement par DO avant congélation améliore la fermeté du fruit et prolonge sa période de conservation par rapport à la DO seule.
Dermesonlouoglou et al., 2008
Goyave La DO sous vide à une température de 40 et 50°C offre le meilleur coefficient de diffusion d’eau.
Panades et al., 2008
Haricot La perte d'eau maximale a été obtenue quand des tranches de fruit de 10 millimètres ont été immergées dans une solution de concentration en sucrose de 60° Brix, maintenue à 60 °C pour 2 h, alors que l'imprégnation maximum était obtenue quand les tranches de 5 mm ont été immergées dans une solution de 50° Brix, maintenue à 60 °C durant 6 h.
Abud-Archila et al., 2008
Oignon Le traitement avec une solution de 40% de saccharose entraine une augmentation de la vitesse de destruction cellulaire au début du procédé. Le maltose et le tréhalose ont un effet protecteur sur la membrane plasmique.
Ferrando et Spiess, 2001
Pommes de terre
Les conditions optimales pour la DO de la pomme de terre sont : température : 22°C ; concentration en saccharose : 54% ; concentration en sel : 14% et le temps de traitement de 329 min.
Eren et Kaymak-Ertekin, 2007
Tomate et tomate cerise
Le traitement physique par perforation du fruit a permit une perte d’eau plus élevée que par traitement chimique.
Shi et al., 1998
(1b)
Dans la littérature, des travaux de modélisation ont recours au coefficient de diffusion
(Deff) issu de l’équation de Fick (Eq. 1) (Crank, 1975) :
xCSDeffm δδφ ..−= (Eq. 1)
Où Φm est le flux de matière traversant la surface S pendant l’unité de temps t (Kg/s) ;
S est la section normale à la direction du flux (m²); C est la concentration (Kg/m3); Deff est le
coefficient de diffusion (m²/s) ; x est la distance sur un axe parallèle à la direction du flux (m).
L’approche fine qui se base sur l’étude des paramètres déterminés à partir de la
calorimétrie différentielle à balayage et la résonance magnétique nucléaire (RMN). Ces
techniques permettent d’approcher les liaisons de l’eau dans le produit.
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Calorimétrie différentielle à balayage
La calorimétrie différentielle à balayage (« differential scanning calorimetry », DSC) est
une méthode thermo-analytique permettant la mesure de propriétés physiques d’un
échantillon soumis à un programme de température. Le principe de la DSC est de comparer le
flux de chaleur nécessaire pour maintenir la température d’un échantillon (capsule contenant
l’échantillon) égale à celle d’une référence (capsule vide), chauffés ou refroidis à une vitesse
contrôlée. Ce flux de chaleur à une température donnée est directement proportionnel à la
chaleur spécifique (Cp) du matériau à cette température (Eq. 2).
T)f(t,dtdT
pCdtdQ
+= (Eq. 2)
Où Q est le flux de chaleur absorbé ou libéré par l’échantillon (mW g-1), Cp est la
chaleur spécifique de l’échantillon (J g-1), T est la température (°C), t le temps et f(t,T) est
une fonction dépendante du temps et de la température.
Des transformations thermodynamiques de 1er ordre, comme la cristallisation ou la fusion,
vont se traduire respectivement par un pic exothermique de cristallisation ou un pic
endothermique de fusion. La position de ces pics indique la température de transformation
(T°fusion ou T°cristallisation); leur aire est proportionnelle à l'enthalpie du processus. Une
transformation de 2ème ordre sera caractérisée par une marche, trahissant un saut de la chaleur
spécifique (Cp), dont le point d’inflexion correspond à la température de transition vitreuse
(Tg). Cette transition correspond durant le chauffage, au changement réversible de la phase
amorphe d’un polymère d’une forme vitreuse (relativement durs, à faible mobilité de l’eau) en
une forme visqueuse ou caoutchoutique (mou, à plus forte mobilité de l’eau). La
transformation est inverse au refroidissement. La détermination de la température de
transition vitreuse, la température de fusion et de cristallisation et de l’enthalpie de fusion et
de cristallisation au cours du temps permet de quantifier la teneur en eau libre et liée dans le
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produit ainsi que d’étudier la cinétique de déshydratation du produit (Cornillon, 2000;
Ohkuma et al., 2008).
Résonance magnétique nucléaire (RMN)
La méthode par RMN repose sur l’analyse de l’interaction entre l’eau et la matrice
macromoléculaire. Les molécules d’eau peuvent interagir de manière labile ou permanente,
définissant ainsi des compartiments distincts dits d’eau faiblement liée (ou vacuolaire) et
d’eau d’hydratation (ou de surface). Au cours des processus d’échange, les protons de l’eau
interagissent avec ceux de la matrice soit par échange chimique direct, soit par une interaction
magnétique (dipôle-dipôle). Pendant la détection RMN (base résolution impulsionnelle) de
l’aimantation des noyaux, certains protons peuvent changer d’état (de compartiment) et cette
modification affecte le signal RMN enregistré. Le retour à l’équilibre de l’aimantation est
caractérisé par deux temps de relaxation des protons de l’eau. La relaxation T1 ou relaxation
spin-réseau caractérise le retour à l’équilibre des populations spins. La relaxation T2 ou spin-
spin représente l’amortissement de l’aimantation dans le plan transversal. La détection et
l’analyse de cette perturbation (transfert d’aimantation) sont à la base de la quantification des
proportions d’eau libre et d’eau d’hydratation, et de l’analyse de la cinétique des échanges
entre ces deux systèmes (Riggs et al., 2001). Actuellement, d’autres méthodes comme
l’imagerie par résonance magnétique (IRM) sont mises à profit afin d’étudier la structure du
produit et de quantifier les échanges internes (Derossi et al., 2008).
2.2. Principaux facteurs influençant les performances de la DO
Les cinétiques de transfert d’eau et de solutés dépendent de trois facteurs (Rastogi et
Raghavarao, 2004; Dermesonlouoglou et al., 2008 ; Garcia-Segovia et al., 2010):
Les propriétés intrinsèques des tissus traités: la structure poreuse, la taille, la forme, la
superficie du produit;
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Les conditions opératoires de traitement: temps, température de traitement, pression,
agitation de la solution, composition de la solution;
Le mode de mise en contact des phases entre aliment solide et solution liquide.
2.2.1. Propriétés des tissus biologiques
Tout ce qui est préjudiciable à l’intégrité des tissus, telle qu’une maturation trop
avancée, la mise en œuvre de pré-traitements thermiques, chimiques ou enzymatiques peut
entraver la perte en eau tout en favorisant le gain en soluté. La grande variabilité observée
dans le comportement des végétaux au cours d’un traitement de DO est généralement
attribuée aux différentes propriétés tissulaires. Ces dernières incluent la compacité des tissus,
l’importance relative des espaces intra- et extra-cellulaires, la porosité et la teneur initiale en
matières sèches (Lenart, 1996). En effet, la porosité de l’aliment affecte sa texture et influence
sa fermeté. Les changements de porosité causés par le processus osmotique favorisent l’action
des forces d’entraînement non-diffusionelles, tels que des gradients de pression (Nieto et al.,
2004). La majorité des produits végétaux (Tableau 1a-1b) sont découpés en cube ou en sphère
avant le traitement de déshydratation osmotique, ce qui facilite le transfert de matière grâce à
un contact direct entre les cellules et la solution (Kowalska et al., 2008).
Lors d’une DO, quelques structures cellulaires peuvent devenir endommagées, tandis
que les autres restent pratiquement inchangées. Les traitements osmotiques impliquent ainsi
un stress cellulaire par suite de la réduction de l’eau disponible dans les cellules, ce qui
modifie leur physiologie (Rastogi et Raghavarao, 2004).
2.2.2. Concentration et composition de la solution osmotique
La différence de concentration en soluté entre le produit à traiter et la solution est le
moteur du transfert de masse en DO. La perte en eau est plus importante lorsque cet écart est
initialement élevé (Raoult-Wack, 1994). En effet, Vial et al. (1990) ont montré que toute
augmentation de la concentration en sucre se traduit principalement par une augmentation des
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vitesses de transfert d’eau, les transferts de soluté n’étant que peu affectés. Pour la
déshydratation des fruits, des solutions de sucre concentrées de 50 à 70°Brix ont été
employées (Lerica et al., 1985; Corrêa et al., 2010). Néanmoins, il existe une concentration
seuil (entre 50 et 65°Brix) au-delà de laquelle l’imprégnation décroît (Raoult-Wack, 1994;
Lenart, 1996).
La composition des solutions (type, masse moléculaire du soluté) mises en œuvre en
DO est un facteur clé du procédé (Corrêa et al., 2010). Les solutions sont préparées à partir de
solutés cristallins solubles ou de solvants miscibles à l’eau, utilisés seuls ou en mélange. Les
constituants doivent être dépourvus de toute toxicité, présenter une solubilité suffisante et
être, idéalement, bon marchés (Rastogi et Raghavarao, 2004). Le choix du soluté est le
résultat d’un compromis entre les exigences technologiques et la qualité du produit final,
c'est-à-dire ses caractéristiques physicochimiques (pH, structure…), ses propriétés
nutritionnelles et organoleptiques (texture, couleur…), ses propriétés fonctionnelles
spécifiques (pouvoir aromatique, sucrant, colorant, état de surface collant ou brillant - dans
l'exemple du glucose) et son pouvoir dépresseur de l’activité en eau. Les solutions à base de
sucres (saccharose), sont les plus couramment utilisées dans la DO des fruits (Lenart, 1996).
Toutefois, il est essentiel de prendre en compte le coût de ces solutés, qui peut se révéler
prohibitif (Giraldo et al., 2003).
Utiliser différents solutés en mélange permet de tirer parti de l’effet respectif de
chacun (masse molaire, propriétés de diffusion...), mais aussi de développer des interactions
spécifiques (soluté/soluté et soluté/aliment) pour mieux maîtriser les niveaux de
déshydratation et d’imprégnation (Giraldo et al., 2003). En pratique, l’utilisation de sucres de
masse molaire élevée (hydrolysats d’amidon de faible indice d'équivalent dextrose ), en
mélange avec le saccharose, conduit à des niveaux de déshydratation plus élevés et des
niveaux d’imprégnation plus faibles que ceux obtenus avec une solution de saccharose. Au
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contraire, l’utilisation de solutés de masse molaire plus faible que le saccharose, tels que les
sucres invertis (fructose, glucose), permet d’obtenir des niveaux d’imprégnation plus élevés
(Saurel et al., 1995 ; Torreggiani et Bertolo, 2001 ; Corrêa et al., 2010). Pour intégrer au
mieux ces deux impératifs, et optimiser la déshydratation tout en limitant l’imprégnation, il
faut utiliser une solution mixte mettant en œuvre deux solutés de masse molaire bien
distinctes (Saurel et al., 1995). L’intérêt de solutions ternaires, ou plus complexes, associant le
saccharose avec d’autres sucres de masse molaire différente, ou associant sucres et chlorure
de sodium, a été mis en évidence expérimentalement (Lenart, 1996). L’addition de NaCl à une
solution osmotique semble augmenter la force d’entraînement lors de la déshydratation. Ce
phénomène est attribué à la capacité du NaCl d'abaisser l’activité de l’eau (aw) (Kowalska et
al., 2008). LeMaguer et Sharma, (1997) ont montré que l’utilisation d’une solution osmotique
contenant 44% de saccharose et 7% de NaCl permet d’optimiser les conditions de
déshydratation osmotique des carottes.
2.2.3. Température de la solution osmotique
Le rôle de la température en DO a été étudié sur un large éventail de températures (5 -
85°C), le domaine de travail devant être adapté pour chaque famille de produit (Lerica et al.,
1985, Floury et al., 2008). Une température opératoire comprise entre 20 et 40°C est souvent
considérée comme optimale sur le plan qualitatif (Lerica et al., 1985). A ces températures, la
semi-perméabilité des membranes cellulaires de différents végétaux est à peine affectée.
L’extraction de l’eau est alors possible seulement par des processus osmotiques. Les transferts
d’eau sont favorisés par des températures élevées (Floury et al., 2008). Aussi, le sucrage et
confisage des fruits sont habituellement réalisés à 60°C. Cependant, une température trop
élevée n’est pas souhaitable car la température est l’un des facteurs responsables de la rupture
des tissus végétaux et des membranes. Par exemple, les membranes plasmatiques
commencent à subir des dommages irréversibles et une perte de sélectivité à 55°C (Thebud et
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Santarius, 1982). Ceci provoque une modification de la structure et de la texture du matériau,
mais aussi le développement de réactions de brunissement enzymatique et de dégradation de
la couleur. Pour chaque fruit et légume existe en outre une température seuil, au-delà de
laquelle la qualité du produit est affectée et les transferts de soluté prennent le pas sur le
transfert d’eau (Floury et al., 2008).
2.2.4. Durée du traitement
La durée du traitement est un facteur important à considérer, quels que soient les
produits traités. Généralement, la perte d’eau, la réduction de masse et le gain en solides
augmentent avec le temps de traitement (Rastogi et Raghavarao, 2004 ; Kowalska et al.,
2008).
Marchal et al. (2005) ont rapporté un changement de sélectivité au cours de la
déshydratation, c'est-à-dire que le rapport de la perte en eau sur le gain en solide (WL/SG)
décroît au cours du temps. Ce phénomène déjà mentionné, est attribué à la mort des cellules
qui accompagne l’augmentation de la concentration en sucre dans le tissus (Mavroudis et al.,
2004). Ceci conduit à la perte de fonctionnalité de la membrane cellulaire et peut affecter la
qualité du produit. Lenart, (1996) a montré que la durée de déshydratation de morceaux de
pomme ne doit pas dépasser une durée de 15 min, à une température comprise entre 70 et
90°C.
2.2.5. Mode de mise en contact des phases, effet de l’agitation et du rapport
solide/solution
Dans le cas de tranches de fruits, les coefficients de transfert d’eau libre et de
saccharose de la solution de déshydratation augmentent, non seulement avec la concentration
en saccharose, mais aussi avec l’agitation, (Vial et al., 1990). Marouzé et al. (2001) ont
montré que le transfert de masse nécessite une agitation entre la solution et le produit, qui,
pouvant être discontinue, permet un gain d’énergie. En effet Mavroudis et al. (2004) ont
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mesuré les effets de l’agitation sur le transfert de masse en terme de nombre de Reynolds, et
ont montré qu’une forte agitation augmente la perte en eau. Au cours du temps, la perte en
eau s’avère moindre lorsque la DO est réalisée en écoulement laminaire plutôt que turbulent.
Le gain en solide n’est que peu affecté par le niveau d’agitation, ce qui s'explique par
l’existence d’une couche limite diluée autour de l’aliment (Giroux et Marouzé, 1994).
Les transferts de matière dépendent fortement de la manière dont sont mises en contact
les phases solide (fragile et « légère »; l'aliment) et liquide (« lourde » et visqueuse, la
solution hypertonique). Une viscosité élevée du liquide augmente la résistance externe (à
l’interface solide/liquide) aux transferts de matière et nécessite la mise en œuvre d’un système
de brassage adapté, compatible avec la fragilité des produits.
Des études ont montré qu’un rapport pondéral, solution de déshydratation/tranches de
fruit, trop grand (facteur de dilution trop marqué), rend difficile la détermination des
différentes substances diffusées et le suivi efficace du phénomène osmotique. Par contre, un
rapport petit ralentit le taux de diffusion (Adamrounou et al., 1994).
Pour une meilleure efficacité de la DO, les systèmes de mise en contact des phases
doivent permettre de réduire la dispersion des temps de séjour, forcer l’immersion des
produits, réduire les effets de couche limite et préserver la forme et la fragilité des produits.
2.2.6. Mise en œuvre de la déshydratation osmotique
Chaque fruit ou légume présentant ses propres conditions optimales de déshydratation
osmotique, il est difficile de mettre en avant une méthodologie universelle.
Typiquement, tant pour les fruits que pour les légumes, le régime turbulent est préféré,
avec des temps de séjour compris entre 5 et 480 min. L'optimum de température se trouve
habituellement entre 25°C et 80°C, et le rapport produit/solution évolue généralement entre
1/2 et 1/20.
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Le soluté le plus fréquemment rencontré est le saccharose, employé à des
concentrations comprises entre 38°Brix et 65°Brix pour les fruits, entre 35°Brix et 60°Brix
pour les légumes. L'usage du sel comme soluté est souvent réservé à la déshydratation des
légumes.
Il apparait ainsi nécessaire, industriellement, d'optimiser les conditions de traitement
spécifiquement pour chaque produit. Les tableaux 2a et 2b fournissent à ce sujet quelques
exemples représentatifs.
Tableau 2: Les conditions optimales de déshydration osmotique des fruits (2a) et légumes (2b)
Fruits
Paramètres Ananas Châtaigne Grenade Kiwi Melon cantaloup
Mangue
Concentration en soluté :
Saccharose
62°Brix 60°Brix 50°Brix 60°Brix 38°Brix 65°Brix
Température 30°C 25°C 50°C 40°C 41°C 35°C
Temps 360 min 480 min 20 min 150 min 132 min 360 min
Rapport (produit/solution) 1:6 - 1:4 - 1:20 -
Références Singh et al., 2008a
Chenlo et al., 2007
Bchir et al., 2009
Cao et al., 2006
Corzo et Gomaz,
2003
Madamba et Lopez,
2002
(2a)
Légumes
Paramètres Carotte Chou-fleur Pois patate
Poivre vert
Radis Tomate
Saccharose 52°Brix - 60°Brix - - 35°Brix
Sel - 12°Brix - 5.5°Brix 25°Brix 5°Brix
Concentration en solutés
Sorbitol - - - 6% - -
Température 49°C 80°C 60°C 30°C 36°C 60°C
Temps 150 min 5 min 120 min 240 min 95 min 120min
Rapport (produit/solution) 1:10 2:4 1:8 1:3 1:15 1:4
Références Singh et al.,
2008b
Vijayanand et al., 1995
Abud-Archila et al., 2008
Ozdemir et al., 2008
Petchi et Manivasagan,
2009
Souza et al., 2007
(2b)
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2.3. Application de la déshydratation osmotique
La DO est un procédé relativement lent. Il est donc important de trouver des méthodes
qui augmentent le transfert de masse sans affecter la qualité du produit. Ainsi, un traitement
permettant d’augmenter la perméabilité des membranes cellulaires et de faciliter la libération
de l’eau pendant la DO est obligatoire. Parmi les pré-traitements utilisés, on peut citer les
méthodes thermiques de blanchiment et de congélation (Kowalska et al., 2008). D'autres
procédés consistent à remplacer un traitement unique (ici, la DO) par la combinaison de
plusieurs techniques de conservation modérées, respectueuses du produit, et pouvant accélérer
les transferts. Ces techniques peuvent être l'application du vide, les hautes pressions
hydrostatiques, l’ultrason, l’irradiation, la centrifugation, le champ électrique pulsé etc.
2.3.1. Pré-traitement thermique
2.3.1.1. Blanchiment
Le blanchiment est un traitement thermique, réalisé par immersion du produit dans un
bain d’eau chaude, par passage dans une atmosphère de vapeur ou par chauffage ohmique. Sa
durée est de quelques minutes, dans une gamme de 85°C à 100°C. Il permet de détruire les
enzymes susceptibles d’altérer les légumes ou les fruits avant leur traitement ultérieur (dans
notre cas c’est la déshydratation). Ce procédé prévient ainsi un certain nombre d’altérations
organoleptiques, telles que des modifications de flaveurs et de couleurs (dégradation de la
chlorophylle, brunissement des pommes, etc.). Il limite également certaines pertes
nutritionnelles comme la destruction des vitamines, et permet l’élimination de l’air et des gaz
occlus dans les tissus végétaux facilitant la réhydratation (Dermesonlouoglou et al., 2008).
Vis-à-vis de la DO, le blanchiment facilite le transfert des matières dissoutes, comme
il l’a été rapporté pour les tranches de pomme et de tomate précédemment blanchies à la
vapeur (Dermesonlouoglou et al., 2008 ; Kowalska et al., 2008). En revanche, le blanchiment
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est opéré à des températures trop élevées pour les constituants cellulaires, causant un risque
accru d'altération qualitative et de pertes par dissolution.
2.3.1.2. Congélation
L’eau est le principal composant de la majorité des aliments congelés du commerce.
Une part notable de cette eau est liée à divers degrés: dans des complexes colloïdaux
macromoléculaires, dans des structures gélifiées ou fibreuses à l’intérieur des cellules, et dans
les hydrates. Lors de la congélation, la nucléation de la glace et la croissance des cristaux
apportent de nombreuses modifications au produit (Talens et al., 2003). Les composantes
cellulaires solubles peuvent atteindre la saturation et précipiter, détruisant ainsi la turgescence
des tissus; des modifications de pH peuvent affecter les complexes colloïdaux ; des
changements très marqués de pression osmotique peuvent rompre les membranes semi-
perméables, ce qui facilite le transfert de masse au cours de la déshydratation osmotique
(Floury et al., 2008 ; Kowalska et al., 2008 ; Dermesonlouoglou et al., 2008 ; Bchir et al., In
press). La vitesse de congélation et la température finale de conservation sont des points
critiques pour le maintien des propriétés sensorielles, fonctionnelles ou biologiques après la
congélation. Une congélation très lente peut conduire à un exsudat excessif à la
décongélation, alors qu’une congélation très rapide permet de préserver la texture de certains
produits (Talens et al., 2003).
2.3.2. Méthodes combinées à la DO
2.3.2.1. Imprégnation sous vide
Le procédé de DO pour les produits végétaux est généralement mis en œuvre à
pression atmosphérique (Raoult-Wack, 1994). Toutefois, l’application d’une dépression
stationnaire augmente la vitesse de déshydratation (Corrêa et al., 2010). La présence de gaz
occlus dans les espaces intercellulaires de la structure poreuse du produit traité apparaît
comme la cause principale de la modification des cinétiques de transferts de matières (Fito,
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1994; Wu et al., 2009). Pendant la DO sous vide, le gaz est expulsé du tissu tandis que
l’écoulement capillaire augmente. L’augmentation de la vitesse du transfert d’eau est
principalement attribuée à l’action combinée du vide et de l’écoulement capillaire, qui dépend
lui-même du volume de gaz occlus dans le tissu. Par conséquent, l’accélération des transferts
de matière en DO sous vide est d’autant plus marquée que la porosité du produit est plus
importante (Corrêa et al., 2010). Cependant, sous vide pulsé, c’est l’imprégnation qui est
favorisée. En effet, lorsque le produit revient à pression atmosphérique, la solution concentrée
pénètre massivement dans les pores du produit alimentaire, ce qui a pour conséquence
ultérieur d’augmenter la surface de contact entre le produit et la solution et d’accélérer ainsi
les transferts de matières (Fito, 1994). Corrêa et al. (2010) ont obtenu par cette technique une
perte d’eau plus élevés que dans le cas d'une DO ordinaire, en utilisant le vide seulement
pendant 15 min. La technologie du sous vide partiel cyclique trouve tout son potentiel lorsque
l’on souhaite formuler le produit à l’aide d’additifs. Ainsi la texture des fruits peut-elle être
améliorée par immersion sous vide dans une solution contenant de la pectine-méthylestérase
ou différentes solutions salines, par exemple à base de chlorure de calcium ou de nitrate de
calcium (Javeri et al., 1991).
La DO sous vide permettrait d’obtenir des produits de qualité organoleptique et
physicochimique supérieure à celle des produits traités par DO à pression atmosphérique. De
plus, cette alternative réduit les coûts énergétiques globaux (Fito, 1994).
2.3.2.2. Haute pression hydrostatique
Les traitements à haute pression permettent une stabilisation microbiologique
significative des aliments, tout en préservant les qualités organoleptiques et nutritionnelles de
manière plus importante que les traitements thermiques. Certaines études (Taiwo et al., 2002;
Rastogi et Raghavarao, 2004) ont mis en évidence le fait que le pré-traitement à haute
pression crée un compactage de la structure cellulaire accompagné d'une libération de
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composants cellulaires. Ce phénomène a pour conséquence la formation d’un gel par liaison
d'ions divalents avec la pectine estérifiée qui limite le coefficient de diffusion des solides. A
haute pression (jusqu’à 600 Mpa) les membranes cellulaires sont réversiblement
perméabilisées. Ce phénomène est imputable aux transitions de phase des bicouches
lipidiques de la membrane cellulaire. Cet effet est recherché pour l’élaboration rapide des
fruits sucrés tout en préservant l’aspect, les qualités organoleptiques et nutritionnelles du
produit frais.
Cependant, la technologie haute pression reste coûteuse, du fait des contraintes de
fabrication des enceintes et par leur capacité limitée.
2.3.2.3. Ultrasons
L’application d’ultrasons (onde à fréquence supérieur à 20 000 Hz) a déjà fait ses
preuves dans l’augmentation du taux de transfert de masse pour la déshydratation osmotique
de tissus poreux comme des cubes de pomme. Simal et al. (2001) ont obtenu par cette
technique une perte d’eau de 27% et un gain de solide de 23% plus élevés que dans le cas
d'une DO ordinaire. L’application d’ultrasons produit un phénomène de cavitation, qui
consiste en la formation de bulles de gaz dans le liquide, qui engendrent, en éclatant, des
fluctuations de pression (Fernandes et al., 2009). Cet effet facilite la diffusion pendant le
processus osmotique et accélère le dégazage du produit, tout en préservant la saveur, la
couleur, et les composants nutritifs les plus sensibles à la chaleur (Simal et al., 2001). Cette
technique permet aussi l’inactivation des enzymes et des bactéries en cassant leurs membrane
cellulaires (Jambrak et al., 2010).
2.3.2.4. Irradiation
La structure intérieure du tissu des produits agricoles peut être lysée par γ-irradiation.
Il en résulte une plus grande perméabilité des cellules, d’où un transfert de masse amélioré
pendant un séchage à l’air (Wang et Sastry, 2000). Rastogi et Raghavarao, 2004, ont rapporté
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que la combinaison de la γ-irradiation avec la DO peut résoudre le problème de la diminution
des transferts de masse pendant un séchage convectif.
2.3.2.5. Chlorure de sodium
Comme il l'a déjà été mentionné, le type d’agent osmotique utilisé, et par conséquent
sa masse moléculaire, affecte fortement la cinétique d’extraction d’eau et de gain en soluté.
D’après Simal et al. (2001), le chlorure de sodium est un excellent agent osmotique, en raison
de sa faible masse moléculaire qui se reflète par sa grande mobilité pendant le transfert de
masse.
Dans le cas des solutions sucrées-salées (Tableau 3), des effets fortement antagonistes
sur le gain en solutés ont été identifiés. L’imprégnation en sel est en particulier limitée par la
présence de sucre. Cet effet « barrière » du sucre sur la pénétration du sel a été mis en
évidence sur des produits végétaux (Lenart, 1996). Il serait dû à la formation, dans l’aliment,
d’une couche périphérique fortement concentrée en sucre. En même temps, la présence de
sucre diminuerait fortement le coefficient de diffusion du NaCl (Lenart, 1996). La présence de
sel empêche par ailleurs dans certains cas la formation d’une croûte superficielle (croûtage du
produit) causée par le sucre.
Tableau 3 : Différents travaux utilisant une solution ternaire pour le traitement des fruits par déshydratation osmotique.
Solutés Concentration
totale (g/100g solution)
Saccharose/NaCl (p/p)
Produit/Soluté (p/p)
Références
Saccharose 50 35/15 1/20 Hawkes et Filk, 1978
50 40/10 1/20 Hawkes et Filk, 1978
+ 50 47.5/2.5 1/20 Biswal et Le Maguer, 1989
50 45/5 1/20 Biswal et Le Maguer, 1989
NaCl 60 59/1 1/5 Lerica et al., 1985
61 59/2 1/5 Lerica et al., 1985
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2.3.2.6. Centrifugation
Cette technique peut être utilisée pour augmenter la perte en eau tout en retardant le
gain en solide. Azuara et al., (1998) ont appliqué une force centrifuge de 64 g pendant la
déshydratation osmotique à 30°C de disques de pomme et de pomme terre. Ils ont observé que
ces conditions augmentent le transfert de masse (perte en eau) de 15% tout en retardant
considérablement (80%) le gain en solide.
2.3.2.7. Traitement par champ électrique pulsé
Le traitement par champ électrique pulsé, pour une intensité de champs électrique
comprise entre 0.5 et 15 kV/cm, entraîne une augmentation de la perméabilité des membranes
végétales. Son action instantanée, sa courte durée d’application (moins d’une seconde), et la
possibilité de traiter des aliments solides à basse température, rendent le champ électrique
pulsé plus promoteur qu’un traitement thermique dans une perspective de diffusion ou
d’extraction d’eau (séchage) ou de métabolites (Ade-Omowaye et al., 2003). Un autre
avantage du champ électrique pulsé réside dans le fait qu’il n’augmente pratiquement pas la
température du produit. Ade-Omowaye et al. (2003) ont montré que la cinétique de DO à
20°C du paprika traité à 2.5 kV/cm est comparable à celle d’une DO réalisée à 55°C sur le
même produit sans application de champ électrique.
2.3.3. Stabilisation des produits déshydratés osmotiquement par des traitements
physiques
Les produits issus du procédé de déshydratation osmotique sont classés parmi les
produits à humidité intermédiaire (PAI), à taux d’humidité élevé (Garcia-Martinez et al.,
2002). Aussi le produit n’est pas encore microbiologiquement stabilisé, et l’activité de l’eau
peut y être élevée. Plusieurs traitements ont été proposés pour parfaire le processus: séchage,
congélation, pasteurisation, friture etc. Les plus communs sont le séchage par air (Ade-
Omowaye et al., 2003) et la congélation (Agnelli et al., 2005).
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2.3.3.1. Séchage
Le séchage provoque un abaissement de l’activité d’eau du produit, c'est-à-dire que
l’eau reste peu disponible pour les micro-organismes et pour les réactions chimiques. On
considère généralement qu’un produit est stable lorsque son activité de l’eau est inférieure ou
égale à 0,65 (Thebud et Santarius, 1982).
L’utilisation du séchage dans les industries agroalimentaires a de multiples objectifs :
accroître la durée de conservation des produits; stabiliser les produits agricoles (maïs, luzerne,
riz, lait, ...) pour amortir le caractère saisonnier de certaines activités; et transformer les
produits par des réactions biochimiques ou biologiques (produits de salaison, touraillage de
malt, etc.). Cependant, cette technique est couteuse en énergie: le séchage des produits
végétaux nécessite environ 5000 kJ/kg d’eau évaporée (Mujumdar, 2006).
La combinaison de la déshydratation osmotique avec le séchage permet d’améliorer la
qualité des produits (Fernandes et al., 2006) et de réduire le coût énergétique global de
l’élimination de l’eau. En effet, la pré-déshydratation diminue le temps de séchage et le
besoin énergétique du séchage complémentaire (Fernandes et al., 2006). En effet, la DO
n'exige qu'entre 100 et 2400 kJ/kg d’eau enlevée, selon les applications (Mujumdar, 2006).
Après une période de mise en régime, la cinétique de séchage par l'air est caractérisée par une
période à vitesse constante, correspondant à l’évaporation de l’eau de surface qui est
constamment renouvelée par transport interne et qui se traduit par une variation linéaire de la
teneur en eau en fonction du temps. Cette première période est suivie d’une ou plusieurs
étapes à vitesses décroissantes, où les forces capillaires n’acheminent plus suffisamment
d’eau en surface pour compenser l’évaporation (Ade-Omowaye et al., 2003). Pour les produits
alimentaires et biologiques, le séchage est limité par la résistance des parois cellulaires, par la
migration des solutés qui obstruent les pores et par le croûtage de la surface (Wang et Sastry,
2000; Ade-Omowaye et al., 2003).
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2.3.3.2. Congélation
La congélation des aliments est un excellent moyen de maintenir pendant longtemps,
presque inchangées, leurs valeurs nutritionnelles. Cette préservation de la qualité s’explique
tant par l’abaissement de la température qui ralentit les réactions biochimiques et inhibe les
activités microbiennes que par la réduction de l’activité de l’eau du substrat (Floury et al.,
2008).
La possibilité de prétraiter les produits par une déshydratation partielle (voire par
imprégnation) avant congélation semble prometteuse (Talens et al., 2003 ; Wu et al., 2009).
Cette technique, dite de déshydro-congélation, permet la réduction de la quantité d’eau dans
le produit afin de diminuer la quantité de cristaux formés, le temps de congélation et de
décongélation. Il en résulte une meilleure conservation des propriétés du fruit. La
déshydratation osmotique constitue de ce point de vue un pré-traitement efficace (Talens et
al., 2003 ; Dermesonlouoglou et al., 2008).
Au point de vue énergétique, on notera que, sans déshydratation préalable, la
congélation des fruits nécessite entre 250 et 340 kJ/kg d’eau congelée, et entre 150 et 320
kJ/kg d’eau congelée pour les légumes (Mujumdar, 2006).
2.4. Équipements pour la DO
Diverses configuration de mise en contact des phases ont été proposées et étudiés
(Figure 1). En pratique, l’opération peut être réalisée en contacteur continu ou discontinu,
avec ou sans agitation. Certains auteurs proposent simplement une immersion forcée des
morceaux dans le liquide (Marouzé et al., 2001). Cette solution est d’ailleurs utilisée dans la
plupart des entreprises de semi-confisage de fruits. D’autres systèmes plus élaborés ont été
proposés en vue d’une mise en œuvre du procédé en continu (contact permanent entre le
solide et le liquide) (Raoult-Wack, 1994; Giroux et Marouzé, 1994).
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Immersion sans agitation
Immersion avec agitation
Immersion avec agitation intermittente
* Mélange hydraulique * Mélange hydraulique et mécanique * Déplacement aliment/solution * Arrosage monocouche * Arrosage multicouche
Film de solution entourant l’aliment
* Immersion avec agitation continue mécanique * Lit infiltré fixe en lot * Lit infiltré en lot * Lit infiltré mobile avec lent co-courant.
Mise en contact des phases
Figure 1. Les différents systèmes de mise en contact des phases (solution osmotique et l’aliment)
2.5. Qualité des produits végétaux traités par DO
La déshydratation osmotique permet le maintien des qualités nutritionnelles, voire
l’amélioration des qualités organoleptiques de produits souvent fragiles, ainsi qu’une
meilleure résistance à des traitements ultérieurs (séchage, stockage…) (Albagnac et al., 2002).
En effet, en tenant compte de la possibilité de transferts de masse dans les deux sens (gain de
solutés et perte de solutés), la déshydratation osmotique permet la formulation de nouveaux
produits (Albagnac et al., 2002). Selon Raoult-Wack, (1994), la déshydratation osmotique
permet de modifier les propriétés fonctionnelles des produits en les imprégnant des solutés
souhaités. La DO augmente le rapport sucre/acide, améliore la texture et préserve la couleur
pendant la déshydratation et le stockage (Raoult-Wack, 1994). Toutefois, il convient de noter
que les apports en soluté, et notamment en sucres, ne vont pas toujours dans le sens d'une
amélioration des propriétés nutritionnelles.
En évitant le contact avec l’oxygène de l’air, la DO limite les réactions d’oxydation,
mais aussi les pertes de composés volatils par entraînement. Elle est efficace même à
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température modérée (souvent inférieure à 50°C), ménageant ainsi les composés
thermosensibles tels que les arômes, pigments et vitamines (Vial et al., 1990).
L’effet de la DO sur les différents attributs de la qualité est détaillé ci-après.
2.5.1. Saveur
Au cours de la DO, l’introduction de soluté modifie inévitablement le rapport
acides/sucre, ce qui adoucit la saveur du produit final. Cependant, en séchage ou en DO,
l'élimination d'eau ne doit pas se faire au détriment de la saveur (i.e. des arômes). En règle
générale, tout facteur tendant à augmenter la viscosité de la solution osmotique diminue la
diffusivité relative des arômes. Ainsi, il vaut mieux en DO diminuer la température et
augmenter la concentration du produit en matières sèches pour conserver les arômes
(Torreggiani et Bertolo, 2001).
2.5.2. Couleur
La couleur est un attribut très important des aliments, car elle influence l’acceptabilité
par le consommateur (40% du critère d’acceptabilité) (Falade et al., 2007). Des couleurs
anormales, suggérant la détérioration de la qualité ou du caractère comestible, sont des causes
de rejet par le consommateur. Beaucoup de réactions peuvent affecter la couleur pendant le
traitement thermique des fruits et de leurs dérivés. Les plus communes sont la dégradation des
pigments (chlorophylle, β-carotène,…) et les caroténoïdes et la chlorophylle, et les réactions
de brunissement telles que la réaction de Maillard des hexoses, et l’oxydation de l’acide
ascorbique. Au cours de la DO, les solutés introduits réduisent les modifications de la couleur
du produit, notamment en limitant la dégradation des pigments chlorophylliens et
caroténoïdes. L’activité enzymatique de polyphenol-oxydase responsable du brunissement
enzymatique est alors inhibée. La sensibilité des produits au brunissement non enzymatique
est également limitée (Torreggiani et Bertolo, 2001).
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2.5.3. Texture
Le départ d’eau, ainsi que son remplacement par d’autres molécules, implique des
contraintes mécaniques qui modifient la conformation du matériau. Ainsi, le produit se
rétracte sous l’action des fortes densités de flux (Castello et al., 2009; Garcia-Segovia et al.,
2010). D’autre part, au cours du processus de déshydratation, les polysaccharides (pectine,
hémicelluloses, cellulose) qui constituent la membrane cellulaire sont partiellement
solubilisés, modifiant ainsi la fermeté du produit (Nunes et al., 2008). Ces modifications sont
quantifiables par l’analyse de la texture du produit; ces mesures sont basées sur la résistance à
la pénétration par une sonde (Torreggiani et Bertolo, 2001).
2.5.4. Réhydratation
Les produits déshydratés sont souvent conçus pour être réhydratés ultérieurement. La
capacité et la vitesse de réhydratation, qui est souvent longue (plusieurs heures), sont décrites
comme les principaux critères de la qualité des produits finis. La réhydratation des fruits et
légumes a été bien étudiée (Taiwo et al., 2002; Rastogi et Raghavarao, 2004). Une étude de la
cinétique de réhydratation peut être effectuée pour mesurer l’ampleur nette des dommages
subis par le produit pendant les étapes de transformation antérieures (Rastogi et Raghavarao,
2004).
La réhydratation est influencée par plusieurs facteurs, groupés en tant que facteurs
intrinsèques (composition chimique du produit, traitement de pré-séchage, formulation de
produit, techniques et conditions de séchage) et extrinsèques (composition du milieu
d’immersion, température, conditions hydrodynamiques). Certains de ces facteurs induisent
des changements de structure et de composition du tissu végétal, ce qui influence les
propriétés de reconstitution lors de la réhydratation (Taiwo et al., 2002). Par exemple, plus la
concentration en sucre est grande ou plus la période de la DO est longue avant séchage,
meilleure est la réhydratation, le sucre empêchant probablement le rétrécissement du tissus
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végétal lors du séchage à l’air (Neumann, 1972). Dans l’étude sur le céleri sec, Neumann
(1972) a rapporté que la réhydratation à des températures élevées diminue le temps exigé pour
atteindre la capacité maximum de sorption d’eau et le contenu d’humidité finale.
3. VALORISATION DES FRUITS CONSERVÉS EN SOLUTION SUCRÉES
Les fruits ont été depuis longtemps conservés par le sucre. La fabrication de produits
déshydratés osmotiquement remonte à la haute antiquité. Aujourd’hui, la conservation des
fruits par les sucres est devenue une industrie à part entière, et est appliqué à une vaste gamme
de produits, comme les fruits confits (Espiard, 2002), les compotes et purées (Espiard, 2002),
la confiture (Espiard, 2002 ; Albagnac et al., 2002), les marmelades et pâtes de fruits (Espiard,
2002 ; Albagnac et al., 2002). On trouve sur le marché un nombre de produits apparentés
ayant tous subis des transformations visant non seulement à obtenir des fruits stables à
température ambiante (grâce à une réduction de leur activité d’eau), mais à aussi leur conférer
une texture et une saveur spécifique (Albagnac et al., 2002).
4. CONCLUSION
A travers cette étude bibliographique, nous espérons apporter des bases scientifiques et
techniques pour l’étude de la conservation des produits végétaux par déshydratation
osmotique. La maîtrise de cette technique revêt une importance primordiale pour les
industries alimentaires car elle permet un gain d’énergie et, moyennant la prise en compte de
l’effet des apports en solutés par imprégnation, une préservation de la qualité nutritionnelle
des produits traités. Ainsi, l’utilisation de la DO permet un meilleur contrôle et une maîtrise
de la qualité des produits finis. Il en découle un élargissement de la gamme des fruits traités,
une diversification des caractéristiques des produits obtenus, et le développement de produits
nouveaux. En effet, cette technique permet de définir des voies de valorisation de plusieurs
fruits et légumes. Prenons l'exemple des graines de grenade, fruit jusqu’ici consommé presque
exclusivement frais, pendant la période de récolte. De récents travaux (Bchir et al., 2009;
35
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Bchir et al., In press) ont évalué l’aptitude des graines de grenade en DO, en fonction de la
température et du choix des solutés. Ces travaux ouvrent la voie à de nouveaux modes de
consommation, comme les graines confites, qui pourraient être introduites dans d’autres
produits transformés. De la sorte, il devient possible d’exploiter bien mieux les excellentes
propriétés nutritionnelles, biologiques et thérapeutiques de ce fruit.
Soulignons enfin que l’introduction de la DO dans le processus de transformation des
fruits permet en général une réduction du nombre d’étapes et/ou de durée totale du traitement,
et de bénéficier de l’effet protecteur des solutés incorporés. La littérature souligne aussi que,
d’une manière générale, la déshydratation osmotique doit être complétée par un traitement
thermique, chimique, mécanique ou physique afin de parfaire la stabilisation du produit fini.
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5. REFERENCES BIBLIOGRAPHIQUES
Abud-Archila M., Vazquez-Mandujano D., Ruiz-Cabrera M., Grajales-Lagunes A., Moscosa-
Santillan M., Ventura-Canseco L., Gutierrez-Miceli F. & Dendooven L. 2008.
Optimization of osmotic dehydration of yam bean (Pachyrhizus erosus) using an
orthogonal experimental design. Journal of Food Engineering, 84, 413-419.
Adamrounou L.T., Conway J. & Castaigne F. 1994. Influence de la déshydratation partielle
par osmose sur la composition de tranches de pomme. Sciences des Aliments, 14, 75-
85.
Ade-Omowaye B.I.O., Rastogi N.K., Angersbach A. & Knorr D. 2003. Combined effects of
pulsed electric fierld pre-treatment and partial osmotic dehydration on air drying
behaviour of red bell pepper. Journal of Food Engineering, 60, 89-98.
Agnelli M.E., Marani C.M. & Mascheroni R.H. 2005. Modelling of heat and mass transfer
during (osmo) dehydrofreezing of fruits. Journal of Food Engineering, 69, 415-424.
Albagnac P.G., Varoquaux J. & Montigaud coord C.I. 2002. Technologies de transformation
des fruits /Paris.-Londres.-New York : Ed. Tec & Doc,cop.-XXII-498 pages.
Azuara E., Beristain C.I. & Gutiérrez G.F. 1998. A method for continuous kinetic evaluation
of osmotic dehydration. Lebensm-Wiss.U. –Technol, 31, 317-321.
Bchir b., Besbes S., Attia H., & Blecker C. 2009. Osmotic dehydration of pomegranate seeds:
Mass transfer kinetics and DSC characterisation. International Journal of Food
Science and Technology, 44, 2208-2217.
Bchir b., Besbes S., Attia H., & Blecker C. Osmotic dehydration of pomegranate seeds
(Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process
Engineering. In press (DOI: 10.1111/j.1745-4530.2010.00591.x).
Biswal R.N. & Le Maguer M. 1989. Mass transfer in plant material in contact with aqueous
solution of ethanol and sodium chloride: equilibrium data. Journal of Food Process
Engineering, 11,159-176.
Cao H., Zhang M., Mujumdar A., Wh D. & Sun J. 2006. Optimization of osmotic dehydration
of kiwifruit. Drying Technology, 24, 89-94.
37
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Castello M., Igual M., Fito P. & Chiralt A. 2009. Influence of osmotic dehydration on texture,
respiration and microbial stability of apple slices (Var. Granny Smith). Journal of
Food Engineering, 91, 1-9.
Chenlo F., Moreira R., Fernandez-Herrero C. & Vazquez G. 2007. Osmotic dehydration of
chestnut with sucrose: Mass transfer processes and global kinetics modelling. Journal
of Food Engineering, 78, 765-774.
Cornillon, P. 2000. Characterization of osmotic dehydrated Apple by NMR and DSC.
Lebensmittel-Wissenschaft und-Technologie-Food Science and Technology, 33, 261-
267.
Corrêa J., Pereira L., Vieira G. & Hubinger M. 2010. Mass transfer kinetics of pulsed vacuum
osmotic dehydration of guavas. Journal of Food Process Engineering, 96, 498-504.
Corzo O. & Gomez E. 2003. Optimization of osmotic dehydration of cantaloupe using desired
function methodology. Journal of Food Engineering, 64, 213-219.
Crank, J. 1975. The mathematics of diffusion (2nd ed). Oxford: Clarendon Press.
Dermesonlouoglou E.K., Pourgouri S. & Taoukis P.S. 2008. Kinetic study of the effect of the
osmotic dehydration pre-treatment to the shelf life of frozen cucumber. Innovative
Food Science and Emerging Technologies, 9, 542-549.
Derossi A., De Pilli T., Severini C. & McCarthy M. J. 2008. Mass transfer during osmotic
dehydration of apples. Journal of Food Engineering, 86, 519-528.
Eren I. & Kaymak-Ertekin F. 2007. Optimization of osmotic dehydration of potato using
response surface methodology. Journal of Food Engineering, 79, 344-352.
Espiard E. 2002. Introduction à la transformation industrielle des fruits. TEC&DOC-
Lavoisier. pp. 181 - 182.
Falade K., Igbeka J. & Ayanwuyi F. 2007. Kinetics of mass transfer and colour changes
during osmotic dehydration of watermelon. Journal of food engineering, 80, 979-985.
Fernandes F., Gallao M. I. & Rodrigues S. 2009. Effect of osmosis and ultrasound on
pineapple cell tissues structure during dehydration. Journal of Food Engineering, 90,
186-190.
38
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Fernandes F., Rodrigues S., Gaspareto O. C. P. & Oliveira E.L. 2006. Optimization of
osmotic dehydration of papaya followed by air-drying. Food Research International,
39, 492-498.
Ferrando M. & Spiess W. 2001. Cellular of plant tissue during the osmotic treatment with
sucrose, maltose and trehalose solutions. Journal of Food Engineering, 49, 115-127.
Ferrari C. & Hubinger M. 2008. Evaluation of the mechanical properties and diffusion
coefficients of osmodehydrated melon cubes. International Journal of Food Science
and Technology, 43, 2065-2074.
Fito, P. 1994. Modelling of vacuum osmotic dehydration of food. Journal of Food
Engineering, 22, 313-328.
Floury J., Le bail A. & Pham, Q.T. 2008. A three-dimensional numerical simulation of the
osmotic dehydration of mango and effect of freezing on the mass transfer rates.
Journal of Food Engineering, 85, 1-11.
Garcia-Martinez E., Martinezmonzo J., Camacho M.M. & Martineznavarrete, N. 2002.
Characterisation of reused osmotic solution as ingredient in new product formulation.
Food Research International, 35, 307-313.
Garcia-Segovia P., Mognetti C., André-Bello A. & Martinez-Monzo J. 2010. Osmotic
dehydration of Aloe vera (Aloea barbadensis Miller). Journal of Food Engineering,
97, 154-160.
Giraldo G., Talens P., Fito P. & Chiralt A. 2003. Influence of sucrose solution concentration
on kinetics and yield during osmotic of mango. Journal of Food Engineering, 58, 33-
43.
Giroux, F. & Marouzé, C. 1994. Etude de dispositifs permettant l’agitation des produits dans
les procédés de déshydratation imprégnation par immersion. In : GFGP. Agitation et
mélange en biotechnologies alimentaire et industrielle. Editions TEC & Doc Lavoisier,
Paris, pp. 29-34.
Hawkes J. & Flink J.M. (1978). Osmotic concentration of fruits slices prior to freeze
dehydration. J Food Process Preser, 2, 265-284.
Heng K. Guilbert S. & Cuq J.L. 1990. Osmotic dehydration of papaya: influence of process
ariables on the product quality. Sci Alim, 10, 831-848.
39
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Islam M.N. & Flink J.M. 1982. Dehydration of potato. II Osmotic concentration and its
effects on air drying behaviour. Journal of Food technology, 17, 387-403.
Jambrak A., Herceg Z., Subaric D., Babic J., Brnic M., Brncic S., Bosiljkov T., Cvek D.,
Tripalo B. & Gelo J. 2010. Ultrasound effect on physical properties of corn starch,
Carbohydrate Polymers, 79, 91-100.
Javeri H., Toledo R. & Wicker L. 1991. Vacuum infusion of citrus pectinmethylesterase and
calcium effects on firmness of peaches. Journal of Food Science, 56, 739-742.
Jena S. & Das H. 2004. Modelling for moisture variation during osmo-concentration in apple
and pineapple. Journal of Food Engineering, 66, 425-432.
Jiokap Nono, Y., Nuadjea G.B., Raoult Wack A.L. & Giroux, F. 2001. Comportment de
certains fruits tropicaux traités par déshydratation imprégnation par immersion dans
une solution de saccharose. Fruits, 56, 75-83.
Khoyi M. & Hesari J. 2007. Osmotic dehydration kinetics of apricot using sucrose solution.
Journal of Food Engineering, 78, 1355-1360.
Kowalska H., Lenart A. & Leszczyk D. 2008. The effect of blanching and freezing on
osmotic dehydration of pumpkin. Journal of food engineering, 86, 30-38.
Krokida M.K., Karathanos V.T. & Maroulis Z.B. 2000. Effect of osmotic dehydration on
colour and sorption characteristics of apple and banana. Drying Technology, 18, 937-
950.
LeMaguer H. & Sharma S. 1997. Design and selection of processing conditions of pilot scale
contactor for continuous osmotic dehydration of carrots. Journal of Food Process
Engineering, 21, 75-88.
Lenart A. 1996. Osmo-convective drying of fruits and vegetables: technology and application.
Drying Technology, 14, 391-413.
Lerica C.R., Pinnavaia T.G., Dalla Rosa M. & Bartolucci L. 1985. Osmotic dehydration of
fruit: influence of osmotic agents on drying behaviour and product quality. Journal of
Food Engineering, 50, 1217-1226.
Madamba P. & Lopez R. 2002. Optimization of the osmotic dehydration of mango
(Mangifera indica L.) Slices. Drying Technology, 20, 1227- 1242.
40
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Marchal L., Allali H. & Vorobiev E. 2005. Blanchiment de fraise par chauffage ohmique:
incidence sur la cinétique de déshydratation imprégnation par immersion, Récents
Progrès en Génies des procédés, Numéro 92- ISBN 2-910239-66-7, Ed. Lavoisier,
Paris, France.
Marouzé C., Giroux F., Collignan A. & Rivier M. 2001. Equipment design for osmotic
treatments. Journal of Food Engineering, 49, 207-221.
Mavroudis N.E., Dejmek P. & Sjoholm I. 2004. Osmotic treatment induced cell death and
osmotic processing kinetics of apples with characterised raw material properties.
Journal of Food Engineering, 63, 47-56.
Mayor L., Pissarra J. & Sereno A. 2008. Microstructural changes during osmotic dehydration
of parenchymatic pumpkin tissue. Journal of Food Engineering, 85, 326-339.
Mujumdar, A. S. 2006. Handbook Of Industrial Drying, 3rd Edition. Taylor and Francis
Group, LLC, 688-700
Neumann H.J. 1972. Dehydrated Celery: effects of pre-drying treatment and rehydration
procedures on reconstitution. Journal of Food Science, 37, 437-441.
Nieto A.B., Salvatori D.M., Castro M.A. & Alzamora S.M. 2004. Structural changes in apple
tissue during glucose and sucrose osmotic dehydration: Shrinkage, porosity, density
and microscopic features. Journal of Food Engineering, 61, 269-278.
Nunes C., Santos C., Pinto G. Lopes-da-silva J.A., Saraiva J.A. & Coimbra M.A. 2008. Effect
of candying on microstructure and texture of plums (Prunus domestica L.). LWT- Food
Science and Technology, 41, 1776-1783.
Ohkuma C., Kawai K., Viriyarattanasak C., Mahawanich T., Tantratian S., Takai R. & Suzuki
T. 2008. Glass transition properties of frozen and freeze-dried surimi products: effect
of sugar and moisture on the glass transition temperature. Food Hydrocolloids, 22,
255-262.
Ozdemir M., Ozen B., Dock L. & Floros J. 2008. Optimization of osmotic dehydration of
diced green peppers by response surface methodology. Food Science and Technology,
41, 2044-2050.
Panades G., Castro D., Chiralt A., Fito P., Nunez M. & Jimenez R. 2008. Mass transfer
mechanisms occurring in osmotic dehydration of guava. Journal of Food Engineering,
87, 386-390.
41
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
Petchi M. & Manivasagan R. 2009. Optimization of Osmotic Dehydration of Radish in Salt
Solution Using Response Surface Methodology. International Journal of Food
Engineering, DOI: 10.2202/1556-3758.1584.
Raoult-Wack A.L., 1994. Recent advances in the osmotic dehydration of foods. Food Science
and Technology, 5, 255-260.
Rastogi N.K. & Raghavarao K.S.M.S. 2004. Mass transfer during osmotic dehydration of
pineapple: considering Fickian diffusion in cubical configuration. Lebensmittel-
Wissenschaft und-Technologie, 37, 43-47.
Riggs P.D., Kinchesh P., Braden M. & Patel P.M. 2001. Nuclear magnetic imaging of an
omotic water uptake and delivery process. Biomaterials, 22, 419-427.
Riva M., Campolongo S., Leva A.A., Maestrelli A. & Torreggiani D. 2005. Structure property
relationships in osmo-air-dehydrated apricot cubes. Food Research International, 38,
533-542.
Saurel R., Raoult-Wack A.L. & Rios Guilbert S. 1995. Approches technologiques nouvelles
de la déshydratation- imprégnation par immersion (DII). Industries Alimentaires et
agricoles, 2, 7-13.
Shi J., Le Maguer M., Wang S. & Liptay A. 1998. Application of osmotic treatment in tomato
processing effect of skin treatments on mass transfer in osmotic dehydration of
tomatoes. Food Research International, 30, 669-674.
Simal S., Sanchez E.S., Bon J., Femenia A. & Rossello C. 2001. Water and salt diffusion
during cheese ripening: effect of the external and internal resistances to mass transfer,
Journal of Food Engineering, 48, 269-275.
Singh B., Panesar P. & Nanda V. 2008b. Optimization of osmotic dehydration process of
carrot cubes in sucrose solution. Journal of Food Process Engineering, 31, 1-20.
Singh C., Sharma H. & Sarkar B. 2008a. Optimization of process conditions during
dehydration of fresh pineapple. Journal of Food Science and Technology-Mysore, 45,
312-316.
Souza J., Medeiros M., Magalhaes M., Rodrigues S. & Fernandes F. 2007. Optimization of
osmótica dehydration of tomatoes in a ternery system followed by air-drying. Journal
of Food Engineering, 83, 501-509.
42
Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________
43
Sunjka P. & Raghavan G. 2004. Assessment of preatreatment methods and osmotic
dehydration for cranberries. Canadian Biosystems Engineering, 46, 335-340.
Taiwo K.A., Angersbach A. & Knorr D. 2002. Influence of high intensity electric field pulses
and osmotic dehydration on the rehydration characteristics of apple slices at different
temperatures. Journal of Food Engineering, 52,185-192.
Talens P., Escriche I., Martinez-Navarrete N. & Chiralt A. 2003. Influence of osmotic
dehydration and freezing on the volatile profile of kiwi fruit. Food research
international, 36, 635-642.
Thebud R. & Santarius K.A. 1982. Effects of high-temperature stress on various bio-
membranes of leaf cells in situ and in vitro. Plant Physiology, 70, 200-205.
Torreggiani D. & Bertolo G. 2001. Osmotic pre-treatments in fruit processing: chemical,
physical and structural effects. Journal of Food Engineering, 49, 247-253.
Tortoe C., Orchard J., Beezer A. & O’Neil M. 2007. Potential of calorimetry to study osmotic
dehydration of food materials. Journal of Food Engineering, 78, 933-940.
Uddin M., Ainsworth P. & Ibanoglu S. 2004. Evaluation of mass exchange during osmotic
dehydration of carrots using response surface methodology, Journal of Food
Engineering, 65, 473-477.
Vial C., Guilbert S. & Cuq J. 1990. Osmotic dehydration of kiwi-fruits: influence of process
variables on the colour and ascorbic acid content. Sci Alim., 11, 63-84.
Vijayanand P., Chand N. & Epieson W. 1995. Optimization of osmotic dehydration of
cauliflower. Journal of Food Processing and Preservation, 19, 229-242.
Wang W.C. & Sastry S.K. 2000. Effects of thermal and electrothermal pretreatments on hot
air drying rate of vegetable tissue. Journal of Food Process Engineering, 23, 299-319.
Wu L., Orikasa T., Tokuyasu K., Shina T. & Tagawa A. 2009. Applicability of vacuum-
dehydrofreezing technique for the long-term preservation of fresh-cut eggplant: effects
of process conditions on the quality attributes of the samples. Journal of Food Process
Engineering, 91, 560-565.
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Chapitre 2:
Cinétique de transfert de masse durant la déshydratation osmotique des graines de grenade
Ce travail a fait l’objet de la publication suivante :
Bchir, B., Besbes, S., Attia, H., & Blecker, C. (2009). Osmotic dehydration of
pomegranate seeds: mass transfer kinetics and differential scanning calorimetry
characterization. International Journal of Food Science and Technology, 44,
2208–2217.
44
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Résumé *
Titre : Cinétique de transfert de masse durant la déshydratation osmotique des graines de
grenade.
Objectif et stratégie expérimentale :
L’objectif de ce premier volet visait à optimiser le procédé de déshydratation
osmotique (DO) des graines de grenade. La DO a été menée durant 120 min en utilisant
différentes températures (30, 40, et 50°C) et solutions sucrées (saccharose, glucose, et
saccharose/glucose 50:50 w/w) présentant un extrait sec soluble de 55°Brix. L’étude de la
cinétique de transfert de masse a été basée essentiellement sur la détermination de la perte en
eau, du gain en solides et de la réduction en poids au cours du temps. D’autre paramètres tels
que la température de transition vitreuse, la température de fusion, et l’enthalpie de fusion ont
été déterminés par calorimétrie différentielle afin d’étudier l’évolution des différentes
fractions d’eau (libre, liée) dans la graine au cours du procédé.
Les différentes étapes du procédé sont reprises de façon synoptique dans la figure 1’.
_________________________________________________________________________ * Ce résumé permet de présenter de façon synthétique, en français, l’axe de recherche de l’article qui a été publié en anglais.
45
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Graines congelées :
(-50°C)
Conditions
Traitement : déshydratation osmotique
Solution (55°Brix): - Saccharose, - Glucose - Saccharose/glucose
Température (°C) : - 30 - 40 - 50
Temps (min): - 0 - 80 - 20 - 100 - 40 - 120 - 60
Rapport : graine/solution: - 1/4
Paramètres
Paramètres physico-chimiques : - pH, aw - MS, Deff, conductivité - °Brix, couleur (L*, a*, b*)
Paramètres de transfert de masse : - Perte en eau - Gain en solides - Réduction en poids
Paramètres thermiques : - Température de transition vitreuse - Température de fusion - Enthalpie de fusion - Teneur en eau congelable et non congelable
Figure 1’ : Différentes étapes du procédé de déshydratation osmotique.
46
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Principaux résultats :
Les transferts de masse les plus significatifs sont intervenus pendant les 20 premières
minutes du traitement. A l'issue de cette période, les pertes mesurées en eau des graines
étaient de 46%, 37%, et 41%, en utilisant les solutions respectives de saccharose, glucose et
saccharose/glucose (50:50 p/p).
L'augmentation de la température s'accompagne d'une augmentation du transfert de
masse, attribué à un effet sur le coefficient de diffusion de l’eau et du soluté.
La calorimétrie différentielle à balayage (differential scanning calorimetry, DSC) a
fourni des informations complémentaires sur les changements de mobilité de l'eau et du soluté
dans la graine au cours de la déshydratation. Le rapport eau libre / eau liée a été divisé par dix
(de 5 à 0,5 à la fin du procédé), ce qui peut contribuer à une meilleure conservation du fruit.
L'analyse DSC révèle également que la température de transition vitreuse dépend du type de
sucre utilisé pour la déshydratation des graines de grenade.
47
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Osmotic dehydration of pomegranate seeds: Mass transfer
kinetics and DSC characterization
Brahim Bchira, Souhail Besbesb, Hamadi Attiab, Christophe Bleckera, *
a Department of Food Technology, Gembloux Agricultural University, Passage des Déportés,
2, B- 5030 Gembloux, Belgium
b Laboratory of Food Analyses, Sfax National of School Engineers, Route de Soukra, 3038
Sfax, Tunisia
*Corresponding authors Tel: +32(0)81/62.23.08
*Fax: +32(0)81/60.17.67
*E-mail address: [email protected]
48
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Abstract
Osmotic dehydration of pomegranate seeds was carried out at different temperatures
(30, 40, 50°C) in a 55°Brix solution of sucrose, glucose, and mixture sucrose & glucose
(50:50 wt/wt). The most significant changes of water loss and solids gain took place during
the first 20 min of dewatering. During this period, seeds water loss was estimated to 46% in
sucrose, 37% in glucose and 41% in mix glucose/sucrose solution. The increase of
temperature favoured the increase of water loss, weight reduction, solids gain and effective
diffusivity. Differential scanning calorimetry data provided complementary information on
the mobility changes of water and solute in osmodehydrated pomegranate seeds. The ratio
between % frozen water and % unfreezable water decreased from 5 to 0.5 during the process.
That involving the presence of very tightly bound water to the sample, which is very difficult
to eliminate with this process. It also appeared that glass transition temperature depends on
the types of sugar.
Keywords: Osmotic dehydration; water loss; solids gain; Differential scanning
calorimetry; pomegranate.
49
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 1. Introduction
Pomegranate (Punica granatum L.) is one of the most important fruits in Tunisia. Its
total production in 2008 reached more than 70,000 tons. Pomegranate is composed by a non
edible part formed by 30% of skin (external part) and 13% of internal lamel and an edible part
formed by seeds (50-70%). Pomegranate seeds are composed by 15% pips (woody part), this
part determines the hardness, and 85% pulp (the juicy part) depending on cultivar (Al-
Maiman & Ahmad, 2002).
The edible part of the fruit contains considerable amounts of sugars, vitamins, organic
acids, phenolic compounds and minerals (Espiard, 2002). In Tunisia the research of addition
value to pomegranate seeds is very limited and presents a traditional feature such as jam
preparation or direct consummation of fruit during the crop season (between September and
December). However, other perspectives of transformation and exploitation of the
pomegranate seeds should be undertaken to give value addition to this typical fruit. As
reported in the literature, seeds could be used for preparation of grenadine (Adsule & patill,
1995), fresh juice (Espiard, 2002), jelly (Maestre et al., 2000), jam (Espiard, 2002), wine
(Altan & Maskan, 2004), spice (Adsule & Patill, 1995), paste, flavoring and coloring drinks
and mainly in some new cosmetics application (Espiard, 2002). Moreover, recently, more
than 475 new products containing pomegranate (food and drinks) were born on the American
market. These included, chewings-gum called pomegranate Power, sausage of chicken to the
pomegranate, ices, breads, and biscuit with pomegranate... (Storey, 2007).
The demand for healthy, natural and tasty processed fruits increased continuously, not
only for finished products, but also for ingredients that can be included in some food
formulation such as ice-cream, cereals, dairy, confectionery and bakery products. In fact, over
the last few decades, a lot of research studies about processing of fruits and vegetables
(Vivanco et al., 2004), meat and fish (Ivan et al., 2007) were developed using osmotic
50
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ dehydration (OD). This process consists in the immersion of the product in a concentrated
solution (sugar, salt, sorbitol, glycerol), generating a partially dehydrated and impregnated
product (Torreggiani & Bertolo, 2001). Osmotic dehydration has a lot of benefit, like the use
of a low energy and cost compared to other dehydration methods. In addition, it involves
effective inhibition of polyphenoxidase, prevention of loss of volatile compounds, even under
vacuum and reduction of heat damage to color and flavor during dehydration (Krokida et al.,
2001). Raoult-Wack et al. (1991) noticed that OD did not strongly deteriorate the texture of
fruits. This effect was explained by the protective function of sugars in the fruit tissue.
Nowadays, the industry uses this technique for some previously cutted fruit like apple,
banana, mango, apricot, between others. This process has not been used for the conservation
of whole pomegranate seeds, neither by scientifics nor by industrials.
The aim of this work was firstly to investigate the kinetics of osmotic dehydration and
to determine the influence of osmotic conditions, such as temperature and osmotic solutions
on mass transfer during osmotic dehydration of whole pomegranate seeds. And secondly to
characterize the internal changes in osmotically dehydrated pomegranate seeds in sucrose,
glucose and mixture sucrose & glucose using differential scanning calorimetry (DSC). Also to
quantify the different states of water in the seeds, and to study the influence of osmotic
dehydration process on different parameters like the glass transition temperature.
2. Material and Methods
2.1. Preparation of pomegranate seeds
Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were obtained from
a local research centre in Gabes, Tunisia. Pomegranates fruits were collected at full ripeness
stage, having the same size. The fruits (20 kg) were washed in cold tap water and then frozen
at -50°C. Pomegranates were thawed, during 1hour at room temperature, and seeds were
51
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ recuperated in bottles just at the moment of osmodehydrated process. During the thawing of
the seeds, 24ml of juice per 100g of fresh matter were percolated.
2.2. Osmodehydration process
Sucrose, glucose and their mixture (50:50 wt/wt) were dissolved in water in order to
obtain 55°Brix solutions. About 10 g of seeds was soaked in the sugar solution and were
placed in bottles (Schott) of 100 ml. The volume ratio between the seeds and the sugar
solution was kept at one part of seeds and four parts of solution (1:4). Osmotic dehydration
process was conducted during 20 to 120 min in a shaking water bath (GFL instrument D
3006, Germany; oscillation rate 160 rpm) at different temperatures (30, 40, & 50°C).
2.3. Mass transfer kinetics
Seeds were removed from the immersion solution at selected time intervals (0, 20, 40,
60, 80, 100, and 120 min) and were quickly rinsed (with distilled water) and the excess of
solution at the surface was removed with absorbent paper. Water activity and soluble solids
were then measured as described below. The material was weighed before and after
osmodehydration to calculate the percentage of weight reduction (WR). The moisture content
was determined to calculate water loss (WL) and solids gain (SG), based on the following
equations (Mavroudis et al., 1998):
WR (%) = 100.)(
i
fi
WWW −
(1)
(3) WL (%) = SG + WR
SG (%) = 100.)(
i
sisf
WWW −
(2)
Where Wi is the initial weight of the sample (g), Wf the final weight of the sample (g),
Wsi the initial total solids content (g) and Wsf the final total solids content (g). Each value is
the mean of three determinations.
52
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 2.4. Mathematical modelling
Peleg’s equation parameters were obtained using eqn (4) (Peleg, 1988). This two-
parameter model was redefined by Palou et al. (1994) in terms of soluble solids and moisture
content and describes sorption curves that approach equilibrium asymptotically.
tkktMCtMC
210)(
+±= (4)
Where )(tMC is the amount of water or solids at the instant t (g/g dry matter (DM)),
is the initial amount of water or solids (g/g DM), k1 and k2 are Peleg’s parameters and t
is the time (s).
0MC
The value of the amount of water loss or solids gain at the equilibrium was then
calculated using eqn 5 (Park et al., 2002).
20
210
1)(limk
MCtkk
tMCMCteq ±=
+±=
∞→ (5)
Pomegranate seeds do not have a spherical shape, Alvarez et al. (1995) pointed out that
diffusion problem for any geometry can be reduced to the analytical solution corresponding to
a sphere, by modifying the Fourier number , using shape factor. 2/0 RtDFffe=
In order to determine the water and solutes effective diffusion coefficient the following
assumptions considerations were taken into account: homogeneous body, the external
resistance to mass transfer is negligible compared with internal resistance, the initial moisture
content was uniform throughout the sample, and the diffusion coefficient is constant (Crank,
1975).
The solution for Fick’s equation law for diffusion out a sphere is given by equation 6,
with using the following boundary conditions of internal resistance (Crank, 1975; Alvarez et
al., 1995):
53
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Uniforme initial amount : t = 0, 0<r<R, MC(t)=MC0; Symmetry of concentration: t >0, r =
0, 0)(=
∂∂
rtMC ; Equilibrium content at surface : t >0, r = R, MC(t)=MCeq
[ ]∑∞
=
−=−
−=
10
2
0
exp)(
nnn
eq
eqAouS FB
MCMCMCtMC
W μ (6)
Where: Bn= 6/μn2; μn= nП; F0= Deff, AorS t/R2; n = 1, 2, 3,…
Where Deff, AorS is the effective diffusivity of water loss or solids gain (m2 s-1); n is the
number of series terms, R is the equivalent radius of sphere (m), r is the distance in the radius
direction (m), and t is the time (s). WA and WS are the dimensionless amount of water loss and
solids gain, respectively; MCeq is the equilibrium amount of water loss or solids gain (g/g
DM) calculated using eqn 5.
As stated earlier, in this work pomegranate seeds were assumed to be ellipsoids, having
three characteristic diameters (2rM1~2rM2≤2RM ). According to Alvarez et al. (1995) the shape
factor (Ψ) eqn 7 is defined as Ss/Sp, and Ss is the surface area of a sphere of volume equal to
that of seeds with surface area Sp, which is assumed to be an ellipsoid. The intrinsic
diffusivity Deff is given by Ψ2 D’eff. It can be concluded that the diffusion coefficient
calculated from eqn (6) is D’eff and that it must be corrected by the factor Ψ2 when the product
shape can be assumed as an ellipsoid.
21
2
2
2
)/(1sin)/(1
22
4
MM
MM
MM
e
p
s
RrRr
Rrr
RSS
−⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−+
==−ππ
πψ (7)
2.5. Physico-chemical analysis of seeds
All analytical determinations were performed in triplicate. Values were expressed as the
mean± standard deviation.
The dry matter was calculated according to AOAC (1995). Approximately, 5 g of seeds
were oven dried at 103°C ± 2°C, until constant weight.
54
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Total nitrogen was determined by the Kjeldahl method. Protein was calculated using the
general factor (6.25) (AOAC, 1995).
To determine total lipid content, about 5g of seeds were mixed with chloridric acid. Fat
was then extracted with a soxtherm automatic S 306 AK solvent extractor equipped with six
Soxhlet posts (Gerhardt soxtherm, Switzerland) and command unit (Gerhardt Variostat,
Switzerland) using petroleum ether 40-60°C in each Soxhlet post. The result was expressed as
the percentage of lipids in the dry matter.
To determine ash content, about 5 g of seeds were incinerated in a muffle furnace (type
Gelman, Germany) at about 550°C for 8h. The total ash content was expressed in dry weight
percentage (AOAC, 1995).
Carbohydrate content was estimated by difference of mean values, 100-(Sum of
percentages of moisture, ash, proteins and lipids) (AOAC, 1995).
aw was measured using an aqualab (Switzerland) instrument at 20 °C.
The soluble solids of seeds were determined according to AOAC (1995) methods. It
was measured by an ATGO digital refractometer (DBX-55, Switzerland) at 20°C and
expressed in °Brix.
pH measurements were performed using a Hanna instrument 8418 pH meter
(Switzerland) at 20°C.
The CieLab coordinates (L*, a*, b*) were directly read with a spectrophotocolorimetre
Mini Scan XE (Germany) with a lamp (D 65). In this coordinate system, the L* value is a
measure of lightness, ranging from 0 (black) to +100 (white), the a* value ranges from -100
(greenness) to +100 (redness) and the b* value ranges from -100 (blueness) to +100
(yellowness).
Conductivity was measured using a conductimeter (LF 597-5; Germany) instrument at
20°C.
55
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Absorbance was measured using a spectrophotometer (Shimadzu UV 240, Cambridge,
USA) and the wave length used (λ ) was between 200 and 700 nm.
Differential scanning calorimetry (DSC) was performed on the pulp previously
separated from pip. A 2920 TA Instruments (New Castle, Delaware, USA) with a
Refrigerated Cooling Assessory and modulated capability was used. The cell was purged with
70 ml min−1 of dry nitrogen and calibrated for baseline on an empty oven and for temperature
using two temperature and enthalpy standards (indium, Tonset: 156.6 °C, ΔH: 28.7 J g−1;
eicosane, Tonset: 36.8 °C, ΔH: 247.4 J g−1). Specific heat capacity (Cp) was calibrated using a
sapphire. The empty sample and reference pans were of equal mass to within ±0.10 mg. DSC
curves were recorded during heating from –50 to 40°C at a scan rate of 5°C/min. All these
DSC experiments were made using hermetic aluminium pans. The analysed sample mass was
about 3.50 ±0.25mg.
2.6. Statistical Analysis
Statistical analyses were carried out using a statistical software program (SPSS for windows
version 11.0). The data was subjected to analysis of variance using the general linear model
option (Duncan test) to determine significant differences between samples (P<0.05).
3. Results and discussion
Chemical composition of pomegranate seeds before osmotic dehydration is shown in
table 1. Pomegranate seeds are rich in carbohydrate (~85%) followed by protein (~8%), lipid
(~5%) and Ash (~3%). This composition is quite similar to pomegranate seeds cultivated in
Egypt (El-Nemr et al., 1990).
56
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Table 1. Chemical characteristic of pomegranate seeds
Seeds Dry matter (DM %) 16.00 ± 0.05 Protein g/100g DM 7.79 ± 0.86 Lipid g/100g DM 4.55 ± 0.40 Ash g/100g DM 2.87 ± 0.19 Carbohydrate g/100g DM 84.93 ± 0.25 pH 4.17 ± 0.20 Aw 0.989 ± 0.002 °Brix 15.50 ± 0.09
As in previous works, three different parameters were followed during osmotic
dehydration: water loss (WL), solids gain (SG) and weight reduction (WR). They were
responsible for total mass change, shrinkage and changes in the fruit liquid phase
concentration that defines the water activity, the quality and the stability of the final product
(Kowalska & lenart, 2001 and Falade et al., 2007).
3.1. Osmodehydration using sucrose solution
3.1.1. Mass transfer kinetics
The effect of dehydration time on WL, WR, and SG was studied in pomegranate seeds
at different temperatures (30, 40 and 50°C). The most significant changes took place during
the first 20 min of dewatering as shown in Fig. 1a. During this time, WL in seeds was 37, 39,
and 46 % respectively for 30, 40 and 50°C. After this period of dehydration, the percentage of
water loss varied slightly and ranged on average close to 35, 38, and 43 % for 30, 40 and
50°C respectively. The same trend was also observed for WR (Fig. 1b). Under the same
conditions, SG was also increased significantly during the first 20 min (Fig. 1c) reaching 5.4,
6.9, and 7.2 % respectively for 30, 40, and 50°C, and tend to be stable at the end of the
process. A similar curve has been reported in the osmotic dehydration of watermelon (Falade
et al., 2007). Statistical analysis showed that WL, SG and WR varied significantly with time.
However, the significant difference founded at course of time was not higher. This fact could
be because the majority of the transfer was done during the first 20 min of the process. As
57
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ consequence we suggest to stop the process after 20 min as it implies no addition of thermal
energy to the system.
Table 2. Evolution of osmotic dehydration parameters in sucrose solution at different temperatures 30, 40, and 50°C
30°C
0 min 20 min 40 min 60 min 80 min 100 min 120 min
°Brix of solution 55.00±0.00a 51.10±0.63b 49.80±0.10b 49.10±0.70b 49.40±0.10b 49.40±0.20b 49.30±0.20b
°Brix of seeds 15.50±0.09a 39.55±0.63b 42.85±0.07c 44.55±0.07d 45.10±0.28d 46.25±0.07e 46.60±0.14e
pH of solution 8.27±0.03a 4.89±0.09b 4.68±0.08c 4.54±0.06cd 4.52±0.04cd 4.48±0.05d 4.46±0.08d
Conductivity of solution
(μs/cm)
0.90±0.01a 31.75±0.63b 36.05±1.62c 39.80±0.84d 40.80±0.70de 42.20±1.27de 43.20±1.27e
Dry matter of seeds (%) 16.00±0.05a 39.30±1.30b 41.20±1.40b 45.20±2.60c 45.20±0.10c 46.80±0.70c 48.20±0.10c
40°C
°Brix of solution 55.00±0.00a 50.00±0.84b 50.40±0.84bc 49.40±0.28cd 49.20±0.14d 49.15±0.12d 49.05±0.14d
°Brix of seeds 15.50±0.09a 40.85±0.07b 43.35±0.21c 45.40±1.31e 45.60±1.13e 45.85±0.35e 46.85±0.63e
pH of solution 8.27±0.03a 4.70±0.04b 4.50±0.02bc 4.46±0.09c 4.42±0.17c 4.43±0.12c 4.40±0.04c
Conductivity of solution
(μs/cm)
0.90±0.01a 33.00±1.83b 37.35±1.76bc 40.60±2.97cd 43.95±3.74d 42.15±2.89cd 44.00±1.27d
Dry matter of seeds (%) 16.00±0.05a 40.90±2.97b 44.20±0.18bc 45.90±1.02c 47.30±0.15c 47.80±0.20c 49.00±0.06c
50°C
°Brix of solution 55.00±0.00a 49.70±0.07b 49.30±0.01bc 49.10±0.21c 48.90±0.28c 49.00±0.21c 49.00±0.21c
°Brix of seeds 15.50±0.09a 41.60±0.14b 45.30±0.14c 46.40±0.14d 46.90±0.14d 48.70±0.28e 49.10±0.07e
pH of solution 8.27±0.03a 4.60±0.27b 4.50±0.20b 4.50±0.01b 4.40±0.02b 4.30±0.10b 4.30±0.07b
Conductivity of solution
(μs/cm)
0.90±0.01a 35.50±0.35b 39.00±1.06c 40.10±0.07de 40.50±0.21e 41.00±0.49e 41.20±0.28e
Dry matter of seeds (%) 16.00±0.05a 42.70±0.08b 46.80±0.65c 47.80±0.83cd 48.30±0.41cde 48.60±0.73de 49.50±0.76e
All values given are means of three determinations. Means in line with different letters are significantly different (P<0.05)
58
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
The rapid loss of water in the beginning at various temperatures (30, 40, and 50°C) is
due to the large osmotic driving force between the dilute sap of seeds and the surrounding
hypertonic medium. Then, slower water transfer is mainly influenced by the reduction of the
difference in concentration between the seeds and osmotic solution which could involve a
slower driving force. Indeed, the reverse trend of °Brix observed in seeds and osmotic
solution confirms these facts (Table 2). The trend observed in SG for the different
temperatures studied (Fig. 1c) could be explained by migration of sucrose to the seeds through
their cell membranes due to the important gradient of sugar between the seeds and the osmotic
solution.
From the results shown in Fig. 1, it can be concluded that the increase of temperature
from 30 up to 50 °C lead to an increase of water loss, weight reduction, and solids gain. The
increases of temperature at 40°C involved the same evolution of WL as using 30°C. However
a significant difference was found in comparing to 30°C from time superior to 60 min. At
50°C significant difference was already observed after 20 min. A similar evolution for WR
and SG as function of time and temperature was found. Nevertheless, the increase of
temperature to 40 and 50°C showed a significant difference of SG compared to 30°C from 20
min. A Higher value of different parameters was always observed using 50°C. As
consequence we suggest using 50°C to have the best dehydration. Ferrari & Hubinger (2008)
showed that the increase of temperature leads to irreversible damage and a loss of selectivity
of cell membrane involving a higher osmotic pressure at the product/solution interface.
Moreover, higher temperature raised diffusion coefficients indiucing higher mass transfer
rates (table 3). This behavior has also been reported in the osmotic dehydration of apricots
(Khoyi & Hesari, 2007) and melon (Ferrari & Hubinger, 2008).
59
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Table 3. Water and solids effective diffusivities calculated by Fick’s model
Water loss Solids gain Sugar T (°C) Deffw (m2s-1) R2 (%) Deffs (m2s-1) R2(%)
50°C 9.44 × 10-12 99.92 4.81 × 10-12 99.72 40°C 8.09 × 10-12 99.89 4.72 × 10-12 99.88
Sucrose 30°C 7.43 × 10-12 99.66 4.21 × 10-12 99.80
50°C 9.10 × 10-12 99.93 5.44 × 10-12 99.92 40°C 7.88 × 10-12 99.77 4.54 × 10-12 99.89
Sucrose/glucose 30°C 6.98 × 10-12 99.77 4.48 × 10-12 99.87
50°C 8.74 × 10-12 99.83 9.54 × 10-12 99.88 40°C 6.69 × 10-12 99.68 6.52 × 10-12 99.89 Glucose
30°C 5.41 × 10-12 99.65 4.94 × 10-12 99.50
In this study, the increase of temperature had a more significant effect on WL than on
SG (Figs 1a and 1c). Indeed, after 120 min when the temperature increased from 30 to 40°C,
the gain of WL and SG was 4.4% and 0.5%, while when the temperature rised from 40°C to
50°C, the gain was 5.7% and 1.1% respectively. In addition effective diffusivity of water was
also higher than that of the solids (Table 3). This effect is generally attributed to the influence
of natural tissue membranes and to the diffusive properties of water and solutes as a function
of their respective molar mass (Falade et al., 2007). In a study on osmotic dehydration of
model foods (aqueous agar-agar gels), Raoult-Wack et al. (1989) suggested that this apparent
contradiction could be due to a reciprocal influence between the water and solute transfers
where sugar penetration by diffusion and sugar release within the water out flow are
combined.
60
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
0
5
10
15
20
25
30
35
40
45
50
0 20 40 60 80 100 120
Time (min)
% W
L
30°C 40°C 50°C
0
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100 120
Time (min)
%W
R
30°C 40°C 50°C
0
2
4
6
8
10
12
0 20 40 60 80 100 120Time (min)
%S
G
30°C 40°C 50°C
a
b
c
Figure 1: Variation of water loss (WL) (a) weight reduction (WR) (b) and solids gain (SG) (c) with time and temperature (30, 40, 50°C) using sucrose solution during osmotic
dehydration.
61
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 3.1.2. Evaluation of the Peleg and Fick mathematical models
The Peleg’s equation parameters (K1 and K2) were determined for water loss and solids
gain (eqn 5), as shown in table 4. This model showed a good fit to the experimental data, with
correlation coefficients (R2) close to 0.99. The parameters K1 and K2 did not exhibit a clear
trend with the increase in temperature. Ferrari & Huninger (2008), Khoyi & Hesari (2007)
and Park et al. (2002) verified the same fact in similar studies with melon, apricots and pears,
respectively, using sucrose as the osmotic agent.
Table 4. Values of Peleg’s equation parameters for water loss and solids gain
Water loss Solids gain Sugar T (°C) k1 k2 R2 k1 k2 R2
30 37.8782 0.2362 0.9993 15.8206 0.0300 0.9988 40 36.1447 0.2316 0.9999 13.4587 0.0302 0.9996
Sucrose 50 28.0531 0.2317 0.9999 12.0808 0.0286 0.9992
30 62.8484 0.2328 0.9999 12.6530 0.0308 0.9967 40 44.9577 0.2315 0.9995 8.9232 0.0360 0.9997
Glucose 50 25.5221 0.2346 0.9999 4.8449 0.0299 0.9997
30 44.4009 0.2301 0.9999 13.6050 0.0289 0.9995 40 34.7331 0.2317 0.9998 13.6054 0.0288 0.9995
Sucrose/glucose 50 29.7106 0.2311 0.9998 10.5526 0.0286 0.9998
Table 3 shows the effect diffusivity values for water and solids calculated using Fick’s
model (eqn 6), which also presented a good fit to experimental data, showing an average
correlation coefficients (R2) close to 0.99. The experimental values for effective diffusivity
were to an order of magnitude between 4×10-12 to 9×10-12 for solids gain and 5×10-12 to 9×10-
12 m2s-1 for water loss. Park et al. (2002) working with pear cubes found that Deff ranged from
0.35×10-9 to 1.92×10-9 m2s-1 for water loss and from 0.20×10-9 to 3.60×10-9 m2s-1 for solids
gain at different temperature (40-60°C). Lazarides et al. (1997) found values ranging from
1.42×10-10 to 4.69×10-10 for moisture diffusivity and from 0.73×10-10 to 2.41×10-10 m2s-1
solute diffusivity of apple slices at different temperature (20-50°C) and sucrose solution
concentrations (45-65%). So, a comparison of the diffusivities values to those reported in the
62
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ literature showed that our results were lower. This comparison should however take into
account the experimental conditions, the different estimation methods employed, the variation
in food composition and its physical structure. Indeed, given their small size, pomegranate
seeds can be kept intact and do not need to be cut. On the contrary, pear, apple or kiwis, due
to their large sizes, have to be cut in small volumes. Thus, cutting these fruits creates more
externals lesion leading to a higher contact of cells with the osmotic solution in a shorter time
inducing a higher diffusion.
3.1.3. Physico-chemical Characteristics of the osmodehydrated fruit preparation
The changes that occurred in pomegranate seeds and in the osmotic solution, a function
of time and temperature, are shown in tables 2 and 5. As it was expected all parameters
(°Brix, pH, conductivity…) evolved in the same trend like mass transfer parameters (WL, SG,
WR). In fact, statistical analysis shows a significant difference (P<0.05) at the beginning of
the process (20 min). The increase of temperature showed that 50 °C gave the lowest °Brix,
and pH of the solution, and the highest °Brix in the seeds. At the beginning of the process (20
min) the °Brix in the solution decreased as the °Brix of seeds increased, after that °Brix
tended towards an equilibrium (Table 2). This was a consequence of osmosis, inducing a
balance of concentration between the seeds and the sucrose solution. The diffusion of some
solutes from pomegranate seeds to the aqueous solution, could explain the decrease of pH and
the increase of the conductivity in the osmotic solution. The measure of colour parameters L*,
a*, b* showed a slight reduction of L* and a slight increase of a* and b* (Table 5). This
variation could be explained by a migration of pigment from pulp to solution. Indeed
absorbance reached a peak at 510 nm. According to the literature, this peak corresponded to
the pink pigmentation (flavonoids) in pomegranate seeds (Sestili et al., 2007). Table 6 shows
that water activity was reduced from 0.989 to 0.903, confirming water loss during the process.
Moreover, using 50°C involved the lowest water activity (0.903) compared to the other
63
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ temperatures (30°C: 0.925 and 40°C: 0.915). Therefore, water activity is very temperature and
time dependent.
Table 5. CieLab coordinates of sucrose solution at different temperatures 30, 40, and 50°C 30°C CieLab
coordinate 0 min 20 min 40 min 60 min 80 min 100 min 120 min
L* 65.76±0.01a 63.04±1.04ab 61.41±2.05b 60.18±2.04b 61.33±1.47b 61.00±2.12b 60.13±0.62b
a* -0.60±0.02a 0.54±0.07b 1.35±0.42c 3.44±0.29e 2.29±0.38d 3.44±0.16e 3.15±0.49e
b* 3.60±0.01a 6.78±0.15c 5.92±0.58bc 6.25±0.68bc 5.95±0.35bc 5.44±0.08b 6.79±0.36c
40°C
L* 65.76±0.01a 59.64±1.73bc 59.58±0.49bc 60.96±1.05b 59.33±0.58bc 57.47±0.03c 58.76±0.65c
a* -0.60±0.02a 3.82±0.34c 4.62±0.70c 2.31±0.02b 4.58±0.73c 5.81±0.27d 6.56±0.15d
b* 3.60±0.01a 4.38±0.36ab 4.85±0.63ab 5.70±0.57bc 6.27±0.891bc 7.01±0.08de 8.18±1.55e
50°C
L* 65.76±0.01a 61.40±0.96b 60.90±0.40b 60.50±0.47bc 59.80±0.75bc 59.60±1.32bc 58.80±0.15c
a* -0.60±0.02a 2.40±0.92b 2.90±0.134bc 3.30±0.46bcd 4.10±0.30cd 3.70±1.14bcd 4.60±0.24d
b* 3.60±0.01a 6.50±0.26b 7.80±0.07bc 8.10±0.17bc 9.60±1.98c 8.90±0.04c 9.30±0.09c All values given are means of three determinations. Means in line with different letters are significantly different (P<0.05)
3.1.4. Thermal properties of seeds as measured by DSC
Osmotic dehydration of pomegranate seeds was investigated by DSC to determine the
kinetic of seeds dehydration and the state of water during the process at 50°C. DSC
thermograms (Fig. 2) presented 2 main thermal events between -50°C and 30°C. The first was
a change in heat capacity (∆Cp) and the second was an endothermic peak. ∆Cp can be related
to a freeze-concentration glass transition temperature (Tg’) due to the presence of sucrose,
protein, fibre (pectin, lignin, hemicellulose and cellulose), and water in the sample. As
reported in such products, carbohydrates and proteins can be described as amorphous food
polymers constituent by not arranged chains (Roos, 1995). The endothermic transition could
be attributed to the melting of crystallized water (free and freezing bound water). In fact,
during the cooling only free water and freezing bound water were crystallized to give ice. And
during the heating, frozen water undergoes a fusion of ice and unfreezable water does not
64
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ undergo any change. The endothermic transition was used to calculate the amount of
unfreezable water (UFW) estimated as follows:
DMHHW
UFW watersamplewater )/( ΔΔ−= (8)
Where ∆Hsample is the heat of melting expressed in j/g, ∆Hwater is the normalized heat of
melting of pure water (351.2+/- 1.2 j/g), Wwater is the total water content of sample expressed
in g of water and DM is the dry matter content expressed in g DM (Goni et al., 2007).
Time: 20 min AW: 0.954
Time: 40 min AW: 0.946
Time: 60 min AW: 0.942
Time: 80 min AW: 0.923
Time: 100 min AW: 0.905
Time: 120 min AW: 0.903
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Hea
t Flo
w (W
/g)
-60 -40 -20 0 20 40
Temperature (°C)Exo Up Universal V3.0G TA Instruments
Figure 2: DSC thermogram obtained for pomegranate seeds soaked in sucrose solution at 50°C.
65
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
A considerable increase in Tg’ was observed up to 20 min and after this period Tg’
varied much slower and reached -34.87 °C at the end of the process (Table 6). Moreover
melting point (Tf), unfrozen water (UFW) and enthalpy of fusion (∆Hfus), strongly decreased
during the first 20 min and after this time, Tf and ∆Hfus showed a slight decrease but UFW
tended to be constant for the rest of the process. A similar evolution was observed by
Cornillon, (2000) using apple soaked in 63 g sucrose/100 g solution for osmotic dehydration.
The increase of the glass transition (Tg’), the lower enthalpy of fusion (∆Hfus) and water
activity (aW) were consistent with the fact that less and less water was present in the seeds as
the dehydration process occurred (Cornillon, 2000). Sá et al. (1999) found that Tg’ for
polysaccharides water systems reach to a maximum with decreasing water content, inducing
the decreased mobility of the polymer chains. Indeed, it is well known that water has a
negative Tg’ (-173°C) as opposed to the different constituents of seeds that have a positive
Tg’ (sucrose: 62°C; pectin: 160°C; Hemicellulose: 150 - 220°C; cellulose 220 - 250°C;
protein: 77 - 112°C) (Roos, 1995). Thus, the presence of water permits to reduce the Tg’ of
sample. It is well know establish that water drastically decreased the Tg’ of food, due to its
plasticizing effect on amorphous polymers (Roos, 1995). A comparison with published
literature data on water-sucrose solutions and Tg’ seeds (at the end of the process) showed
close values, indicating that sucrose was predominant in seeds. In fact Tg’ of a sucrose
solution (49°Brix) and Tg’ of seeds (also 49°Brix), at the end of the process, were
respectively -32.21 and -34.87°C. It is well known establish previously that diffusion of
sucrose in the seeds induced an increase of Tg’ (Sá et al., 1999). Moreover pomegranate seeds
present a decrease of freezing temperature, which can be explained by the depressing effect of
solute, such as sugars, diffusing in the seeds. Indeed, several works reported that the increase
of solutes in fruits was closely related to the decrease of the freezing temperature (Ohkuma et
66
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ al., 2008). The increase of the relative amount of sucrose and the reduce amount of water in
the cell corresponded to the shift of Tg’.
On analyzing the DSC data, we observed that the amount of unfreezable water (water
that did not freeze) declined in the beginning of the process from 3.98 to 1.09 g.g-1 DM and
tended to stabilize in the course of time reaching ~ 0.90 g.g-1 DM. This fraction is strongly
associated with the polymer matrix and showed neither exothermic nor endothermic peak on
DSC curves. The decrease of UFW in seeds could be due to the higher percentage of water
loss at the beginning of the process. In fact, dry matter increased from 16% to 43% min and
water activity decreased from 0.989 to 0.954 at 50 °C after 20 min. The stability of UFW was
the result of water loss and solids gain, and implicate that water become more bound in seeds.
Moreover, the analyzing of the % of frozen water (calculated by dividing the enthalpy of
fusion of sample by the enthalpy of fusion of pure water) and the % of UFW (calculated by
subtracting the % of total water per the % of frozen water) showed different evolution in
course of time. In fact, the % of frozen water decreased 3.5 times contrary the % of UFW that
increased 2.5 times. This was a consequence of water loss and sugar gain during the process.
Many authors found that the increase in the unfreezable water weight fraction in fruit can
mainly be attributed to a significant accumulation of osmolites such as soluble sugars (Goni et
al., 2007 and Ohkuma et al., 2008). Moreover, the most significant changes took place during
the first 20 min of the process, that confirming previous results. During this time, the % of
frozen water decreased from 70% to 28% whereas the % of UFW increased from 14% to
29%. After this period, the % of frozen water and UFW slightly varied and ranged on average
close to 20 and 34% respectively. The same trend was also observed for water activity
indicating that less and less free water was available in seeds. The final product presented a
higher % of UFW than % of frozen water; this is an advantage for a better conservation of
seeds. Nevertheless, water activity of osmodehydrated seeds was higher (superior to 0.9)
67
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ involving certain undesirable reactions, such as non-enzymatic browning, fat oxidation,
vitamin degradation, enzymatic reactions, and protein denaturation. As a consequence, other
treatments (pasteurization, freezing, drying) will be necessary to ensure a good conservation
of the seeds.
Table 6. Differential scanning calorimetry results for pomegranate seeds over soaking time in sucrose, glucose and mixture sucrose & glucose solution at 50°C
Time (min) Tg’ (°C)
midpoint
Tf (°C)
onset
∆Hfus
(J/g)
unfreezable water
(g.g-1 DM)
AW
Sucrose solution
0 -41.88 0.22 233 3.98 0.989a
20 -34.19 -4.90 93.86 1.09 0.954b
40 -33.64 -5.65 81.17 0.91 0.946c
60 -33.61 -5.92 79.02 0.88 0.942c
80 -34.43 -8.84 58.74 0.92 0.923d
100 -35.35 -9.48 47.52 0.92 0.905e
120 -34.87 -8.56 61.33 0.85 0.903e
Sucrose and glucose solution
0 -41.88 0.22 233 3.98 0.989a
20 -35.39 -7.43 78.88 1.08 0.958b
40 -35.62 -6.49 83.20 0.96 0.945c
60 -35.42 -7.78 70.04 0.94 0.931d
80 -35.50 -7.36 76.19 0.88 0.919e
100 -34.74 -7.39 75.47 0.84 0.910ef
120 -35.03 -6.92 82.96 0.80 0.906f
Glucose solution
0 -41.88 0.22 233 3.98 0.989a
20 -40.65 -8.32 83.99 1.25 0.956b
40 -40.03 -10.04 82.87 0.93 0.947b
60 -40.65 -10.92 72.74 0.95 0.930c
80 -40.08 -11.25 70.43 0.89 0.925c
100 -40.87 -11.42 67.22 0.90 0.911d
120 -40.60 -11.88 65.20 0.93 0.910d
Means in column with different letters are significantly different (P<0.05)
68
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 3.2. Osmodehydration using glucose and mixture sucrose & glucose solution
Using glucose and mixture sucrose & glucose solution time and temperature had a
similar effect on different parameters (WL, SG, WR, Deff, °Brix, DM, pH …) as with the
sucrose solution. With regard to the kind of solute employed, comparing the values of WL,
SG and Deff at the same temperature, osmotic dehydration with glucose leads a decrease of
water loss and a slightly increased solids gain (Figs 3a and 3b). Moreover the difference on
°Brix between seeds and solution was very weak using glucose solution. In addition glucose
solution induces a lower Deff of water and a higher Deff of solids contrary to sucrose solution
(Table 3). Fig 3a shows that the sucrose solution allowed a better water loss followed by the
sucrose/glucose mix and the glucose solution. These dissimilarities were attributed to the
specific surface of seeds and the differences between molecular weight of glucose and
sucrose. In fact, mass transport can be described by the Fick’s second Law, depending on the
coefficient (Deff) which is influenced by the radius of solute (Saurel et al., 1994). Briois et al.
(1998) showed that the speed of diffusion of sugar molecules in fruit membrane cells was
negatively correlated with their molecular size. Indeed with a low molar mass (glucose)
solute, the effect of dilution was responsible for the faster reduction of the difference in
average concentration between liquid phase and solid phase, inducing decrease of the water
loss in the course of time. As consequence glucose allowed easily diffusion (Masmoudi et al.,
2007).
69
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
0
5
10
15
20
25
30
35
40
45
50
0 20 40 60 80 100 120
Time (min)
% W
L
Sucrose Glucose Sucrose/Glucose
a
0
2
4
6
8
10
12
14
0 20 40 60 80 100 120Time (min)
% S
G
Sucrose Glucose Sucrose/glucose
b
Figure 3: Comparison of water loss (WL) (a) and solids gain (SG) (b) using different osmotic
solutions (sucrose, glucose and mixture sucrose & glucose) at 50°C.
70
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
Tf, ∆Hfus, aW and UFW evolve in the same way as with the sucrose solution except the
Tg’ for glucose seeds that slightly increase as a function of time (Table 6). As it was shown in
sucrose solution in this case, Tg’ of water-glucose and seeds (at the end of the process)
showed close values, indicating that glucose was predominant in seeds. In fact Tg’ of a
glucose solution (55°Brix) and Tg’ seeds, at the end of the process, were respectively -40.6
and -44.9°C. Roos, (1995) and Liu et al. (2007) showed that Tg’ was strongly dependent on
the molecular weight, it decreased with decreasing molecular weight. Table 6 shows that the
addition of small molecules decreased the value of Tg’. In fact, the use of a glucose solution
showed the lowest Tg’ and Tf. Thus glucose solution induces reducing of water loss compared
to the others osmotic solution. Roos (1995) found that higher sucrose content in the mix
induced a higher Tg’. Table 6 shows that the amount of UFW at the end of the process was
very closed. In fact, UFW varied between 0.80 and 0.93 involving the presence of very tightly
bound water to the sample, which is very difficult to eliminate with this process even after
120 min.
4. Conclusion
Osmotic dehydration process could be used for the conservation of pomegranate seeds.
The rate of different parameters was directly related to temperature, time, and solute. In fact
process showed that operating 20 min at 50°C offered the best result. As a consequence, it
could be better to stop the process after 20 min as it implies no addition of thermal energy to
the system. Osmotic dehydration reduced water activity from 0.989 to an average of 0.900. At
this aW value a complementary treatment such as drying, freezing and pasteurization should
be necessary to ensure its good conservation.
The nature of sugar used for the dehydration solution involves modifications in the
evolution of mass transfers and effective diffusivity during the process. In fact, sucrose
(higher molecular weight) induced the best effective diffusivity of water involving the best
71
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ dehydration of seeds, contrary to glucose (lower molecular weight) that induced the best
effective diffusivity of solids and than impregnation. Therefore we suggest using sucrose
solution to have the best dehydration. On the other hand, the study of physico-chemical
characteristics of osmodehydrated pomegranate seeds showed the lost of solutes from the
seeds to the osmotic solution during the osmotic dehydration process. So these solutions could
be used as natural additives (flavour and color) in the industry.
DSC data provide complementary information on the mobility changes of water and
solute in osmotically dehydrated pomegranate seeds. Indeed, it was possible to determine a
strong decrease in water mobility involving an increase of glass transition as more solute and
water migrated respectively into and out of seeds. It also appeared that Tg’ depends on the
types of sugar. In fact Tg’ of seeds using a sucrose solution was higher than Tg’ using glucose
or glucose/sucrose mix solution. After osmotic dehydration, the product presented a higher %
of UFW than % of frozen water, this is an advantage for a better conservation of seeds.
The finished product has an attractive colour and presents a good texture in mouth, a
pleasant sugar taste and a good aroma. It would be interesting for the continuation of this
work to assess the sensory properties (texture, flavour, etc.) and to substitute the standard
solutions by new solutions (date juice) that bring new organoleptic properties to the finished
product.
72
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ References:
Adsule, N.R., & Patil, N.B. (1995). Pomegranate. In: Salunkhe, D.K., Kadam, S.S., ed.
Handbook of Fruit Science and Technologies: Production, Composition, Storage and
Processing. New York, USA: 455-463.
Al-Maiman, S.A., & Ahmad, D. (2002). Changes in physical and chemical properties during
pomegranate (Punica granatum L.) fruit maturation. Food Chemistry, 76, 437-441.
Altan, A., & Maskan, M. (2004). Rheological Behavior of pomegranate (Punica granatum L.)
juice and concentrate. Journal of Texture Studies, 36, 68-77.
Alvarez, A .C., Aguerre, R., Gomez, R., Vidales, S., Alzamora, S.M., & Gerschenson, L.N.
(1995). Air Dehydration of Strawberries: effects of Blanching and Osmotic
Pretreatments on the Kinetics of Moisture Transport. Journal of Food Engineering,
167-178.
AOAC (1995) Official methods of analysis. 15th edn. Association of Official Analytical
Chemists.
Briois, E., Dusautois, C., & Heno, P. (1998). Applications alimentaires des sirops de glucose
à haute teneur en maltose. Industries Alimentaires et Agricoles, 7/8, 79-81.
Cornillon, P. (2000). Characterization of osmotic dehydrated Apple by NMR and DSC.
Lebensmittel-Wissenschaft und-Technologie-Food Science and Technology, 33, 261-
267.
Crank, J. (1975). The mathematics of diffusion (2nd ed). Oxford: Clarendon Press.
El-Nemr, E., Ismail, A,. & Ragab, M. (1990). Chemical composition of juice seeds of
pomegranate fruit. Journal of Food Science, 7, 601–606.
Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC&DOC-
Lavoisier (pp 181-182).
Falade, K., Igbeka, J., & Ayanwuyi, F. (2007). Kinetics of mass transfer and colour changes
during osmotic dehydration of watermelon. Journal of food engineering, 80, 979-985.
73
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Ferreari, C.C., & Hubinger, D.M. (2008). Evaluation of the mechanical properties and
diffusion coefficients of osmodehydrated melon cubes. International journal of Food
Science and Technology, 43, 2065-2074.
Goni, O., Munoz, M., Cabello, J., Escribano, M., & Merodio, C. (2007). Changes in water
status of cherimoya fruit during ripening. Postharvest Biology and Technology, 45,
147-150.
Ivan, L., Nicholas, J., Gary, & D. (2007). A simple centrifugal dehydration force method to
characterize water compartments in fresh and post-mortem fish muscle. Cell Biology
International, 31, 516-520.
Khoyi, M., & Hesari, J. (2007). Osmotic dehydration kinetics of apricot using sucrose
solution. Jouranl of food engeneering, 78, 1355-1360.
Kowalska H., & Lenart A. (2001). Mass exchange during osmotic pretreatement of
vegetables. Journal of food engineering, 49, 137-140.
Krokida, M. K., Oreopoulou, V., Maroulis, Z.B., & Marinos-Kouris, D. (2001). Effect of
osmotic dehydration pretement on quality of French fries. Journal of food
Engineering, 49, 339-345.
Lazarides, H.N., Gekas, V., & Mavroudis, N. (1997). Apparent mass diffusivities in fruit and
vegetable tissues undergoing osmotic processing. Journal of food Engineering, 31,
315-324.
Liu, Y., Bhandari, B., & Zhou, W. (2007). Study of glass transition and enthalpy relaxation of
mixtures f amorphous sucrose and amorphous tapioca starch syrup solid by differential
scanning calorimetry. Journal of food Engineering, 81, 599-610.
Maestre, J., Melgarejo, P., Tomas, A. & Garcia-Viguer, C. (2000). New food products drived
from pomegranate. Cahiers Options méditerranéennes. 243 - 245.
Masmoudi, M., Besbes, S., Blecker, C., & Attia, H. (2007). Preparation and characterization
of osmotidehydrated Fruits from Lemon and Date By-products. Journal of Food
Science and Technology international, 13, 405-412.
74
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Mavroudis, N. E., Gekas, V., & Sjohlm, I. (1998). Osmotic dehydration of apples, shrinkage
phenomena and the significance of initial structure on mass tranfer rates. Journal of
food Engineering, 38, 101-123.
Ohkuma, C., Kawai, K., Viriyarattanasak, C., Mahawanich, T., Tantratian, S., Takai, R., &
Suzuki, T. (2008). Glass transition properties of frozen and freeze-dried surimi
products: effect of sugar and moisture on the glass transition temperature. Food
Hydrocolloids, 22, 255-262.
Palou, E., Lopes-Malo, A., Argaiz, A. & Welti, J. (1994). The use of Peleg’s equation to
model osmotic concentration of papaya. Drying Technology, 12, 965-978.
Park, K.J., Bin, A., Bord, F.P.R. & Park, T.H.K.B. (2002). Osmotic dehydration Kinetics of
pear D’anjou (Pyrus communis L.). Journal of Food Engineering, 52, 293-298.
Peleg, M. (1988). An empirical model for the description of moisture sorption curves. Journal
of Food Science, 53, 1216-1219.
Raoult-Wack, A. L., Guilbert, S., Le Maguer, M., & Rios, G. (1991). Simultaneous water and
solute transport in shrinking media-part 1: application to dewatering and impregnation
soaking process analysis (osmotic dehydration). Drying Technnology, 9, 589-612.
Raoult-Wack, A. L., Lafont, F., Rios, G., & Guilbert, S. (1989). Osmotic dehydration: study
of mass transfer in terms of engineering properties. In Drying 89, ed. A. S. Mujumdar
and M. Roques, Hemisphere Publishing Corporation (pp 487 – 495). New York.
Roos, Y. (1995). Characterisation of Food Polymers Using State Diagrams. Journal of food
Engineering, 24, 339-360.
Sá, M.M., Figueiredo, A.M., & Sereno, A.M. (1999). Glass transitions and state diagrams for
fresh and processed apple. Thermochimica Act, 329, 31-38.
Saurel, R., Raoult-Wack, A.L., Rios, G., & Guilbert, S. (1994). Approches technologiques
nouvelles de la déshydrataion-impregnation par immersion. Cahier Scientifique,
Volume N°112.
75
Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________
76
Sestili, P., Martinelli, C., Ricci, D., Fraternale, D., Bucchi, A., Giamperi, L., Curcio, R.,
Piccoli, G., & Stocchi, V. (2007). cytoprotection effect of preparations from various
parts of Punica granatum L. fruits in oxidatively injured mammalian cells in
comparison with their antioxidant capacity in cell free systems. Pharmacological
research, 56, 18-26.
Storey, T. (2007). La grenade le fruit médicament, Magazine NEXUS, Santé, 51, 46-54.
Torreggiani, D., & Bertolo, G. (2001). Osmotic pre-treatments in fruit processing: chemical,
physical and structural effects. Jouranl of food engineering, 49, 247-253.
Vivanco, P. D, Hubinger, M. D., & Sobral, P. J. A. (2004). Mass transfer in osmotic
dehydration of Atlantic Bonito (Sarda) fillets under vacuum and atmospheric pressure.
Internationnal Drying Symposium, 14, 2105-2112.
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Chapitre 3:
Effet de la congélation sur la cinétique de transfert de masse durant la déshydratation osmotique des graines de
grenade
Ce travail a fait l’objet de la publication suivante :
Bchir, B., Besbes, S., Attia, H., & Blecker, C. (2010). Osmotic dehydration of
pomegranate seeds (Punica granatum L.): Effect of freezing pre-treatment.
Journal of Food Process Engineering, DOI: 10.1111/j.1745-4530.2010.00591.x
77
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Résumé *
Titre : Effet de la congélation sur la cinétique de transfert de masse durant la déshydratation
osmotique des graines de grenade
Objectif et stratégie expérimentale :
Ayant démontré précédemment que l’utilisation d’une solution de saccharose à
55°Brix et d’une température de 50°C, favorise la perte en eau au cours du procédé de DO,
cette partie visait, tout en appliquant ces conditions, à déterminer l’impact du pré-traitement
de congélation sur la cinétique de DO et la qualité organoleptique des graines de grenade,
durant sept heures de traitement. Pour ce faire, nous avons comparé l’évolution des
paramètres physico-chimiques et de transfert de masse des graines fraiches et congelées au
cours de la DO. Nous avons aussi mis en œuvre une technique fine de microscopie
électronique à balayage afin d’observer les modifications structurales des cellules, et
développer une procédure particulière d’analyse de texture des graines de grenade.
Les différentes étapes du procédé sont reprises de façon synoptique dans la figure 1’.
_________________________________________________________________________ * Ce résumé permet de présenter de façon synthétique, en français, l’axe de recherche de l’article qui a été publié en anglais.
78
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Graines de grenade congelées
Graines de grenade fraîches
Conditions
Traitement : déshydratation osmotique
Solution (55°Brix): - Saccharose
Temps (min): - 0 - 60 - 240 - 10 - 80 - 300 - 20 - 120 - 360 - 40 - 180 - 420
Température (°C): - 50
Rapport : graine/solution: - 1/4
Paramètres
Paramètres physico-chimiques : - pH, aw, - MS, Deff, conductivité - °Brix, couleur (L*, a*, b*) - Texture (hardness et toughness) - Structure cellulaire (microscopie électronique à balayage)
Paramètres de transfert de masse : - Perte en eau - Gain en solides - Réduction en poids
Figure 1’ : Différentes étapes du procédé de déshydratation osmotique des graines fraîches et
congelées.
79
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Principaux résultats :
Le pré-traitement de congélation des graines a entraîné une réduction du temps de
déshydratation attribuée à une augmentation des coefficients de diffusion de l’eau et de soluté.
Dans le cas des graines congelées, les principales pertes en eau (46% de réduction) et gain en
solide (7% d'accroissement) surviennent pendant les 20 premières minutes du traitement. La
DO des graines fraîches est caractérisée par une cinétique plus lente, mais une modification
finale plus importante (62% de perte en eau). Par conséquent, la congélation est un pré-
traitement intéressant pour favoriser la DO.
La microscopie électronique à balayage ainsi que l'analyse de texture ont montré une
altération physique de la structure des graines après congélation, ayant une répercussion
directe sur la fermeté du fruit. Les mêmes techniques ont également indiqué une modification
de texture/structure induite par le processus de déshydratation osmotique.
80
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Osmotic dehydration of pomegranate seeds (Punica
granatum L.): Effect of freezing pre-treatment
*Brahim Bchira, Souhail Besbesb, Hamadi Attiab, Christophe Bleckera,
a Gembloux Agro-Bio Tech, University of liège, Passage des Déportés, 2, B- 5030 Gembloux,
Belgium
b Unité Analyses Alimentaires, Ecole Nationale d’Ingénieurs de Sfax, Route de Soukra, 3038
Sfax, Tunisia
*Corresponding authors Tel: +32(0)81/62.23.03
*Fax: +32(0)81/60.17.67
*E-mail address: [email protected]
81
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Abstract
Osmotic dehydration of pomegranate seeds was compared using fresh and frozen seeds.
The process was carried out at 50°C in a 55°Brix solution of sucrose. Freezing pomegranate
seeds before osmotic dehydration involved an increase of effective diffusivity and a reduction
of dehydration time. The most significant changes of water loss (46 g/100g of fresh seeds
(FS)) and solids gain (7 g/100g of FS) took place during the first 20 min for frozen seeds.
After this period, seeds water loss and solids gain ranged on average close to 43 and 8 g/100g
of FS, respectively. Osmotic dehydration was slower starting from fresh fruits but led to a
higher rate of water loss (62 g/100g of FS) at the end of the process. Both scanning electron
microscopy and texture analysis showed a destruction of cell structure and seed texture during
the pretreatment (freezing). The same techniques also revealed a texture/structure
modification induced by the osmotic dehydration process.
Practical applications
In Tunisia the research of addition value to pomegranate seeds is very limited and
presents a traditional feature such as jam preparation or direct consummation of fruit during
the crop season (between September and December). However, other perspectives of
transformation and exploitation of the pomegranate seeds should be undertaken to give an
added value to this typical fruit. Osmotic dehydration can be an alternative process to increase
the conservation time of fruit and to give a new osmodehydrated fruit that can be included in
some food formulation such as ice-cream, cereals, dairy, confectionery and bakery products.
Keywords: Osmotic dehydration; water loss; solids gain; effective diffusivity; scanning
electron microscopy; texture analysis; pomegranate.
82
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ 1. Introduction
Pomegranate (Punica granatum L.) is one of the most important fruits in Tunisia. Its
total production in 2008 reached more than 70,000 tons. Pomegranate is composed by a non
edible part formed by 30% of skin (external part) and 13% of internal lamel and an edible part
formed by seeds (50-70%). Pomegranate seeds are composed by 15% pips (woody part), this
part determines the hardness, and 85% pulp (the juicy part) depending on cultivar (Al-
Maiman and Ahmad, 2002).The edible part of the fruit contains considerable amounts of
sugars, vitamins, organic acids, phenolic compounds and minerals (Espiard, 2002).
Pomegranate seeds need preservation methods to increase their shelf-life due to insect
attacks and microorganism growths. Among these methods we notes; drying, freezing,
pasteurization, osmotic dehydration etc. (Raoult-Wack et al., 1991). Osmotic dehydration has
received a considerable attention due to its low energy and cost compared to other
dehydration methods. Osmotic dehydration can be an alternative process to increase the
conservation time of fruits (Kowalska et al., 2008). Therefore, this method allows to consume
pomegranate seeds during the off season (other than September -December). Other benefits of
osmotic dehydration include effective inhibition of polyphenoxidase, prevention of loss of
volatile compounds, even under vacuum and reduction of heat damage to color and flavor
during dehydration (Krokida et al., 2001). The other major application is to reduce the water
activity of many food materials so that microbial growth will be inhibited (Bolin et al., 1983).
Osmotic dehydration gives two major simultaneous counter-current mass transfer fluxes,
namely water flow from the product to the surrounding solution and solute infusion into the
product (Escriche et al., 2000; Lewicki et al., 2005). There is a third flow of natural solutes
such as sugars, organic acids, minerals and salts leaching from the food into the solution
(Waliszewski et al., 1997), which is quantitatively negligible, but may have an important
effect on the organoleptic and nutritional value of the product.
83
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Mass transfer in osmodehydrated tissues was described by many scientifics (Lazarides
et al., 1995 and Jiokap Nono et al., 2001). Nowadays, the industry uses osmotic dehydration
for some previously cut fruit like apple, banana, mango, and apricot, amongst others.
However, this process has not been used for the conservation of whole pomegranate seeds by
industrials. Moreover, to our Knowledge, we are the first to have previously studied the
osmotic dehydration of pomegranate seeds (Bchir et al., 2009).
The cellular membrane exerts high resistances to transfer and slows down the rate of
osmotic dehydration (Erle and Shubert, 2001). Therefore pre-treatments such as freezing,
high-pressure, high intensity electric field pulse have been reported to enhance mass transfers
(Tedjo et al., 2002; using mangos fruit).
The aim of this research was to investigate the kinetics of osmotic dehydration and to
determine the influence of freezing, on mass transfer during osmotic dehydration and to
characterize textural and structural change in osmotically dehydrated pomegranate seeds.
2. Material and Methods
2.1. Preparation of pomegranate seeds
Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were obtained from
a local research centre in Gabes, Tunisia. Pomegranates fruit were collected at the same
ripening stage, having the same size. The fruit (20 kg) were washed in cold tap water and then
frozen at -50°C. Some pomegranates (20 kg) were conserved at 4°C until analysis. Seeds were
recuperated immediately prior to the osmodehydration process.
2.2. Osmodehydration process
Sucrose was dissolved in water in order to obtain 55°Brix solutions. About 10 g of
seeds was soaked in the sugar solution and were placed in bottles (Schott) of 100 ml. The
volume ratio between the fruit and the sugar solution was kept at one part of fruit and four
parts of solution (1:4). Osmotic dehydration process was conducted during 10 to 420 min in a
84
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ shaking water bath (GFL instrument D 3006, Germany; oscillation rate 160 rpm) at 50°C
(Bchir et al., 2009).
2.3. Mass transfer kinetics
Seeds were removed from the immersion solution at selected time intervals (0, 10, 20,
40, 60, 80, 120, 180, 240, 300, 360 and 420 min for frozen and fresh) and were quickly rinsed
(with distilled water) and the excess of solution at the surface was removed with absorbent
paper. Soluble solids were then measured as described below. The material was weighed
before and after osmodehydration to calculate the percentage of weight reduction (WR). The
moisture content was determined to calculate water loss (WL) and solids gain (SG), based on
the following equations (Mavroudis et al., 1998):
WR g/100g of fresh seeds = 100.)(
i
fi
WWW −
(1)
WL g/100g of fresh seeds = SG + WR (3)
SG g/100g of fresh seeds = 100.)(
i
sisf
WWW −
(2)
Where Wi is the initial weight of the sample (g), Wf the final weight of the sample (g),
Wsi the initial total solids content (g) and Wsf the final total solids content (g). Each value is
the mean of three determinations.
2.4. Mathematical modelling
Diffusion coefficients were calculated using Fick’s second law equation applied to a
sphere, by modifying the Fourier number , using shape factor, due to an
ellipsoids shape of pomegranate seeds.
2/0 RtDFffe=
The following assumptions were taken into account in order to determine the effective
diffusion coefficient: homogeneous body, the external resistance to mass transfer was
negligible compared with internal resistance, the initial moisture content was uniform
throughout the sample, and the diffusion coefficient was constant (Crank, 1975).
85
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
The solution for Fick’s equation law for diffusion out of a sphere is given by equation 4,
using the following boundary conditions of internal resistance (Luikov, 1968; Crank, 1975):
Uniforme initial amount: t = 0, 0<r<R, MC(t)=MC0
Symmetry of concentration: t >0, r = 0, 0)(=
∂∂
rtMC
Equilibrium content at surface: t >0, r = R, MC(t)=MCeq
[ ]∑∞
=
−=−
−=
10
2
0
exp)(
nnn
eq
eqAouS FB
MCMCMCtMC
W μ (4)
Where: Bn= 6/μn2; μn= nП; F0= Deff, AorS t/R2; n = 1, 2, 3,…
Where Deff, AorS is the effective diffusivity of water loss or solids gain (m2 s-1); n is the
number of series terms, R is the equivalent radius of sphere (m), r is the distance in the radius
direction (m), and t is the time (s). WA and WS are the dimensionless amount of water loss and
solids gain, respectively; MCeq is the equilibrium amount of moisture or solids content (g/g
DM) calculated using Peleg’s equation.
Peleg’s equation parameters were obtained using eqn (5) (Peleg, 1988). This two-
parameter model was redefined by Palou et al. (1994) in terms of soluble solids and moisture
content and describes sorption curves that approach equilibrium asymptotically.
tkktMCtMC
210)(
+±= (5)
Where )(tMC is the amount of moisture or solids at the instant t (g/g dry matter
(DM)), is the initial amount of moisture or solids (g/g DM), k1 and k2 are Peleg’s
parameters and t is the time (s).
0MC
The value of the amount moisture or solids content at the equilibrium was then
calculated using eqn 6 (Park et al., 2002).
20
210
1)(limk
MCtkk
tMCMCteq ±=
+±=
∞→ (6)
86
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
As stated earlier, in this work pomegranate seeds were assumed to be ellipsoids, having
three characteristic diameters (2rM1~2rM2≤2RM ). According to Alvarez et al. (1995) the
diffusion coefficient (D’eff) must be corrected by the factor Ψ2 when the product shape can be
assumed as an ellipsoid. The shape factor (Ψ) eqn 7 is defined as Ss/Sp, and Ss is the surface
area of a sphere of volume equal to that of fruit with surface area Sp, which is assumed to be
an ellipsoid (Alvarez et al., 1995). The intrinsic diffusivity Deff is given by Ψ2 D’eff (Alvarez et
al., 1995).
21
2
2
2
)/(1sin)/(1
22
4
MM
MM
MM
e
p
s
RrRr
Rrr
RSS
−⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−+
==−ππ
πψ (7)
2.5. Physico-chemical analysis
All analytical determinations were performed in triplicate. Values were expressed as the
mean± standard deviation.
Dry matter and moisture contents
The dry matter was calculated using oven drying, according to AOAC method 934.01
(1990). Approximately, 5 g of seeds were oven dried at 103°C ± 2°C, until constant weight.
Moisture content was estimated by difference of mean values, 100% - % of dry matter
(Chenlo et al., 2007).
Protein content
Total nitrogen was determined by the Kjeldahl method. Protein was calculated using the
general factor (6.25) (AOAC, 1990, method number: 920.152).
Lipids content
To determine total lipid content, about 5g of seeds were mixed with chloridric acid. Fat
was then extracted with a soxtherm automatic S 306 AK solvent extractor equipped with six
Soxhlet posts (Gerhardt soxtherm, Switzerland) and command unit (Gerhardt Variostat,
87
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Switzerland) using petroleum ether 40-60°C in each Soxhlet post. The result was expressed as
the percentage of lipids in the dry matter.
Ash content
To determine ash content, about 5g of seeds were incinerated in a muffle furnace
(Gelman, Germany) at about 550°C for 8h. The total ash content was expressed in dry weight
percentage (AOAC, 1990, method number: 940.26).
Carbohydrate content
Carbohydrate content was estimated by difference of mean values, 100-(Sum of
percentages of moisture, ash, proteins and lipids) (Barminas et al., 1999).
Total soluble solids and water activity
The soluble solids of seeds and osmotic solution were determined by measuring the
°Brix at 20°C using an ATGO digital refractometer (DBX-55, Switzerland). Water activity
(aw) was measured using an aqualab (Switzerland) instrument at 20 °C.
pH and conductivity
pH measurements were performed using a Hanna instrument 8418 pH meter
(Switzerland) at 20°C. Conductivity was measured using a conductimeter LF 597-5
(Germany) instrument at 20°C.
Color
The CieLab coordinates (L*, a*, b*) were directly read with a spectrophotocolorimetre
Mini Scan XE (Germany) with a lamp (D 65). In this coordinate system, the L* value is a
measure of lightness, ranging from 0 (black) to +100 (white), the a* value ranges from -100
(greenness) to +100 (redness) and the b* value ranges from -100 (blueness) to +100
(yellowness).
88
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Microscopic observations
Microscopic observations were carried out according to Attia et al. (1993). Samples
were deposed on supports and underwent successively three operations. First, the dehydration
was carried out in an increasing ethanol gradient, going from 10 to 100% (volume/volume).
Secondly, the drying was performed at critical point CO2 (75 bars, 40 to 42 °C) using an Bal-
Tec apparatus (Bal-Tec CPD030). Finally, the metallization with gold was carried out using
an Bal-Tec apparatus (Bal-Tec MET 020). The observations were performed with a scanning
electron microscope (SEM) Philips XL 30 (Philips, France).
Texture analysis
Texture analysis were carried out using a texture profile analyzer (TA.XT2; Stable
Micro Systems, UK), with 75 mm compression probe. The operating conditions of the
instrument were as follows: 1.5 mm/s pre-test speed, 0.5 mm/s test speed, 10.0 mm/s post-test
speed, 0.10 N trigger force and 85% sample deformation. The hardness and toughness of
seeds were the means of 20 single seed measurements. Hardness (N) of seed was taken as the
force in compression which corresponded to the breakage of samples, while the toughness (N
mm) is the energy required to crush the sample completely (Kingsly et al., 2006; Al-Said et
al., 2009).
2.6. Statistical Analysis
Statistical analyses were carried out using a statistical software program (SPSS for windows
version 11.0). The data was subjected to analysis of variance using the general linear model
option (Duncan test) to determine significant differences between samples (P<0.05).
89
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ 3. Results and discussion
Chemical composition of pomegranate seeds before osmotic dehydration is shown in
table 1. Seeds are rich in carbohydrate (~85% DM) followed by protein (~8% DM), lipid
(~5% DM) and ash (~3% DM). This composition is quite similar to pomegranate seeds
cultivated in Egypt (El-Nemr et al., 1990). Seeds also had a low pH (~4.17), this could be
attributed to their high content in organic acids such as citric and malic acids, which are
important for sensory properties and preservation (Poyrazoglu et al., 2002).
Table 1. Chemical characteristic of pomegranate seeds
Seeds Dry matter (DM %) 16.00 ± 0.05 Protein g/100g DM 7.79 ± 0.86 Lipid g/100g DM 4.55 ± 0.40 Ash g/100g DM 2.87 ± 0.19 Carbohydrate g/100g DM 84.93 ± 0.25 pH 4.17 ± 0.20 aw 0.989 ± 0.002 °Brix 15.50 ± 0.09
3.1. Mass transfer kinetics
The effect of dehydration time on water loss (WL), weight reduction (WR), and solids
gain (SG) was studied starting from both frozen and fresh pomegranate seeds at 50°C, using
sucrose solution. The water loss increased in time during the osmotic process using fresh
seeds, but mostly before 120 min, reaching 47 g/100g of fresh seeds (FS). After this period,
only a slight increase was observed during the rest of the process reaching 62 g/100g of FS
after 420 min (Fig. 1). The WR and SG for fresh seeds followed also the same evolution
reaching 55 and 7 g/100g of FS at the end of the process, respectively. Several works reported
similar dewatering and impregnation kinetics for osmotic dehydration of many fresh fruits
(Khoyi and Hesari, 2007 and Falade et al., 2007).
90
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
0
10
20
30
40
50
60
70
0 50 100 150 200 250 300 350 400 450
Time (min)
SG
and
WL
(g/1
00g
of fr
esh
seed
s)
Figure 1. Comparison of WL and SG using fresh (× WL, ● SG) and frozen (ΔWL, ○SG) seeds during osmotic dehydration process.
Using frozen seeds, the most significant changes took place during the first 20 min of
dewatering as shown in Fig. 1. During this time, WL in seeds was 46 g/100g of FS, and after
that it varied slightly and ranged on average close to 43 g/100g of FS. The same trend was
also observed for WR. Under the same conditions, SG of frozen seeds was also increased
significantly during the first 20 min reaching 7 g/100g of FS, and tended to be stable at the
end of the process. Similar trends for osmotic process of frozen pumpkin, apple and carrot
were observed by Kowalska and lenart, (2001) and Kowalska et al. (2008).
The increase of WL, as shown in Fig. 1, at the beginning of the process is due to the
large osmotic driving force between the dilute sap of seeds and the surrounding hypertonic
medium. Then after this period, the slower water transfer is mainly influenced by the
reduction of the difference in concentration between the seeds and osmotic solution which
could involve a slower driving force. Indeed, the reverse trend of °Brix observed in seeds and
91
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ osmotic solution confirms these facts (Table 2). During the first 20 min, water loss was the
results of both osmotic process and defrosting of seeds. The trend observed in SG (Fig.1)
could be explained by migration of sucrose to the seeds through the cell wall and
accumulating between the cell wall and the cellular membrane, due to the important gradient
of sugar between the seeds and the osmotic solutions.
Using fresh seeds the dewatering was slow at the beginning of the process compared to
frozen seeds. In fact, during 60 min the percentage of WL in frozen seeds was 11 g/100g of
FS higher than observed in fresh seeds. However, a higher rate of WL (62 g/100g of FS) was
obtained using fresh seeds after 420 min of the process. These results are in accordance with
previous findings (Kowalska et al., 2008) comparing osmotic dehydration of fresh and frozen
pumpkin and carrot. Moreover, Fig. 1 shows that SG in fresh seeds was lower than those of
frozen seeds. In fact after 60 min of the process, SG of frozen seeds was 6 g/100g of FS
higher than fresh seeds. Even after 420 min, the percentage of SG of fresh seeds was lower.
So pretreatment before osmotic dehydration proved to increase solids gain in comparison with
samples without pretreatment. Our results were similar to those observed by Kowalska et al.
(2008) showing that the use of freezing before OD increased the percentage of solids gain.
Differences in water loss and solids gain between fresh and frozen seeds can be
explained by the damaged cell structure of frozen fruit due to freezing. In fact, Delgado and
Rubiolo (2005) showed that during freezing a part of the aqueous fraction freezes out and
forms ice crystals that damage the integrity of the cellular compartments. Thus, the cellular
membranes loose their osmotic status and their semi-permeability, favouring a large osmotic
driving force between the dilute sap of the seeds and the surrounding osmotic solution
(Kovacs and Meresz, 2004; Torreggiani and Bertolo, 2001). This fact was behind the higher
WL and SG for frozen seeds, at the begging of the process. The difference in water lost from
92
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ 180 min was due to the fast reduction of the difference in concentration between seed and
osmotic solution using frozen seeds (Table 2).
Different studies on various fresh fruit dewatering (papaye apple, pumpkin, carrot, and
kiwi) mentioned that the most significant changes were observed between 30 and 100 min
using sucrose solutions (60 and 62 °Brix) at different temperatures (30 and 50°C) (Saurel et
al., 1994; Kowalska et al., 2008). All these authors reported that WL ranged between 39 and
50 g/100g of FS. Results obtained in this study showed that WL in frozen seeds was in
accordance with the literature, however fresh seeds were slightly higher. This comparison
should however take into account the experimental conditions and the differences in states of
the various fruits. Indeed, given their small size, pomegranate seeds can be kept intact and do
not need to be cut. On the contrary, in literature most fruits (apple, kiwis…) used for the
osmotic process have to be cut into small volumes, due to their large sizes. Thus, cutting these
fruit creates more external lesions leading to a higher contact of cells with the osmotic
solution in a shorter time. The longer time needed to dewater pomegranate fresh seeds could
be explained by their external membrane ability to protect cells.
The value of water loss was higher than solids gain and depended on the advancement
of the dewatering process. In addition effective diffusivity of water was also higher than that
of the solids (Table 3). According to Vial et al. (1991) the difference between WL an SG was
essentially due to the diffusional differences between water and sugar as related to their
different molar mass as well as to the presence of “semi-permeable” vegetal membranes.
Raoult-Wack et al. (1989) described an antagonistic effect of water and solute transfer,
signaling that this is probably due to the combination of sugar penetration by diffusion and
sugar transportation by the water out-flow, as a function of the water flow rate.
93
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Table 2. Evolution of osmotic dehydration parameters in sucrose solution using frozen and fresh seeds
Frozen seeds
0min 10min 20min 40min 60min 80min 120min 180min 240min 300min 360 min 420min
°Brix of
solution
55.00
±0.00e
52.30
±0.14d
49.70
±0.07c
49.30
±0.01bc
49.10
±0.21ab
48.90
±0.28ab
49.00
±0.21ab
48.60
±0.21a
48.70
±0.35a
48.60
±0.21a
48.60
±0.14a
49.00
±0.28ab
°Brix of seeds 15.50
±0.09a
29.30
±0.14b
41.60
±0.14c
45.30
±0.14d
46.40
±0.14e
46.90
±0.14e
49.10
±0.07f
49.20
±0.70f
49.30
±0.35f
49.20
±0.07f
49.30
±0.14f
49.00
±0.28f
pH of solution 8.27
±0.03c
4.71
±0.02b
4.60
±0.27ab
4.50
±0.20a
4.50
±0.01ab
4.40
±0.02a
4.30
±0.07a
4.40
±0.01a
4.30
±0.07a
4.40
±0.02a
4.40
±0.01a
4.39
±0.14a
Conductivity
of solution
(μs/cm)
0.90
±0.01a
26.57
±0.04b
35.50
±0.35c
39.00
±1.06d
40.10
±0.07de
40.50
±0.21def
41.20
±0.28defg
42.90
±0.84fg
42.10
±2.19efg
42.90
±1.13fg
43.60
±1.13g
48.2
±2.55h
Dry matter of
seeds (%)
16.00
±0.05a
34.88
±1.05b
42.70
±0.08c
46.80
±0.65d
47.80
±0.83de
48.30
±0.41ef
49.50
±0.76f
49.29
±0.03f
49.30
±0.17f
49.50
±0.16f
49.60
±0.17f
49.60
±0.01f
L* 65.76
±0.01f
62.21
±0.76 e
61.40
±0.96cde
60.90
±0.40cde
60.50
±0.47bcde
59.80
±0.75bcd
58.80
±0.15b
59.90
±0.58bcd
59.90
±0.05bc
56.20
±0.60a
55.40
±1.63a
55.20
±1.98a
a*
-0.60
±0.02a
2.09
±0.02 b
2.40
±0.92b
2.90
±0.13bc
3.30
±0.46 bcd
4.10
±0.30 bcde
4.60
±0.24 bcde
5.30
±0.14bc
5.30
±0.77 cde
5.90
±0.34de
6.30
±0.17e
6.38
±0.79e
CieLab-
coordina
-te of
solution
b* 3.60
±0.01a
5.36
±0.15b
6.50
±0.26bc
7.80
±0.07cd
8.10
±0.17de
9.60
±1.98e
9.30
±0.09de
9.30
±0.10de
9.30
±0.22ed
9.40
±0.43ed
9.40
±0.02ed
9.42
±0.92ed
Fresh seeds
°Brix of
solution
55.00
±0.00g
54.90
±0.25g
54.9
±0.14g
54.75
±0.07g
54.55
±0.21g
53.95
±0.07f
53.30
±0.14e
52.75
±0.35d
52.05
±0.07c
52.40
±0.28cd
51.15
±0.07b
48.45
±0.31a
°Brix of seeds 15.50
±0.09a
17.55
±0.07ab
19.45
±0.07ab
20.30
0.01b
25.45
±0.35b
26.95
±0.07c
30.80
±0.07cd
34.60
±0.63d
40.90
±0.42e
45.80
±0.28f
48.4
±0.56f
48.40
±0.57f
pH of solution 8.27
±0.03e
5.82
±0.03d
5.62
±0.02cd
5.15
±0.64bc
5.39
±0.02cd
5.21
±0.01c
4.70
±0.24ab
4.60
±0.11a
4.60
±0.09a
4.60
±0.08a
4.40
±0.02a
4.38
±0.27a
Conductivity
of solution
(μs/cm)
0.90
±0.01a
3.13
±0.04b
3.61
±0.23bc
4.10
±0.17bc
4.83
±0.07cd
6.00
±0.31de
7.10
±0.96e
9.10
±0.13f
10.70
±0.60g
13.50
±1.45h
15.20
±0.46i
15.50
±0.99i
Dry matter of
seeds (%)
16.00
±0.05a
18.25
±0.35ab
20.79
±0.10b
23.53
±1.01c
30.94
±1.44d
33.06
±0.86d
49.62
±0.24e
50.52
±0.47e
51.73
±1.41ef
53.33
±0.55f
53.94
±0.56f
53.93
±0.45f
L* 65.76
±0.01e
64.68
±0.16cde
64.5
±0.12de
64.00
±0.96cde
64.10
±0.69cd
64.00
±0.51cd
64.20
±0.27c
63.6
±0.12cd
63.60
±0.12bc
63.80
±0.83bc
62.50
±0.18ab
61.33
±1.43a
a*
-0.60
±0.02a
-0.59
±0.14a
-0.59
±0.01a
-0.53
±0.02a
0.16
±0.02b
0.35
±0.02bc
0.80
±0.14cd
0.50
±0.22bc
1.00
±0.33d
1.50
±0.01e
1.80
±0.44e
1.81
±0.24e
CieLab-
coordina
-te of
solution b* 3.60
±0.01a
3.54
±0.01a
3.48
±0.99ab
4.68
±0.01b
5.41
±0.13b
5.22
±0.52b
5.50
±0.19b
5.81
±0.44b
6.03
±0.43b
6.38
±0.63b
6.08
±0.80b
6.12
±0.47b
Means in line with different letters are significantly different (P<0.05)
94
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Table 3. Water and solids effective diffusivities calculated by Fick’s model and values of
Peleg’s equation parameters
Water loss Solids gain Deffw (m2s-1) R2(%) K1 K2 R2 (%) Deffs (m2s-1) R2(%) K1 K2 R2(%)Frozen 9.44×10-12 99.92 28.05 0.23 99.99 4.81×10-12 99.72 12.08 0.03 99.92 Fresh 0.58×10-12 98.96 670.00 0.21 99.66 0.20×10-12 98.12 318.63 0.02 99.51
3.2. Evaluation of the Peleg and Fick mathematical models
The Peleg’s equation parameters (K1 and K2) were determined for water loss and solids
gain (Table 3). This model showed a good fit to the experimental data, with correlation
coefficients (R2) close to 0.99. Effective diffusivity values for water and solids were
calculated using Fick’s model, which also presented a good fit to experimental data, showing
an average correlation coefficients (R2) close to 0.99. Freezing pretreatment involved a strong
increase of effective diffusivity of water and solids compared to untreated samples (Table 3).
3.3. Physico-chemical characteristics of the osmodehydrated fruit preparation
The effect of time on physico-chemical characteristics of the osmodehydrated seeds
(frozen and fresh) and osmotic solution during osmotic process are shown in table 2. Using
frozen seeds, the most significant change occurred during the first 20 min of dewatering. In
fact, statistical analysis shows a significant difference (P<0.05) between 0, 10 and 20 min for
all parameters. The fresh seeds, on the contrary, showed a progressive evolution. At the
beginning of the process the °Brix of the solution decreased as the °Brix of seeds increased,
after that °Brix tended towards an equilibrium. This was a consequence of osmosis, inducing
a balance of concentration between the seeds and the sucrose solution. The decrease of the
osmotic solution pH and the increase of conductivity could be attributed to the diffusion of
some organic acids from pomegranate seeds to the aqueous solution. Moreover, the measure
of color parameters L*, a*, b* showed a slight variation of these parameters. This variation
95
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ could be explained by a migration of pigment from pulp to solution and the non-enzymatic
browning (Masmoudi et al., 2007).
Compared to fresh seeds, the speed of the osmodehydration process using frozen
seeds was higher. Indeed, using frozen seeds the concentration equilibrium (°Brix) was
reached earlier between the two compartments (seeds and osmotic solution) compared to fresh
seeds. In fact, after 60 min °Brix and pH (solution) for frozen and fresh seeds reached 46 and
25°Brix; 4 and 5, respectively. Moreover, conductivity of frozen seeds solution was 6 time
higher than conductivity of fresh seeds solution. These differences could be explained by the
increase of the exchange through the seeds membranes due to the irreversible damage and a
loss of selectivity of cells induced by freezing.
These results showed the utility of freezing for a better transfer of solutes and water
respectively into and out of the fruits.
3.4. Microstructure and texture analysis
To better understand the effect of freezing and osmodehydration treatment at cell level
and on texture characteristics, scanning electron microscopy (SEM) and texture analysis were
used. Fig. 2a shows a control sample of pomegranate seed tissue, which did not receive any
other treatment only the preparation for SEM. The bright regions in the micrograph are
mainly the cytoplasmic membrane and the cell walls; the darker regions are holes where cell
contents were before. Frozen cells have a different appearance than fresh cells (Fig. 2b). In
fact, tissues of fresh seeds showed isodiametric cells with a regular shape and well-organized.
On the contrary, frozen seed cells appeared torn and irregular in shape, due to the loss of
turgor, with the presence of many empty regions (regions which were not occupied by cells).
This difference is due to the freezing treatment. Indeed, empty regions indicated that ice
nucleation and crystal growth damaged the cell wall. Numerous studies described that during
freezing a gradual breakdown in the organization of the protoplasmic structure and, in most
96
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ cases, the rupture of the plasmalemma with subsequent loss of turgor pressure in cells. In
addition, some degradation and separation of cell walls were also noted (Jewell, 1979;
Torreggiani and Bertolo, 2001). Decompartmentalization caused by ice crystals prevents the
return of water to the intracellular medium during thawing, causing loss of turgidity. This has
practical consequences in terms of the loss of the ability to act as a semi-permeable membrane
or diffusion barrier, and also the modification of fruit texture (Kovacs and Meresz, 2004).
a
b
c
d
Figure 2. Scanning electron microscopy photographs of fresh (a), frozen (b) and osmodehydrated fruits prepared with fresh (c) and frozen (d) seeds.
97
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Texture analysis showed three zones characteristic of seed compression (Fig. 3). The
first (a) correspond to the pulp resistance (~13 N). The second (b) correspond to the fracture
of pip (~30 N), and the third zone (c) correspond the crushing of seed characterised by the
increased in force through a short distance. In fact, using the same analytical procedure, Al-
Said et al. (2009) showed that pulp and pip hardness varied between 9-14 N and 24-45 N
respectively, for pomegranate seeds cultivated in Oman. The peak force attained during the
test is referred to as hardness and area under the curve as toughness. Statistical analysis shows
a difference (P<0.05) between fresh and frozen seeds (Table 4). Frozen-thawed seeds have the
lower values of both hardness (47±2 N) and toughness (54±3 N mm) then fresh seeds (53±4
N and 74±3 N mm respectively). This can be explained by cells membrane deterioration
during freezing inducing the loss of binding capacity among cell walls, which is in accordance
with the result observed by SEM. As consequence frozen seeds lose their firmness and reduce
their thickness. Hence the probe penetrates more the seed and touches the pip at a smaller
distance compared to fresh seed. In fact, the distance before pip crushing using fresh seeds
(5.45±0.73mm) was higher than frozen seeds (4.34±0.51mm).
Distance (mm)
Forc
e (N
)
c
b a
Figure 3. Characteristic force-distance curve for texture analysis using fresh seeds.
98
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
Figure 2c and 2d show that osmotic dehydration process changed the tissue structure
compared to an untreated sample. In fact cells appeared irregular in shape, slightly distorted
but well-organized. This fact was probably due to the solubilisation of polysaccharides
(cellulose, hemicellulose and pectin) that composes the cell walls, the water loss and the pre-
concentration of sucrose on the surface of the tissue (Raoult-Wack et al., 1991; Torreggiani
and Bertolo, 2001; Delgado and Rubiolo, 2005). Indeed, pectin is the major constituents of
the middle lamella and thus contributing to the cell adhesion and firmness (Nunes et al.,
2008). Moreover water loss induces the plasmolysis of cells and solids gain gives consistency
to the tissues. Nunes et al. (2008) showed that diffusion of sucrose into the fruit tissue, during
osmotic dehydration process, and its interaction with the cell wall and middle lamella pectic
polysaccharides might result in the formation of a jam-like structure that gives consistency to
the tissues. Likewise, Delgado and Rubiolo (2005) showed that osmotically dehydrated
strawberry tissue with the sucrose concentration used did not greatly affect the tissue
structure.
Textural properties of seeds are closely linked to cellular structure (Torreggiani and
Bertolo, 2001; Sajeev et al., 2004). Indeed, the osmotic dehydration process induced an
increase of textural parameters (toughness and hardness) compared to the untreated sample
(Table 4). Statistical analysis shows a difference (P<0.05) between hardness and toughness of
seeds before and after osmotic process. In fact, hardness and toughness increased to 17 N and
13 N mm using frozen seeds and 36 N-15 N mm for fresh seeds. This could be a consequence
of the exchange (water loss and solids gain) between seeds and osmotic solution. Sajeev et al.
(2004) showed that the increase in textural parameters can be attributed to the dehydration of
cormels (Taro: Colocasia esculenta L.). As a consequence of this exchange, the products will
more or less lose weight and will shrink eventually. Indeed, the peaks of pip crushing using
frozen and fresh seeds were observed at 3.7±0.34 mm and 3.2 ± 0.34 mm, respectively. Thus
99
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ seeds reduce their thickness between an average close to 0.64 and 2.25 mm, after osmotic
dehydration process, using frozen and fresh seeds respectively.
Table 4. Textural properties of pomegranate seeds
Without treatment Osmotic dehydration treatment
Frozen seeds Fresh seeds Frozen seeds Fresh seeds
Toughness (N mm) 54.55±3.96a 74.22±3.45b 67.21±5.55b 89.59±5.85c
Hardness (N) 46.73±2.47a 53.41±4.23a 63.46±3.04b 90.10±5.01c Means in line with different letters are significantly different (P<0.05)
Fresh osmodehydrated seeds had higher texture parameters compared to frozen
osmodehydrated seeds. Table 4 shows a statistical difference (P<0.05) between hardness and
toughness of osmodehydrated fresh and frozen seeds. These results are in accordance with
previous findings (Van Buggenhout et al., 2007) using fresh and frozen carrots. In fact,
hardness and toughtness of osmodehydrated fresh seed was 26 N and 22 N mm higher than
those observed in osmodehydrated frozen seed, respectively. In addition, thickness of
osmodehydrated fresh seeds was 0.6 mm lower then osmodehydrated frozen seeds. These
differences were due to higher water loss using fresh seeds. In fact, Kingsly et al., (2006)
found that hardness and toughness of pomegranate seeds decreased when the moisture content
increased.
4. Conclusion
Freezing treatment prolongs the conservation of pomegranate seeds, however it involves
a destruction of cell structure and seed texture. As a consequence frozen seeds can not be
consumed directly. However, osmotic dehydration process could add value to frozen
pomegranate seeds. Indeed, freezing before osmotic dehydration provided 1.4 and 3.5-times
more water loss and solids gain respectively, than an untreated sample at the beginning of the
process. So as a consequence, the process could be stopped after 20 min, implying a
100
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ substantial gain of time and thermal energy. On the contrary for fresh seeds, it is better to
continue the process up to 420 min.
After the osmotic dehydration process, microstructure and texture analysis revealed a
modification of seed texture and cell structure. This could essentially be due to water loss and
solids gain. On the basis of seed texture, osmodehydrated frozen fruit may be considered
more suitable for incorporating in many foods products while osmodehydrated fresh fruit, due
to their hard texture, could be air-dried to give dried fruit.
The finished product has an attractive colour and presents a good texture in mouth, a
pleasant sugar taste and a good aroma. Osmotic dehydration reduced water activity from
0.989 to an average of 0.900. At this aw value a complementary treatment such as drying,
freezing and pasteurisation should be necessary to ensure its good conservation.
101
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ References:
Al-Maiman, S.A., and Ahmad, D. 2002. Changes in physical and chemical properties during
pomegranate (Punica granatum L.) fruit maturation. Food Chemistry, 76, 437-441.
Al-Said, F.A., Opara, L.U., and Al-Yahyai, R.A. 2009. Physico-chemical and textural quality
attributes of pomegranate cultivars Brown in the Sultanate of Oman. Journal of Food
Engineering, 90, 129-134.
Alvarez, A .C., Aguerre, R., Gomez, R., Vidales, S., Alzamora, S.M., and Gerschenson, L.N.
(1995). Air Dehydration of Strawberries: effects of Blanching and Osmotic
Pretreatments on the Kinetics of Moisture Transport. Journal of Food Engineering, 25,
167-178.
AOAC. 1990. Official methods of analysis of the Association of Official Analytical Chemists.
Edited by K. Helrich. Published by the Association of Official Analytical Chemists,
Inc., 15th edition, Arlington, Virginia; USA. 1298 pp.
Attia, H., Bennasar, M., Lagaude, A., Hugodo, B., Rouviere, J., and Tarodo De La Fuente, B.
1993. Ultrafiltration with microfiltration membrane of acid skimmed and fat-enriched
milk coagula: hydrodynamic, microscope and rheological approaches. Journal of
Dairy Research, 60, 161-174.
Barminas, J.T., James, M.K., Abubakar, U.M. 1999. Chemical composition of seeds and oil of
Xylopia aethiopica grown in Nigeria. Plant Foods for Human Nutrition, 53, 193-198.
Bchir, b., Besbes, S., Attia, H., & Blecker, C. 2009. Osmotic dehydration of pomegranate
seeds: Mass transfer kinetics and DSC characterization. International Journal of Food
Science and Technology, 44, 2208–2217.
Bolin, H.R., Huxsoll, C.C., and Jacko, R. 1983. Effect of osmotic agents and concentrations
on fruit quality. Journal of food Science, 48, 202-205.
Chenlo, F., Moreira, R., Fernandez-Herrero, C., and Vazquez, G. 2007. Osmotic dehydration
of chestnut with sucrose: Mass transfer processes and global kinetics modelling.
Journal of Food Engineering, 78, 765-774.
102
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Crank, J. (1975). The mathematics of diffusion (2nd ed). Oxford: Clarendon Press.
Delgado, A.E., and Rubiolo, A.C. 2005. Microstructural changes in strawberry alter freezing
and thawing processes. Lebensmittel-Wissenschaft und-Technologie-Food Science and
Technology, 38, 135-142.
El-Nemr, E., Ismail, A., and Ragab, M. 1990. Chemical composition of juice seeds of
pomegranate fruit. Journal of Food Science, 7, 601–606.
Erle, U., and Shubert, A. 2001. Combined osmotic and microwave vacuum dehydration of
apples and stawberries. Journal of Food Engineering, 49, 193-199.
Escriche, I., Garcia-Pinchi, R., and Fito, P. 2000. Osmotic dehydration of kiwifruit (Actinidia
chinensis): fluxes and mass transfer kinetics. Journal of Food Process Engineering, 23,
191-205.
Espiard, E. 2002. Introduction à la transformation industrielle des fruits. TECandDOC-
Lavoisier (pp 181-182).
Falade, K., Igbeka, J., and Ayanwuyi, F. 2007. Kinetics of mass transfer and colour changes
during osmotic dehydration of watermelon. Journal of Food Engineering, 80, 979-
985.
Jewell, G.G. 1979. Fruits and vegetable. In J.G. Vaughan (Ed.), Food Micriscopy (pp. 1-34).
London: Academic Press Inc.
Jiokap Nono, Y., Giroux, F., Cuq, B., and Raoult-Wack, A. 2001. Etude de paramètres de
contrôle et de commande du procède de deshydratation-impregnation par immersion,
sur système probatoire automatise: application au traitement des pommes "Golden".
Journal of Food Engineering, 50, 203-210.
Khoyi, M., and Hesari, J. 2007. Osmotic dehydration kinetics of apricot using sucrose
solution. Journal of Food Engineering, 78, 1355-1360.
Kingsly, A.R.P., Singh, D.B., Manikantan, M.R. and Manikantan, Jain, R.K. 2006. Moisture
dependent physical properties of dried pomegranate seeds (Anardana). Journal of
Food Engineering, 75, 492-496.
103
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Kovacs, E., and Meresz, P. 2004. The effect of harvesting time on the biochemical and
ultrastructural changes in Idared apple. Acta Alimentaria, 33, 285-296.
Kowalska, H., Lenart, A., and Leszczyk, D. 2008. The effect of blanching and freezing on
osmotic dehydration of pumpkin. Journal of Food Engineering, 86, 30-38.
Kowalska, H., and Lenart, A. 2001. Mass exchange during osmotic pretreatment of
vegetables. Journal of Food Engineering, 49, 137-140.
Krokida, M. K., Oreopoulou, V., Maroulis, Z.B., and Marinos-Kouris, D. 2001. Effect of
osmotic dehydration pretement on quality of French fries. Journal of Food
Engineering, 49, 339-345.
Lazarides, H.N., Katsanidis, E., and Nickolaidis, A. 1995. Mass transfer during osmotic pre-
concentration aiming at minimal solid uptake. Journal of Food Engineering, 25, 151-
166.
Lewicki, P.P., and Porzecka-Pawlak, R. 2005. Effect of osmotic dewatering on apple tissue
structure. Journal of Food Engineering, 66, 43-50.
Luikov, A.V. 1968. Analytical Heat Diffusion Theory. Academic Press, Inc., New York, NY.
685 pp.
Masmoudi, M., Besbes, S., Blecker, C., and Attia, H. 2007. Preparation and characterization
of osmotidehydrated Fruits from Lemon and Date By-products. Journal of Food
Science and Technology International, 13, 405-412.
Mavroudis, N. E., Gekas, V., and Sjohlm, I. 1998. Osmotic dehydration of apples, shrinkage
phenomena and the significance of initial structure on mass tranfer rates. Journal of
Food Engineering, 38, 101-123.
Nunes, C., Santos, C., Pinto, G. Lopes-da-silva, J.A., Saraiva, J.A., and Coimbra, M.A. 2008.
Effect of candying on microstructure and texture of plums (Prunus domestica L.).
LWT- Food Science and Technology, 41, 1776-1783.
Palou, E., Lopes-Malo, A., Argaiz, A. and Welti, J. (1994). The use of Peleg’s equation to
model osmotic concentration of papaya. Drying Technology, 12, 965-978.
104
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Park, K.J., Bin, A., Bord, F.P.R. and Park, T.H.K.B. (2002). Osmotic dehydration Kinetics of
pear D’anjou (Pyrus communis L.). Journal of Food Engineering, 52, 293-298.
Peleg, M. (1988). An empirical model for the description of moisture sorption curves. Journal
of Food Science, 53, 1216-1219.
Poyrazoglu, E., Gokmen, V., and Artik, N. 2002. Organic Acids Phenolic Compounds in
pomegranates (Punica granatum L.) Grow in Turkey. Journal of Food Composition
and Analysis. 15, 567 - 575.
Raoult-Wack, A. L., Guilbert, S., Le Maguer, M., and Rios, G. 1991. Simultaneous water and
solute transport in shrinking media-part 1: application to dewatering and impregnation
soaking process analysis (osmotic dehydration). Drying Technnology, 9, 589-612.
Raoult-Wack, A. L., Lafont, F., Rios, G., and Guilbert, S. 1989. Osmotic dehydration: study
of mass transfer in terms of engineering properties. In Drying 89, ed. A. S. Mujumdar
and M. Roques, Hemisphere Publishing Corporation (pp 487 – 495). New York.
Sajeev, M.S., Manikantan, M.R., Kingsly. A.R.P., Moorthy, S.N., and Sreekumar, J. 2004.
Texture Analysis of Taro (Colocasia esculenta L. Schott) Cormel during Storage and
Cooking. Journal of Food Science, Vol. 69, Nr. 7.
Saurel, R., Raoult-Wack, A.L., Rios, G., and Guilbert, S. 1994. Mass transfer phenomena
during osmotic dehydration of apple II. Frozen plant tissue. International Journal of
Food Science and Technology, 29, 543-550.
Tedjo, W., Taiwo, K.A., Eshtiaghi, M.N., and Knorr, D. 2002. Comparaison of pretreatment
methods on water and solids diffusion kinetics of osmotically dehydrated mangos.
Journal of Food Engineering, 78, 90-97.
Torreggiani, D., and BERTOLO, G. 2001. Osmotic pre-treatments in fruit processing:
chemical, physical and structural effects. Journal of Food Engineering, 49, 247-253.
Van Buggenhout, S. Grauwet, T. Van Loey, A., and Hendrickx, M. 2007. Effect of high-
pressure induced ice I/ice III-transition on the texture and microstructure of fresh and
pretreated carrots and strawberries. Food Research International, 40, 1276-1285.
105
Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________
106
Vial, C., Guilbert, S., and Cuq, J. 1991. Osmotic dehydration of kiwi fruits: influence of
process variables on the colour and ascorbic acid content. Science des aliments, 11,
63-84.
Waliszewski, K. N., Texon, N.I., Salgado, M.A., and Garcia, M.A. 1997. Mass transfer in
banana chips during osmotic dehydration. Drying Technology, 15, 2597-2607.
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Chapitre 4:
Utilisation du jus de datte comme milieu d’immersion pour la déshydratation osmotique des graines de grenade
Ce travail a fait l’objet de la publication suivante :
Bchir, B, Besbes, S., Karoui, R., Paquot, M., Attia, H., & Blecker, C. (2010).
Osmotic dehydration kinetics of pomegranate seeds using date juice as an
immersion solution base. Food and Bioprocess Technology, DOI:
10.1007/s11947-010-0442-1.
107
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Résumé *
Titre : Utilisation du jus de datte comme milieu d’immersion pour la déshydratation
osmotique des graines de grenade
Objectif et stratégie expérimentale :
Tout en transposant les conditions mises au point et les méthodologies d’analyse des
chapitres précédents, cette partie visait à valoriser une autre agrofourniture tunisienne : le jus
de datte. Ainsi, nous avons substitué la solution de saccharose utilisée comme milieu
d’immersion, par du jus de datte possédant un extrait sec soluble de départ de ~ 20°Brix et
ajusté à 55°Brix par du saccharose. L’étude de la cinétique de transfert de masse a été basée
sur la détermination de la perte en eau, du gain en solides et de la réduction en poids au cours
du temps. Les produits élaborés ont été caractérisés du point de vue physico-chimique (pH,
aw, MS, Deff, °Brix, couleur). D’autres techniques plus fines ont été développées, la
microscopie électronique à balayage et la texturométrie, permettant d’élucider les
modifications structurales des cellules et texturale de la graine avant et après DO.
Les différentes étapes du procédé sont reprises de façon synoptique dans la figure 1’.
_________________________________________________________________________ * Ce résumé permet de présenter de façon synthétique, en français, l’axe de recherche de l’article qui a été publié en anglais.
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Graines congelées :
(-50°C)
Conditions
Traitement : déshydratation osmotique
Solution (55°Brix): Jus de datte: - Deglet-nour - Allig - Kintichi
Température (°C): - 50
Temps (min): - 0 - 20 - 80 - 5 - 40 - 100 - 10 - 60 - 120
Rapport : graine/solution: - 1/4
Paramètres de transfert de masse : - Perte en eau - Gain en solides - Réduction en poids
Paramètres
Paramètres physico-chimiques : - pH, aw, - MS, Deff - °Brix, couleur (L*, a*, b*) - Texture (hardness et toughness) - Structure cellulaire (microscopie électronique à balayage)
Figure 1’ : Procédé de déshydratation des graines de grenade dans du jus de datte.
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Principaux résultats :
Du fait de la richesse en sucre du jus de datte (~ 53 g de saccharose /100 g MS et ~ 35
g de sucre réducteur /100 g MS). Son utilisation a permis d’apporter ~ 35% du sucre pour la
préparation de la solution d’immersion à 55°Brix, et ainsi de réduire la quantité de saccharose
ajoutée.
L’étude de la cinétique de déshydratation osmotique a montré que les changements les
plus cruciaux du transfert de masse sont intervenus pendant les 20 premières minutes du
procédé, indépendamment des variétés de datte utilisées. Ces conditions ont permis une perte
d'eau de ~ 39 %, et un gain en solides de ~ 6 %. Cette période est suivie d'une phase de
stabilisation, plus lente, amenant la perte en eau à ~ 40 % et le gain en solides à ~ 9 %.
Cependant ces valeurs sont légèrement plus faibles que celles retrouvées lors de l’utilisation
d’une solution de saccharose à 55°Brix.
Une lixiviation des solutés de la graine vers la solution a été observée durant le
procédé. Cette perte n'est quantitativement pas négligeable, et pourrait avoir un impact
important sur les caractéristiques sensorielles et valeurs nutritives des graines.
La microscopie électronique a indiqué que le processus de déshydratation osmotique
induit des modifications structurales des cellules, ce qui est confirmé par les analyses
texturales. Les modifications texturales (hardness et toughness) sont plus faibles que celles
enregistrées lors de l’utilisation d’une solution d’immersion à base de saccharose. Ainsi, la
teneur en saccharose est apparue comme le principal facteur influençant la texture de la
graine, mais aussi la viscosité du jus de datte.
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Osmotic dehydration kinetics of pomegranate seeds using
date juice as an immersion solution base
*Brahim Bchira, Souhail Besbesb, Romdhane Karouia, Michel Paquotc, Hamadi Attiab,
Christophe Bleckera
a Gembloux Agro-Bio Tech, University of Liege, Passage des Déportés, 2, B- 5030
Gembloux, Belgium
b Laboratory of Food Analyses, National Engineering School of Sfax, Route de Soukra, 3038
Sfax, Tunisia
c Department of Industrial Biological Chemistry, Gembloux Agro-Bio Tech, University of
Liege, Passage des Déportés, 2, 5030 Gembloux, Belgium
*Corresponding authors Tel: +32(0)81/62.23.03
*Fax: +32(0)81/60.17.67
*E-mail address: [email protected]
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Abstract:
Pomegranate seeds were osmodehydrated using date juice added with sucrose (final
°Brix: 55) as immersion solution. The kinetics of osmotic dehydration showed that the most
significant changes of mass transfer took place during the first 20 min of the process,
regardless of date juice varieties. During this time, seeds water loss and solids gain were
estimated to ~ 39 % and ~ 6 %, respectively. After 20 min of the process, the percentage of
water loss and solids gain varied slightly and ranged on average close to ~ 40 % and ~ 9 %,
respectively. During osmotic dehydration, there was a leaching of natural solutes from seeds
into the solution, which is quantitatively not negligible, and might have an important impact
on the sensorial and nutritional value of seeds and date juices. Both scanning electron
microscopy and texture (compression) analysis revealed that osmotic dehydration process
induced modifications of seed texture and cells structure. Sucrose was found to be the
essential element which influences the texture of seed and the viscosity of date juice.
Additionally, natural sugar present in date juice permits to substitute 35% of the total quantity
of sucrose added to the osmotic solution.
Keywords: Pomegranate; Date juice; Osmotic dehydration; Texture analysis;
Scanning electron microscopy.
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1. Introduction
Pomegranate (Punica granatum L.) and date (Phoenix dactylifera L.) have always
played an important part in the economic, the social lives and food habits in Tunisia. In 2008,
the production reached 70,000 and 126,000 tonnes respectively for pomegranate and date.
Pomegranate seeds contain considerable amounts of sugars (e.g., glucose, fructose),
vitamins (e.g., vitamin c, vitamin B3) , organic acids (e.g., oxalic acid, citric acid), phenolic
compounds (e.g., anthocyanin, flavonoid) and minerals (e.g., Magnesium, potassium)
(Espiard 2002; Yunfeng et al. 2006). In Tunisia, research giving added value to pomegranate
seeds is very limited. Traditional use such as jam preparation and/or direct consumption of
fruit during the crop season, is limited to a period between September and December of each
year.
Tunisia is currently classified the 10th world producer and the first exporter of dates.
During 2007-2009, Tunisian production of date has reached an average of 120.000 tonnes per
year with dominance of Deglet Nour variety constituting about 60 % of the total production
due to its very good sensory quality and a high commercial value. In Tunisia, Deglet Nour,
Allig and Kentichi are the most consumed varieties (Besbes et al. 2009). This production
progress is unfortunately accompanied by a substantial increase of loss (about 30% of the
total production) during picking, storage, commercialization and conditioning processes
(Besbes et al. 2005a). The lost dates, commonly named “date by-products”, are not consumed
by humans due to their inadequate texture (too soft or too hard), or visual properties (Besbes
et al. 2009). Up to now, “date by-products” are discarded or utilized in limited cases for
animal feed. Owing to their high content in sugar, dates were also used for the preparation of
some food products (e.g., date juice, syrup, date preserves or date jellies). Studies concerning
the use of these by-products to develop new products are scarce and are concerned mostly
with metabolites or biomass production (Besbes et al. 2009; Besbes et al. 2005b).
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Bchir et al. (2009) showed the possibility of pomegranate seeds to be dehydrated
osmotically using different osmotic solutions (55°Brix) (Sucrose, glucose and mix
sucrose/glucose solutions). In the research studies of Besbes et al. (2009) a high level of
carbohydrate (total sugar ~ 86 g/100 g dry matter; total dietary fiber ~ 9 g/100 g dry matter) in
date was described. Due to the high sugar content, date is used like source of sugar more than
like fruit (direct consumption) (Barreveld 1993). Indeed, dates were used to confer to milk
and tea a sweetened taste appreciated by the consumers (Munier 1973).
Nowadays, the industry uses osmotic dehydration for some previously cut fruits like
apple, banana, mango, apricot and strawberry among others, to obtain fruits with a good
aroma, and nutritional content. In addition, osmotic dehydration is a pre-drying step for
textural reasons, allowing to reduce convective air drying due to infused sugar binding
available water and thus partially reducing water activity. The infused solids provide a
superior texture to many dried fruits. Regarding tart or astringent fruits, such as cranberry and
tart cherry, the osmotic pre-treatment provides a favorable acid:sugar ratio resulting in an
overall better flavour.
This paper reports on value addition to pomegranate seeds and date juice from the most
produced and consumed Tunisian varieties (Date: Delglet Nour, Allig and Kentichi;
Pomegranate: El Gabsi) through their use in the formulation of dehydrated fruits using
osmotic dehydration process. The kinetics of osmotic dehydration, physico-chemical
characteristics, textural and structural change of osmodehydrated fruits was investigated.
2. Material and methods
2.1. Samples
2.1.1. Pomegranate seeds
Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were obtained
from a research centre in Gabes (Tunisia). Pomegranate fruits were collected at full ripening
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stage, having the same size. Pomegranate is composed of a non edible part formed by 30% of
skin (external part), 13% of internal lamel and an edible part formed by seeds (50–70 %).
Pomegranate seeds are composed of 15 % pips (woody part), that determines the hardness,
and 85 % pulp (the juicy part) (Al-Maiman & Ahmad, 2002). Seeds have an ellipsoids shape,
13±1 mm length, 7±1 mm breadth, 628±2 kg/m3 bulk density, and 2±0.2 mm of pip thickness.
An average weight of an individual seed was 0.504±0.04 g.
Twenty kilograms (20 kg) of pomegranate were washed in cold tap water and then
frozen at -50 °C. A digital thermometer BT20 (Bresso, Italy) was placed in the pomegranate
core to measure the temperature changes during one hour of thawing at room temperature.
Temperature of pomegranate core reached -7.5 °C, after one hour of thawing. Seeds were
recuperated immediately prior to the osmotic dehydration process.
2.1.2. Date by-products
Date by-products (Phoenix dactylifera L.) of ‘‘Deglet Nour’’, ‘‘Kintichi’’and ‘‘Allig’’
varieties were obtained from the National Institute of Arid Zone (Degach, Tunisia). One batch
of 20 kg (of each variety) of date fruits having the same origin and ripening stage was used.
Dates were directly pitted, washed in running tap water at 25 °C, and left for 24 h at an
ambient temperature of 25 °C. Then, dates pulps were milled to obtain date paste, which was
stored at -20 °C until analyses (Masmoudi et al. 2007).
2.1.3. Date Juice Preparation
The date juice was prepared by adding water to date paste at a ratio of 3:1 (v/w) as
described by Masmoudi et al. (2007) and Yousif et al. (1990). The date paste water mixture
was boiled gently with continuous stirring for 5 min. The extract was filtered through fine
mesh cheesecloth. The obtained date juice presented total soluble solids about 21 °Brix. It was
cooled at room temperature and then stored at -20 °C until analyses.
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2.2. Osmodehydrated fruit preparation
2.2.1. Osmotic solution preparation
Sucrose was dissolved in date juice (total soluble solid varied between 19.03 and
24.70 °Brix) in order to obtain 55 °Brix solution at 25°C. It was then kept at 4 °C until
analyses and use. 55 °Brix was selected as an average of the values used in the literature. In
fact, osmotic solution concentration varied between 50 and 70 °Brix, depending on the fruit to
be dried (Saurel et al. 1994; Corrêa et al., 2010).
2.2.2. Osmodehydration process
About 10 g of seeds was soaked in the osmotic solution and were placed in bottles
(Schott) of 100 ml. The volume ratio between the fruit and the sugar solution was kept at one
part of fruit and four parts of solution (1:4). Osmotic dehydration process was conducted
during 5 to 120 min in a shaking water bath (GFL instrument D 3006, Germany; oscillation
rate 160 rpm) at 50°C (Bchir et al. 2009; Bchir et al. 2010).
2.3. Mass transfer kinetics
Seeds were removed from the immersion solution at selected time intervals (5, 10, 20,
40, 60, 80, 100 and 120 min) and were quickly rinsed (with distilled water) and the excess of
solution at the surface was removed with absorbent paper. Soluble solids were then measured
as described below. The material was weighed before and after osmodehydration to calculate
the percentage of weight reduction (WR). The moisture content was determined to calculate
water loss (WL) and solids gain (SG), based on the following equations (Mavroudis et al.
1998):
WR g/100g of fresh seeds = 100.)(
iWfWiW −
(1)
SG g/100g of fresh seeds = 100.)(
iWsiWsfW −
(2)
WL g/100g of fresh seeds = SG + WR
(3)
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Where Wi is the initial weight of the sample (g), Wf the final weight of the sample (g),
Wsi the initial total solids content (g) and Wsf the final total solids content (g). Each value is
the mean of two determinations.
2.4. Physico-chemical analyses
All analytical determinations were performed in duplicate. Values were expressed as the
mean± standard deviation.
Dry matter and moisture contents
The dry matter (DM) was calculated according to AOAC (AOAC 1990) method 934.01
(1990). Approximately, 5 g of samples were oven dried at 103 °C ± 2 °C, until constant
weight. Moisture content was estimated by difference of mean values, 100 % - % of DM
(Chenlo et al. 2007).
Protein content
Total nitrogen was determined by the Kjeldahl method. Protein was calculated using the
general factor (6.25) (AOAC 1990, method number: 920.152).
Lipids content
To determine total lipid content, about 5 g of samples were mixed with chloridric acid.
Fat was then extracted with a soxtherm automatic S 306 AK solvent extractor equipped with
six soxhlet posts (Gerhardt soxtherm, Switzerland) and command unit (Gerhardt Variostat,
Switzerland) using petroleum ether 40-60 °C in each Soxhlet post. The result was expressed
as the percentage of lipids in the DM (Bchir et al. 2009).
Ash content
To determine ash content, about 5 g of samples were incinerated in a muffle furnace
(Gelman, Germany) at about 550 °C for 8h. The total ash content was expressed in dry weight
percentage (AOAC 1990, method number: 940.26).
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Carbohydrate content
Carbohydrate content was estimated by difference of mean values, 100-(Sum of
percentages of moisture, ash, proteins and lipids) (Barminas et al. 1999).
Determination of individual sugars
Sugars were extracted with 10 ml of ethanol solution (80%) according to Bouabidi et al.
(1996). The extracts were centrifuged (2000 tr/min, 30 min) and filtered (0.45 μm). Sucrose,
glucose, and fructose were analyzed with a High Performance Anion Exchange
Chromatography coupled with Pulse Amperometric Detection (HPAEC-PAD) on a Dionex
DX500 chromatographic system. The flow rate was 1 ml/min, the pressure and temperature
were 1000 psi and 80 °C, respectively. External standards of fructose, glucose, and sucrose
were used for quantification.
Total soluble solids and water activity
The soluble solids of pomegranate seeds juice, date juice and osmotic solution were
determined by measuring the °Brix at 25 °C using an ATAGO digital refractometer (DBX-55,
Switzerland). Water activity (aw) of seeds and date juice were measured using an aqualab
(Switzerland) instrument at 20 °C.
pH, conductivity and acidity
pH measurements were performed using a Hanna instrument 8418 pH meter
(Switzerland) at 25 °C. Conductivity was measured using a conductimeter LF 597-5
(Germany) instrument at 25 °C. Acidity was determined by titration with Na OH (0.1N) to pH
= 8.1 ± 0.2 as described by Grigelmo and Martin (Grigelmo and Martin 1999).
Color
The CieLab coordinates (L*, a*, b*) were directly read with a spectrophotocolorimetre
Mini Scan XE (Germany) with a lamp (D 65). In this coordinate system, the L* value is a
measure of lightness, ranging from 0 (black) to +100 (white), the a* value ranges from -100
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(greenness) to +100 (redness) and the b* value ranges from -100 (blueness) to +100
(yellowness). The hue angle (h°ab) and chroma or intensity (C*) were calculated according to
the following equations:
(5) h° = arctan (b*/a*) (4) C*= (a*2+b*2)1/2
Viscosity
Viscosity of osmotic solution was measured at 25 °C with a CV 120 Bohlin rheometer
(Bohlin instruments, UK) using a cone plate geometry (radius=40 mm, angle=4 °). The gap
between the cone and plate geometry was set to 150 μm. Shear rates from 0 to 70 s−1 were
utilized for flow behaviour determination.
Microscopic observations
Microscopic observations were carried out according to Attia et al. (1993). Samples
were deposed on supports and underwent successively the following three operations. First,
the dehydration was carried out in an increasing ethanol gradient, going from 10 to 100 %
(volume/volume). Secondly, the drying was performed at critical point CO2 (75 bars, 40 to 42
°C) using a Bal-Tec apparatus (Bal-Tec CPD030). Finally, the metallization with gold was
carried out using a Bal-Tec apparatus (Bal-Tec MET 020).
The observations were performed with a scanning electron microscope (SEM) Philips
XL 30 (Philips, France).
Texture analysis
Texture analysis were carried out using a texture profile analyzer (TA.XT2; Stable
Micro Systems, UK), with 75 mm compression probe. The operating conditions of the
instrument were as follows: 1.5 mm/s pre-test speed, 0.5 mm/s test speed, 10.0 mm/s post-test
speed, 0.10 N trigger force and 85 % sample deformation. The hardness and toughness of
seeds were the means of 20 single seed measurements. Hardness (N) of seed was taken as the
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force in compression which corresponded to the breakage of samples, while the toughness (N
mm) is the energy required to crush the sample completely (Kingsly et al. 2006; Al-Said et al.,
2009).
2.5. Statistical Analysis
Statistical analyses were carried out using a statistical software program (SPSS for
windows version 11.0). The data was subjected to analysis of variance (ANOVA) using the
general linear model option (Duncan test) to determine significant differences between
samples (P<0.05).
3. Results and discussion
3.1. Physico-chemical properties of seeds and date juices
The composition of pomegranate seeds and date juices from studied cultivars was
investigated. Results showed predominance of carbohydrate in seeds (84.93±0.25 g/100 g
DM) and a significant amount of protein (7.79±0.86 g/100 DM). Pomegranate seeds also
contained a relatively low level of ash (2.87±0.19 g/100 g DM) and lipid (4.55±0.40 g/100 g
DM). Carbohydrate level showed a low content of sucrose (1.09±0.11 g/100 g DM) and a
high level of glucose and fructose (i.e., respectively, 27.95±1.86 and 32.90±2.47 g/100 g
DM), and is in accordance with Fadavi et al. (2005) who found a similar content of glucose
and fructose from pomegranate seeds cultivated in Iran. Seed composition is similar to that
found in literature. Indeed, carbohydrate, protein, lipid and ash values varied respectively
between, 76.0 % - 86.0 %; 4.4 % - 8.7 %; 1.3 % - 9.1 %; 2.6 % - 3.5 % of (DM) (Espiard
2002; Fadavi et al. 2005). Seed had a low pH (4.17±0.20) this could be attributed to their high
content in organic acids such as citric and malic acids, which are important for sensory
properties and preservation (Poyrazoglu et al., 2002).
Date juices exhibited a high content of carbohydrate (~96.14 - 97.60 g /100 g DM) and
a total soluble solid varied between 19.03 and 24.70 °Brix. Lipid, ash protein and acidity
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contents were relatively low (~0.14 - 0.17; ~1.23 - 2.34; ~0.76 - 1.37 g/100 g DM and ~8.3
meq/100g respectively). A slight difference between varieties was observed. The composition
is higher than that found with Yousif & Al-Ghamdi (1999) and Al-Farsi (2003) for Sefri,
Sullag and Habash date varieties. Using the same method of juice extraction and the same
date variety, Masmoudi et al. (2007) showed a relatively high sucrose content (48.43 g/100 g
DM) flowed by fructose and glucose (37.19 g/100 g DM) in juices of Deglet Nour date
variety. The high content of natural sugar could allow an added value to date juice (e.g., used
as immersion solution). In addition, starting with date juice permitted to reduce by 35 % the
amount of sugar added to the osmotic solution.
3.2. Kinetics of osmotic dehydration
3.2.1. Mass transfer kinetics
The osmotic process was studied in terms of water loss (WL), solids gain (SG) and
weight reduction (WR). The values of these parameters were calculated according to Eqs. (1),
(2) and (3). An increase on WL, SG and WR of seeds were observed with the increase of
immersion time for all process conditions (using different date juices), at the beginning of the
osmotic dehydration process. Fig. 1 shows that the most significant changes took place during
the first 20 min of the process. During this time, WL in seeds was estimated to ~ 39 %. After
this period of dehydration, the percentage of water loss varied slightly and ranged on average
close to ~ 40 %. The same trend was observed for WR. Using the same conditions, SG was
increased significantly during the first 20 min reaching ~ 6 %. However, a slight increase of
SG was observed during the rest of process tending to be stable at the end. Statistical analyses
showed a significant difference during the first 20 min, regardless the considered parameters.
Different studies on various osmodehydrated fresh fruit (e.g., papaye apple, pumpkin, carrot,
guavas and kiwi) mentioned that the most significant changes were observed between 30 and
100 min using sucrose solutions (50 and 62 °Brix) at different temperatures (30 and 50 °C)
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(Saurel et al. 1994; Kowalska and Lenart 2001; Kowalska et al. 2008; Corrêa et al. 2010). All
these authors reported that WL ranged between 39 and 50 g/100 g of fresh matter. Results
obtained in this study showed that WL in seeds was in accordance with the literature. Bchir et
al. (2009) showed a slightly higher result (43 % of WL and 9 % of SG after 20 min of the
process) but a similar dewatering and impregnation kinetics for the osmotic dehydration of
pomegranate seeds in sucrose solution at 50°C; this difference could be explained by the
difference in the solids soluble composition of different osmotic solutions.
0
5
10
15
20
25
30
35
40
45
50
5 10 20 40 60 80 100 120
Time (min)
SG, W
L a
nd W
R (g
/100
g fr
esh
seed
s)
Figure 1. Water loss (WL: ♦), weight reduction (WR: ○) and solids gain (SG: ×) from
the osmodehydrated seeds using Deglet Nour date juice as an immersion solution base.
An initial rapid increase on water loss at various osmotic solutions could be due to a
large osmotic driving force between the dilute sap of seeds (15.50 °Brix) and the surrounding
hypertonic medium (55.00 °Brix), and also due to the thawing of seeds during the process.
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Indeed, Raoult-Wack et al. (1991), Shi et al. (2009) and Ruiz-Lopez et al. (2010) showed that
the rate of mass transfer during osmotic dehydration is influenced essentially by the difference
of concentration between samples and the surrounding osmotic solution. In addition, mass
transfer could be influenced by the loss of cell membrane selectivity inducing by the
pretreatment (e.g., Freezing). In fact, Delgado and Rubiolo (2005) and Aguilera et al. (2003)
showed that during slower freezing, a part of the aqueous fraction freezes out and forms ice
crystals that damage the integrity of the cellular compartments. Thus, the cellular membranes
lose their osmotic status and their semi-permeability, favoring a large osmotic driving force
between the dilute sap of the seeds and the surrounding osmotic solution. Slower water
transfer after this period is mainly influenced by the reduction of the difference in
concentration between the seeds and osmotic solution and hence a reduced driving forces
(Allali et al., 2008 and Uribe et al., 2010). Indeed, °Brix observed in seeds and osmotic
solutions tend to be equal at the end process (Table 1). Moreover rapid loss of water and
uptake of solids near the surface in the beginning caused structural changes leading to
compaction of these surface layers and increased mass transfer resistance for water and solids
(Delgado and Rubiolo 2005, Fathi et al., 2009). The trend observed in SG could be explained
by migration of sucrose to the seeds through the cell wall and accumulation between the cell
wall and the cellular membrane, due to the important gradient of sugar between the seeds and
the osmotic solutions. In fact, osmotic solution has a higher content of sucrose than
pomegranate seeds.
3.2.2. Evolution of osmodehydrated fruit parameters
Table 1 shows the change that occurred in seeds and osmotic solutions parameters as
function of time during the process. No difference was observed, regardless the osmotic
solutions used. As it was expected all parameters (°Brix, pH, conductivity,...) evolved in the
same trend like mass transfer parameters. Soluble solids content in pomegranate seeds
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increased as the °Brix solution decreased at the beginning of the process (20 min); after that
they tended towards equilibrium (49.10 °Brix). This was a consequence of osmosis, inducing
a balance of concentration between the seeds and the osmotic solution (Raoult-Wack et al.
1991). An explanation could arise from the migration of the osmotic solution (high sucrose
content) sugars to the pomegranate seeds (low sucrose content) through the cells membrane.
Thus, seeds were progressively concentrated in sugars and lost a part of its water content. The
difference between the total soluble solids in the pomegranate seeds and the osmotic solutions
at the end of the treatments was relatively small. This could support solute impregnation and
limit water loss. In fact, Saurel et al. (1994) reported that low concentrations in solutes
facilitate their impregnation in the fruits, whereas high concentrations allow much more
dehydration and lower water activity.
The increase of conductivity and the decrease of the osmotic solutions pH could be
attributed to diffusion of some solutes and organic acids from pomegranate seeds to the
aqueous solution. Many authors found similar kinetics during the osmotic dehydration of
various fruits (Delgado and Rubiolo 2005; Masmoudi et al. 2007; Fabiano et al., 2008;
Kowalska et al., 2008; Azuara et al., 2009). Moreover, the measure of color parameters L*,
a*, b* showed a slight variation of these parameters. In fact, hue angle (h°) and chroma (C*)
decreased from 1.4 - 27.9 to 1.2 - 23.0 respectively. This variation could be explained by a
migration of pigment from pulp to solution and the enzymatic or non-enzymatic browning
(Monsalve-Gonzalez et al. 1994; Nisha et al. 2010).
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Table 1. Evolution of osmotic dehydration parameters of pomegranate seeds and
osmotic solution (using Deglet Nour date juice)
0 min 5 min 10 min 20 min 40 min 60 min 80 min 100 min 120 min
°Brix of solution 55.00 ±0.00
53.90 ±0.50
52.40 ±0.40
50.30 ±0.35
49.50 ±0.01
49.30 ±0.07
49.20 ±0.01
49.10 ±0.07
49.10 ±0.01
°Brix of seeds
15.50 ±0.09
22.65 ±0.60
30.90 ±0.67
42.90 ±0.28
45.00 ±1.06
47.70 ±0.13
48.80 ±0.49
49.00 ±0.28
49.10 ±0.42
pH of solution 5.55 ±0.24
5.50 ±0.34
5.46 ±0.11
5.40 ±0.05
5.30 ±0.07
5.10 ±0.12
5.10 ±0.07
5.10 ±0.08
5.00 ±0.04
Conductivity of solution (μs/cm)
82.24 ±2.31
95.13 ±1.13
125.00 ±0.51
131.90 ±0.01
134.40 ±2.54
142.5 ±2.19
147.4 ±0.84
147.8 ±0.35
149.10 ±1.55
Dry matter of seeds (%)
16.00 ±0.05
20.80 ±1.05
26.90 ±0.16
35.42 ±1.43
41.45 ±0.64
43.66 ±0.06
45.34 ±0.09
46.43 ±0.18
46.95 ±0.58
aw of seeds 0.9890 ±0.002
0.9760 ±0.001
0.9570 ±0.002
0.9470 ±0.003
0.9445 ±0.001
0.9420 ±0.001
0.9400 ±0.001
0.9385 ±0.002
0.9355 ±0.002
L* 52.99 ±0.96
52.00 ±0.36
51.56 ±0.17
51.10 ±0.08
50.80 ±2.15
50.40 ±2.28
48.00 ±1.12
47.20 ±0.04
47.10 ±0.70
a* 4.30 ±0.27
4.35 ±0.72
4.41 ±0.15
4.50 ±0.18
5.10 ±0.67
5.40 ±0.48
6.80 ±0.64
7.70 ±0.38
7.80 ±0.46
b* 27.58 ±0.50
27.00 ±0.40
26.08 ±0.23
25.70 ±0.19
25.20 ±0.53
24.50 ±1.44
23.30 ±1.39
22.00 ±1.18
21.60 ±1.81
h° 1.41 ±0.25
1.40 ±0.05
1.38 ±0.04
1.35 ±0.10
1.32 ±0.07
1.30 ±0.03
1.29 ±0.10
1.23 ±0.20
1.22 ±0.30
CieLab coordinate of solution
C* 27.91 ±1.50
27.52 ±0.50
26.81 ±1.30
26.10 ±1.10
25.85 ±0.70
25.55 ±0.05
25.18 ±0.70
23.30 ±1.04
23.01 ±0.50
3.3. Osmodehydrated fruits characteristics
3.3.1. Physico-chemical characteristics of the osmodehydrated fruit preparations
The changes that occurred in pomegranate seeds after osmotic dehydration process are
shown in table 2. No significant difference was observed, regardless the osmotic solutions
used. Carbohydrate and total soluble solids in osmodehydrated seeds increased by ~11% DM
and ~34 °Brix, respectively. This increasing is due to the diffusion of sucrose from osmotic
solution (high sucrose content) to the seeds (low sucrose content). In fact the determination of
individual sugars in seeds, showed an increase of sucrose content (59.24±1.03 g/100 g DM).
Contrary to fructose and glucose level, that decreased (10.31±0.51 and 10.75±0.05 g/100 g
DM, respectively). Moreover protein and ash content decreased strongly by ~7.5 % and 1.9 %
DM, respectively; contrary to lipid level that varied slightly (from 4.5 to ~3.8 % DM)
compared to untreated seeds. In this sense, protein, mineral and organic acid diffusion by
125
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
osmosis contributed to a better flavour of the osmotic solutions. Albagnac et al. (2002)
showed that the solute diffusion flow from fruit to osmotic solution have an important effect
on the sensorial and nutritional value of the product.
Table 2. Physico-chemical properties of pomegranate seeds before and after osmotic dehydration process
Deglet Nour Kintichi Allig
Pomegranate seeds
Juices Components Before OD After OD Dry matter (%) 16.00±0.05 46.95±0.58a 47.69±1.36a 47.80±0.38a
Protein g/100g DM 7.79±0.86 0.35±0.08a 0.30±0.07a 0.27±0.01a
Lipid g/100g DM 4.55±0.40 3.57±0.11a 4.11±0.16a 3.74±0.31a
Ash g/100g DM 2.87±0.19 0.90±0.02a 0.88±0.04a 0.86±0.05a
Carbohydrate g/100g DM
84.93±0.25 96.85±0.70a 96.74±0.45a 96.76±0.24a
aw of seeds 0.989±0.002 0.935±0.002b 0.919±0.001ab 0.912±0.005a
°Brix of seeds 15.50±0.09 49.10±0.42a 49.85±0.28a 48.90±0.28a
Means in line with different letters are significantly different (P<0.05)
Water activity (aw) of the osmodehydrated fruit remained at high levels (~0.913). This
could be explained by the final total soluble solids (~49 °Brix) insufficient for an important
decrease of aw values. After osmotic process seeds can be kept in a refrigerator for a short
time and in a freezer for a long period. Rastogi et al. (2002) showed that osmotic dehydration
did not generally produce stable products. As consequence it must be used as pre-processing
stage before a complementary processing such as pasteurization, freezing, drying, etc.
(Fernandez et al. 2006; Mujumdar & Law 2010 ; Devic et al. 2010).
3.3.2. Microstructure and texture analysis
To better understand the effect of osmodehydration treatment at cell level and on
texture characteristics, scanning electron microscopy (SEM) and texture analysis were used.
Microstructural changes during osmotic dehydration are important in order to understand the
changes which occurred in the kinetics parameters and compositional changes of seeds
126
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
(Garcia et al. 2010). Images of transversal cross sections of treated and untreated seeds are
presented in Fig. 2. Fig. 2a shows the control sample of frozen pomegranate seed tissue,
which did not receive any treatment other the preparation for SEM. The bright regions in the
micrograph are mainly the cytoplasmic membrane and the cell walls; the darker ones are
holes where ice and cell contents were before SEM treatment. Cells appeared torn and
irregular in shape, with the presence of many empty regions (regions which were not occupied
by cells) (Fig. 2a). Empty regions indicate that ice nucleation and crystal growth damaged the
cell wall during the pre-treatment (freezing). This had influence in terms of semi-permeable
membrane loss, diffusion barrier, and/or modification of seeds texture (Aguilera et al. 2003;
Maity et al. 2009).
Fig. 2b shows that osmotic dehydration process change the tissue structure compared
to untreated sample. In fact cells appeared shrinking, distorted, and their contour appeared
irregular and wrinkling. This fact was probably due to the solubilisation of polysaccharides
(cellulose, hemicellulose and pectin) that composes the cells walls, the water loss and the pre-
concentration of sucrose on the surface of the tissue during the process (Raoult-Wack et al.
1991; Torreggiani & Bertolo 2001; Delgado and Rubiolo 2005; Garcia et al. 2010). Indeed,
pectin is the major constituents of the middle lamella and thus contributing to the cell
adhesion and firmness (Nunes et al. 2008). Moreover water loss induces the plasmolysis of
cells and solids gain gives consistency to the tissues. In fact, Nunes et al. (2008) showed that
diffusion of sucrose into the fruit tissue, during osmotic dehydration process, and its
interaction with the cell wall and middle lamella pectic polysaccharides might result in the
formation of a jam-like structure that gives consistency to the tissues. There are several
experimental findings in the literature that are consistent with our claims regarding the
occurrence of cells structure modification during osmotic processing (Delgado and Rubiolo
2005; Nunes et al. 2008).
127
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
c
a
b
Forc
e (N
)
c
b
a
Distance (mm)
Figure 2. Scanning electron microscopy photographs of frozen (a) and osmodehydrated seeds (b) and a characteristic force-distance curve for texture analysis using frozen (•) and osmodehydrated (‒) seeds (c)
128
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Textural properties of seeds (composed by pulp and pip) are closely linked to cellular
structure (Torreggiani & Bertolo 2001; Sajeev et al. 2004). Fig. 2c shows a characteristic
force-distance curve for texture analysis using frozen and osmodehydrated seeds. It shows
three zones characteristic of seed compression. The first (a) is attributed to the pulp resistance
(~13 N). The second (b) correspond to the fracture of pip (~30 N), while the third (c) zone is
ascribed to the crushing of seed, characterised by the increase in force through a short
distance. In fact, Al-Said et al. (2009) showed that pulp and pip hardness varied between 9-14
N and 24-45 N respectively, for pomegranate seeds cultivated in Oman. The peak force
attained during the test is referred to as hardness and area under the curve as toughness.
Osmotic dehydration process induced an increase on toughness (61.6±4.7 Nmm) and hardness
(62.8±3.5 N) compared to untreated sample (toughness: 54.5±3.9 Nmm and hardness:
46.7±2.4 N). ANOVA shows a significant difference (P<0.05) between treated and untreated
sample. This could be a consequence of the exchange (water loss and solids gain) between
seeds and osmotic solution. Many authors showed that the hardness and toughness increased
when the moisture content decreased since water act as plasticizer (Sajeev et al. 2004; Kingsly
et al. 2006; Al-Said et al. 2009). No significant differences (P<0.05) in textural properties
(Toughness and hardness) of osmodehydrated fruit using date juice added with sucrose or
only sucrose osmotic solution (55°Brix) were observed. Therefore sucrose was an important
factor influencing fruit texture. In fact, during osmotic process sucrose (major component
present in osmotic solution) passes through the cell wall and accumulates between the cell
wall and the cellular membrane, where it forms a hypertonic solution leading to water out flux
through the cellular membrane (Mascheroni & Spiazzi 1997). As consequence of this
exchange, the products will more or less lose weight and will shrink eventually. Hence the
probe penetrates more in the seed and touches the pip at a shorter distance compared to
untreated seed. Indeed, the peak of pip crushed was observed at a short distance (3.39±0.23
129
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
mm) compared to untreated seeds (4.34±0.51 mm). Thus seeds reduce their thickness between
an average close to ~0.95 mm, after osmotic dehydration process. Delgado and Rubiolo
(2005) and Falade et al. (2007) reported similar results with carrot and banana immersed in
sucrose solution.
3.3.3. Apparent viscosity of the osmotic solutions
An increased in the date juice viscosity (about 83 %) was observed, following the
addition of sucrose. This was due to the high molecular weight of sucrose (Masmoudi et al.
2007). During the osmotic process, the viscosity was reduced by half. This fact was due to the
migration of water from seeds to the osmotic solution. The apparent viscosity of all osmotic
solutions at the end of the process decreased when the shear rate increased until 10s-1,
corresponding to a non-Newtonian rheological behaviour (shear-thinning). After this rate,
viscosity trend to be constant until the end of the test (Fig. 3). Until a shear rate of 10 s-1, the
flow behaviour could be due to the presence of different molecules (carbohydrates, proteins,
sucrose) in osmotic solutions (Aylin & Medeni 2005). After 10 s-1 the flow behaviour was
essentially due to the presence of sucrose. Indeed Fig. 3 shows that sucrose solution (49.4
°Brix) had the same flow behaviour and similar viscosity then osmotic solutions (~49.4 °Brix)
after 10 s-1. At our point of view, the difference between osmotic solutions viscosity was due
to the variation of carbohydrate and proteins level in date juices.
130
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
0
0,01
0,02
0,03
0,04
0,05
0,06
0 0,4 0,8 2 4 10 30 50 70
Shear rate (1/s)
App
aren
t vis
cosi
ty (P
a/s)
Deglet Nour
Allig
Kintichi
Sucrose
Figure 3. Flow behaviour of the osmotic solutions after 120 min of the process
4. Conclusion
Date juice added with sucrose could be considered as a potential good immersion
solution used to the osmodehydration of pomegranate seeds. In fact, the use of date juice
presented two important advantages. It: (i) involves an increasing of an economic gain by
reducing the amount of sucrose added to the osmotic solution, and (ii) allows a high
nutritional value product with high natural sugar content. As consequence, date juice could be
used for the conservation of many fruits using osmotic dehydration process.
The rate of different osmotic dehydration parameters was directly related to time.
There was a leaching of natural solutes from seeds into the solution during the process, which
is quantitatively not negligible, and might have an important impact on the sensory properties
and nutritional value of seeds and date juices. During osmotic treatment not only the
composition of the tissue is changed but also the structure since modification of cells structure
was observed by SEM, which was confirmed by texture analysis.
131
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Osmotic dehydration found to be a good alternative to valorize pomegranate seeds and
date, suggesting the use of these products as ingredients in food formulations.
Acknowledgment
This research was supported by Gembloux Agro-Bio Tech, University of Liege (Belgium) and
National Engineering School of Sfax (Tunisia).
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Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
References
Aguilera, J. M., Chiralt, A., & Fito, P. (2003). Food and product structure. Trends in Food
Science and Technology, 14, 432-437.
Albagnac, P.G., Varoquaux, J., & Montigaud, C.I. (2002). Technologies de transformation
des fruits /Paris.-Londres.-New York, Ed. Tec & Doc, cop.-XXII, pp. 498.
Al-Farsi, M. (2003). Clarification of date juice. International Journal of Food Science and
Technology, 38, 241-245.
Allali, H., Marchal, L., & Vorobiev, E. (2008). Blanching of Strawberries by Ohmic Heating:
Effects on the Kinetics of Mass Transfer during Osmotic Dehydration. Food
Bioprocess Technology, DOI: 10.1007/s11947-008-0115-5, in press
Al-Maiman, S.A. & Ahmad, D. (2002). Changes in physical and chemical properties during
pomegranate (Punica granatum L.) fruit maturation. Food Chemistry, 76, 437-441.
Al-Said, F.A., Opara, L.U., & Al-Yahyai, R.A. (2009). Physico-chemical and textural quality
attributes of pomegranate cultivars Brown in the Sultanate of Oman. Journal of Food
Engineering, 90, 129-134.
AOAC. (1990). Official Methods of Analysis of the Association of Official Analytical
Chemists, 15th Ed., (K. Helrich, ed.), Association of Official Analytical Chemists, Inc.,
Arlington, Virginia, VA.
Attia, H., Bennasar, M., Lagaude, A., Hugodo, B., Rouviere, J., & Tarodo, B. (1993).
Ultrafiltration with microfiltration membrane of acid skimmed and fat-enriched milk
coagula: hydrodynamic, microscope and rheological approaches. Journal of Dairy
Research, 60, 161-174.
Aylin, A., & Medeni, M. (2005). Rheological Behavior of pomegranate (Punica granatum L.)
juice and concentrate. Journal of Texture Studies, 36, 68 - 77.
Azuara, E., Flores, E., & Beristain, C. (2009). Water Diffusion and Concentration Profiles
During Osmodehydration and Storage of Apple Tissue. Food and Bioprocess
Technology, 4, 361-367.
133
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Barminas, J.T., James, M.K., & Abubakar, U.M. (1999). Chemical composition of seeds and
oil of Xylopia aethiopica grown in Nigeria. Plant Foods for Human Nutrition, 53, 193-
198.
Barreveld, W.H. (1993). Date palm products, FAO agricultural services bulletin No. 101.
Bchir, B., Besbes, S., Attia, H., & Blecker C. (2009). Osmotic dehydration of pomegranate
seeds: Mass transfer kinetics and DSC characterization. International Journal of Food
Science and Technology, 44, 2208-2217.
Bchir, B., Besbes, S., Attia, H., & Blecker, C. (2010). Osmotic dehydration of pomegranate
seeds (Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process
Engineering. DOI: 10.1111/j.1745-4530.2010.00591.x, in press
Besbes, S., Hentati, B., Blecker, C., Deroanne, C., Lognay, G., Drira, N.E. & Attia H.
(2005a). Voies de valorisation des sous produits de dattes: Valorisation du noyau.
Hygiène Microbiologie Alimentaire, 49, 1-9.
Besbes, S., Cheikh Rouhou, S., Blecker, C., Deroanne, C., Lognay, G., Drira, N. E., & Attia
H. (2005b). Voies de valorisation des sous produits de dattes: Valorisation de la pulpe.
Microbiologie Hygiène Alimentaire, 18, 3-11.
Besbes, S., Drira, L., Blecker, C., Deroanne, C., & Attia, H. (2009). Adding value to hard date
(Phoenix dactylifera L.): compositional, Functional and sensory characteristics of date
jam. Journal of Food Chemistry, 112, 406-411.
Bouabidi, H., Reyens, M., Roussi, M. (1996). Critères de caractérisation des fruits de
quelques cultivars de palmier dattiers (Phoenix dactylifera L.) du sud tunisien. Annales
de L’INRAT, 69, 73-86.
Chenlo, F., Moreira, R., Herrero, F., & Vazquer, G. (2007). Osmotic dehydration of chestnut
with sucrose: Mass transfer processes and global Kinetics modelling. Journal of Food
Engineering, 78, 765-774.
Corrêa, J., Pereira, L., Vieira, G., & Hubinger, M. (2010). Mass transfer kinetics of pulsed
vacuum osmotic dehydration of guavas. Journal of Food Engineering, 96, 498-504.
134
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Delgado, A.E., & Rubiolo, A.C. (2005). Microstructural changes in strawberry after freezing
and thawing processes. Lebensm. Wiss. u. Technol, 38, 135-142.
Devic, E., Guyot, S., Daudin, J., & Bonazzi, C. (2010). Kinetics of Polyphenol Losses During
Soaking and Drying of Cider Apples. Food and Bioprocess Technology,
DOI: 10.1007/s11947-010-0361-1, in press.
Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC and DOC-
Lavoisier, Paris; France, pp. 181-182.
Fabiano, A.N., Oliveira, F., & Rodrigues, S. (2008). Use of Ultrasoud for Dehydration of
Papayas. Food and Bioprocess Technology, 1, 339-345.
Fadavi, A., Barzegar, M., Azizi, H., & Bayat, M. (2005). Physicochemical Composition of
Ten Pomegranate Cultivars (Punica granatum L.) Grown in Iran. Journal of Food
Science Technology, 11, 113 - 119.
Falade, K., Igbeka, J., & Ayanwuyi, F. (2007). Kinetics of mass transfer and colour changes
during osmotic dehydration of watermelon. Journal of Food Engineering, 80, 979-985.
Fathi, M., Mohebbi, B., & Razavi, S. (2009). Application of Image Analysis and Artificial
Neural Network to Predict Mass Transfer Kinetics and Color Changes of Osmotically
Dehydrated Kiwifruit. Food and Bioprocess Technology, DOI: 10.1007/s11947-009-
0222-y, in press.
Fernandes, F.A.N., Rodrigues, S., Gaspareto, O.C.P., & Oliveira, E.L. (2006). Optimisation of
osmotic dehydration of bananas followed by air-drying. Journal of Food Engineering,
77, 188-193.
Garcia, P., Mognetti, C., André-Bello, A., & Martinez-Monzo, J. (2010). Osmotic dehydration
of Aloe vera (Aloea barbadensis Miller). Journal of Food Engineering, 97, 154-160.
Grigelmo-Miguel, N., & Martin-Belloso, O. (1999). Characterization of dietary fiber from
orange juice extraction. Food Research International, 31,355-361.
135
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Kingsly, A.R.P., Singh, D.B., Manikantan, M.R., & Manikantan, R.K. (2006). Moisture
dependent physical properties of dried pomegranate seeds (Anardana). Journal of Food
Engineering, 75, 492-496.
Kowalska, H., & Lenart, A. (2001). Mass exchange during osmotic pre-treatment of
vegetables. Journal of Food Engineering, 49, 137-140.
Kowalska, H., Lenart, A., & Leszczyk, D. (2008). The effect of blanching and freezing on
osmotic dehydration of pumpkin. Journal of Food Engineering, 86, 30-38.
Mascheroni, R., & Spiazzi, E. (1997). Mass transfer model for osmotic dehydration of fruits
and vegetables-I. Development of the simulation model. Journal of Food Engineering,
34, 387-410.
Masmoudi, M., Besbes, S., Blecker, C., & Attia, H. (2007). Preparation and characterization
of osmotidehydrated Fruits from Lemon and Date By-products. Journal of Food
Science and Technology international, 13, 405-412.
Mavroudis, N. E., Gekas, V., & Sjohlm, I. (1998). Osmotic dehydration of apples, shrinkage
phenomena and the significance of initial structure on mass tranfer rates. Journal of
Food Engineering, 38, 101-123.
Maity, T., Raju, P., & Bawa, A. (2009). Effect of Freezing on Textural Kinetics in Snacks
During Frying. Food and Bioprocess Technology, DOI: 10.1007/s11947-009-0236-5,
in press.
Monsalve-Gonzalez, A., Barbosa-Canovas, G.V., Cavalieri, R.P., McEvily, A.J. & Iyengar R.
(1994). Inhibition of enzymatic browning in apple products by 4-hexylresorcinol. Food
Technology, 49, 110-118.
Mujumdar, A., & Law, C. (2010). Drying Technology: Trends and Applications in
Postharvest Processing. Food and Bioprocess Technology, DOI: 10.1007/s11947-010-
0353-1, in press.
Munier P. (1973). Le palmier dattier. Paris : Maison neuve et Larose, pp. 141-221.
136
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
Nisha, P., Singhal, R., & Pandit, A. (2010). Kinetic Modelling of Colour Degradation in
Tomato Puree (Lycopersicon esculentum L.). Food and Bioprocess Technology,
DOI: 10.1007/s11947-009-0300-1, in press.
Nunes, C., Santos, C., Pinto, G., Lopes-da-silva, J.A., Saraiva, J.A., & Coimbra, M.A. (2008).
Effect of candying on microstructure and texture of plums (Prunus domestica L.).
LWT- Food Science and Technology, 41, 1776-1783.
Poyrazoglu, E., Gokmen, V., & Artik, N. (2002). Organic Acids Phenolic Compounds in
pomegranates (Punica granatum L.) Grow in Turkey. Journal of Food Composition
and Analysis, 15, 567- 575.
Raoult-Wack, A. L., Guilbert, S., Le Maguer, M., & Rios, G. (1991). Simultaneous water and
solute transport in shrinking media-part 1: application to dewatering and impregnation
soaking process analysis (osmotic dehydration). Drying Technnology, 9, 589-612.
Rastogi, N.K., Raghavarao, K.S.M.S., Niranjan, K., & Knorr, D. (2002). Recent
developments in osmotic dehydration methods to enhance mass transfer. Trends in
Food Science and Technology, 13, 48-59.
Ruiz-Lopez, I., Castillo-Zamudio, R., Salgado-Cervantes, M.A., Rodriguez-Jimenes, G.C., &
Garcia-Alvarado, M.A. (2010). Mass Transfer Modelling During Osmotic Dehydration
of Hexahedral Pineapple Silices in Limited Volume Solutions. Food and Bioprocess
Technology, 3, 427-433.
Sajeev, M.S., Manikantan, M.R., Kingsly, A.R.P., Moorthy, S.N., & Sreekumar, J. (2004).
Texture Analysis of Taro (Colocasia esculenta L. Schott) Cormels during Storage and
Cooking, Journal of Food Science, 69, 315-321.
Saurel, R., Raoult-Wack, A.L., Rios, G., & Guilbert, S. (1994). Mass transfer phenomena
during osmotic dehydration of apple II. Frozen plant tissue. International Journal of
Food Science and Technology, 29, 543-550.
Shi, J., Pan, Z., McHugh, T., & Hirschberg, E. (2009). Effect of Infusion Method and
Parameters on Solids Gain in Blueberries. Food and Bioprocess Technology, 3, 271-
278.
137
Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________
138
Torreggiani, D., & Bertolo, G. (2001). Osmotic pre-treatments in fruit processing: chemical,
physical and structural effects. Journal of Food Engineering, 49, 247-253.
Uribe, E., Miranda, M., Vega-Galvez, A., Quispe, I., Claveria, R., & Di Scala, K. (2010).
Mass Transfer Modelling During Osmotic Dehydration of Jumbo Squid (Dosidicus
gigas): Influence of Temperature on Diffusion Coefficients and Kinetic Parameters.
Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0336-2, in press.
Yousif, A.K. & Al-Ghamdi, A S. (1999). Suitability of some date cultivars for jelly making.
Journal of Food Science and Technology, 36, 515-518.
Yousif, A.K., Abou Ali, M. & Abou-Idreese, A. (1990). Processing evaluation and storability
of date jelly. Journal of Food Science and Technology, 27, 264-267.
Yunfeng, L., Changjiang, G., Jijun, Y., Jingyu, W., Jing, X., & Shuang, C. (2006). Evaluation
of antioxidant properties of pomegranate peel extract incomparison with pomegranate
pulp extract. Journal of Food Chemistry, 96, 254–260.
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
Chapitre 5:
Effet des conditions de séchage sur les propriétés physico-chimiques des graines de grenade déshydratées
osmotiquement
Ce travail a fait l’objet de la publication suivante :
Bchir, B., Besbes, S., Karoui, R., Attia, H., Paquot, M., & Blecker, C. (2010).
Effect of air-drying conditions on physico-chemical properties of osmotically
pre-treated pomegranate seeds, Food and Bioprocess Technology, DOI:
10.1007/s11947-010-0469-3.
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Résumé *
Titre : Effet des conditions de séchage sur les propriétés physico-chimiques des graines de
grenade déshydratées osmotiquement
Objectif et stratégie expérimentale :
Nous avons précédemment étudié la déshydratation osmotique des graines de grenade
et l’analyse de l’effet de paramètres tels que la température (30, 40, 50°C), la solution
osmotique (saccharose, glucose, saccharose/glucose, jus de datte) et l’état du fruit (congelé et
frais) sur la cinétique de déshydratation ainsi que sur les caractéristiques physico-chimiques
des graines. D’après les résultats obtenus, on peut conclure que pour une meilleure
déshydratation des graines de grenade, les conditions suivantes sont les plus intéressantes: un
traitement de 20 min, une température de 50°C et une solution de saccharose (55°Brix). Les
graines issues de la DO présentent une activité d’eau proche de 0,9 ce qui permet la
croissance des micro-organismes et d’autres réactions indésirables (oxydation des lipides,
dégradation des vitamines…) au cours de l’entreposage. Ainsi dans un but plus appliqué, un
traitement supplémentaire de séchage par entrainement (2 m/s durant 4 heures) a été mis en
place afin de réduire l’activité d’eau à une valeur inférieure à 0,65. Dans ce cadre, nous nous
sommes intéressé à l’étude de l’effet des conditions du post-traitement de séchage sur les
caractéristiques physico-chimiques des graines de grenade déshydratées osmotiquement.
Avant le procédé de séchage, les graines de grenade ont subi une DO pendant 20
minutes à 50°C en utilisant une solution de saccharose (55°Brix). Ensuite un traitement de
séchage par entrainement (2 m/s durant 4 heures) a été mis en place à différentes températures
(40, 50, et 60°C). En premier lieu nous avons étudié l’effet de la température de séchage du
traitement sur l’évolution de la matière sèche (MS), l’activité d’eau (aw) et le pourcentage de
séchage (DR). Afin de déterminer l’évolution des différentes fractions d’eau dans la graine en
fonction de la température, une méthode plus fine a été adoptée qui consiste à analyser les
propriétés thermiques de la pulpe par calorimétrie différentielle. Le séchage par air chaud est
de moins en moins performant au regard des exigences croissantes en matière de qualité des
produits finis. Ainsi, nous avons étudié l’effet de la température (40, 50, 60°C) de séchage sur
plusieurs paramètres de qualité des graines de grenade tels que : activité antioxydante,
composés phénoliques, anthocyanines, couleur et texture.
Les différentes étapes du procédé sont reprises de façon synoptique dans figure 1’. _________________________________________________________________________
* Ce résumé permet de présenter de façon synthétique, en français, l’axe de recherche de l’article qui a été publié en anglais.
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Graines congelées :
(-50°C)
Conditions
Traitement : déshydratation osmotique
Rapport : graine/solution: - 1/4
Solution (55°Brix): - Saccharose
Temps (min): - 20
Température (°C): - 50
Conditions
Post- traitement : séchage par entrainement
Température (°C): - 40 - 50 - 60
Temps (min): - 0 - 120 - 30 - 180 - 60 - 240
Vitesse de l’air (m/s): - 2
Paramètres
* MS, aw, DR, Deff , Tg’, ∆H * Activité antioxydante, composés phénoliques, anthocyanines * Couleur (L*, C*, h°), texture (hardness, toughness) * Activité de la polyphénol oxydase (PPO), teneur en hydroxy-méthyl-furfural (HMF).
Figure 1’ : Différentes étapes des procédés de déshydratation osmotique et de séchage des
graines de grenade.
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Principaux résultats :
Le procédé de séchage mis en œuvre permet d’atteindre l’activité de l’eau cible de
0,65. Le temps nécessaire pour atteindre cette valeur diminue avec la température (240, 120,
et 60 min pour respestivement des températures de 40, 50, et 60°C), en relation avec
l’accroissement du coefficient de diffusion de l’eau et du taux de séchage.
D’autre part, la déshydratation osmotique et le séchage exercent une influence
significative sur la qualité nutritionelle des graines. En effet, le procédé de DO entraîne une
réduction de l'activité de scavenging du radical diphenylpicryl-hydrazyl (DPPH). Cette
réduction est suivie par une diminution des teneurs en composés phénoliques, en
anthocyanines. Ce phénomène est accentué par des températures de séchage de plus en plus
élevées. Cependant, ces valeurs restent comparables (voires supérieures) à celles observées
chez d’autres fruits n’ayant subi aucun traitement.
Les paramètres chromatiques (L*, C* et h°), ainsi que l’indice de brunissement, sont
affectés par le procédé de séchage qui conduit à une variation de la couleur des graines de
grenade. La faible activité PPO et la teneur réduite en HMF indiquent que les réactions
enzymatiques et non-enzymatiques n'ont pas une influence majeure sur le brunissement de la
graine. Cela confirme les faibles valeurs de l’indice de brunissement. Outre la couleur, la
combinaison entre la DO et le séchage a influencé la forme et la texture, puisque les graines
ont perdu jusqu'à 55% de leurs épaisseurs après séchage à 60°C.
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Effect of air-drying conditions on physico-chemical properties of
osmotically pre-treated pomegranate seeds
*Brahim Bchira, Souhail Besbesb, Romdhane Karouia, Hamadi Attiab, Michel Paquotc,
Christophe Bleckera
a Gembloux Agro-Bio Tech, University of Liege, Passage des Déportés, 2, B- 5030
Gembloux, Belgium
b Laboratory of Food Analyses, National Engineering School of Sfax, Route de Soukra, 3038
Sfax, Tunisia
c Department of Industrial Biological Chemistry, Gembloux Agro-Bio Tech, University of
Liege, Passage des Déportés, 2, 5030 Gembloux, Belgium
*Corresponding authors Tel: +32(0)81/62.23.03
*Fax: +32(0)81/60.17.67
*E-mail address: [email protected]
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Abstract
The drying of pomegranate seeds was investigated at 40, 50, and 60 °C with air
velocity of 2 m/s. Prior to drying, seeds were osmodehydrated in 55 °Brix sucrose solution for
20 min at 50 °C. The drying kinetics and the effects of osmotic dehydration (OD) and air-
drying temperature on antioxidant capacity, total phenolics, colour, and texture were
determined. Analysis of variance (ANOVA) revealed that OD and air-drying temperature
have a significant influence on the quality of seeds. Both anthocyanin and total phenolic
contents decreased when air-drying temperature increased. The radical diphenylpicril-
hydrazyl activity showed the lowest antioxidant activity at 60 °C. Both chromatic parameters
(L*, C* and h°) and browning index were affected by drying temperatures, which contributed
to the discolouring of seeds. The final product has 22, 20, 16 % of moisture; 0.630, 0.478,
0.414 of aw; 151, 141, 134 mg gallic acid equivalent/100g Fresh Matter (FM) of total
phenolics; 40, 24, 20 mg/100g FM of anthocyanins; and 46, 39, 31 % of antioxidant activity,
for drying temperatures of 40, 50 and 60 °C respectively. In view of these results, the
temperature of 40 °C is recommended as it has the lowest impact on the quality parameters of
the seeds. Differential scanning calorimetry data provided complementary information on the
mobility changes of water during drying. Glass transition temperature (Tg’) depends on
moisture content and as consequence on drying conditions. In fact, Tg’ of seeds dried at 60 °C
(Tg’= -21 °C) was higher than those dried at 50 °C (Tg’= -28 °C) or 40 °C (Tg’= -31 °C) and
osmodehydrated seeds (Tg’= -34 °C). During OD and drying process, the texture of seeds
changed. The thickness of seeds shrank by 55 % at 60 °C.
Keywords: pomegranate; osmotic dehydration; drying; antioxidant activity;
differential scanning calorimetry; texture.
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Introduction
Pomegranate (Punica granatum L.) presents a virtual explosion of interest as a
medicinal and nutritional product. Recently, more than 475 new products containing
pomegranate (food and drinks) were created in United State, including a chewing gum named
“pomegranate power”, a chicken sauce containing pomegranate, etc. (Storey 2007).
The edible part of the fruit (seeds) contains a considerable amount of sugars, vitamins,
polysaccharides, minerals and polyphenols (Espiard 2002). Due to their polyphenol
compounds (e.g., anthocyanins), condensed tannins (e.g., proanthocyanidins) and
hydrolysable tannins (e.g., ellagitannins and gallotannins)) pomegranate seeds possess anti-
oxidative properties (Hernandez et al. 1999; Jaiswal et al. 2010). In fact, these compounds are
able to reduce the formation of free radicals compounds that cause oxidation reactions
associated with biological complications such as aging, cardiovascular disease, and
carcinogenesis (Rosenblat and Hayek 2006).
Despite all these advantages, the consumption of pomegranate seeds is limited to the
crop season due to problems of preservation (Defilippi et al. 2006). Indeed, the major cause
limiting the potential of pomegranates is the development of decay, which is often caused by
the presence of fungal inoculum in the blossom end of the fruit. During long term storage,
rinds scalds symptoms appear as a superficial browning (Defilippi et al. 2006).
Preservation methods can be used to increase the shelf-life of fruits, among them there
are drying, pasteurization, osmotic dehydration etc. (Raoult-Wack et al. 1991). Freezing is
also a preservation method; however, this treatment involves modifications of the texture and
cell structures (Bchir et al. 2010a; Bchir In press). As consequence, frozen fruit cannot be
consumed directly after thawing. Nevertheless, freezing could be an excellent pre-treatment
for osmotic dehydration of fruit and vegetable, improving significantly mass transfer during
osmotic process. Our previous investigations showed that freezing before osmotic dehydration
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provided 1.4 and 3.5 times more water loss and solids gain, respectively, than an untreated
pomegranate seeds (Bchir et al. 2010a). Osmotic process has received considerable attention
as a pre-drying treatment to reduce energy consumption and improve food quality (El-Aouar
et al. 2003; Ruiz-Lopez et al. 2010). According to Pokharkar et al. (1997) and Uribe et al.
(2010) the main advantages of the osmotic dehydration process are: retention of natural colour
without addition of sulphites and high retention of volatile compounds during subsequent
drying.
After osmotic process, water activity of sample was found to be higher than 0.900,
allowing development of microorganisms (e.g. bacteria, fungi), and some undesirable
reactions such as; enzymatic and non-enzymatic browning reactions, fat oxidation, vitamin
degradation, and protein denaturation during storage (Bchir et al. 2009; Bchir et al. 2010b).
As a consequence, a complementary treatment such as drying may enable better conservation
of pomegranate seeds.
Drying is the most commonly used method for food dehydration since it is the most
rapid process; it inhibits enzymatic degradation, limits microbial growth and produces a
uniform dried product (Harbourne et al. 2009; Uribe et al. 2009). In this context, various
fruits and vegetables such as onions (Singh and Sodhi 2000), red pepper (Doymaz 2007),
garlic cloves (Sharma et al. 2003), ear corn (Friant et al. 2004), apricots (Doymaz 2007), and
mulberry (Doymaz 2007) have been dried, despite several negative reactions such as
shrinkage, loss of colour, texture and nutritional-functional properties (Arabhosseini et al.
2009; Miranda et al. 2009).
The aim of the present study was to: (i) investigate the kinetics and influence of air-
drying temperature on mass transfer; and (ii) determine the impact of drying temperatures on
antioxidant activity, phenolic, anthocyanin content, colour development and texture of
pomegranate seeds.
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Materials and methods
Materials
Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were grown and
harvested in Gabes during autumn (2009). Tunisia pomegranate fruits were collected at full
ripeness stage (weight: ~ 500 g; skin colour: red; juice colour: pink; juice pH: ~ 4.4; °Brix: 15
g/100g; skin thickness: ~ 4 mm). Pomegranate is composed of a non edible part composed of
30% skin (external part), 13 % internal lamel, and an edible part formed of 50 – 70 % seeds.
The seeds are composed of about 15 % pips (woody part), which determines hardness, and 85
% pulp which contains juice (Espiard 2002). The investigated seeds presented the following
characteristics; shape: ellipsoids; length: 13±1 mm; breadth: 7±1 mm; pip thickness: 2±0.2
mm; average weight of an individual seed: 0.504±0.04 g; bulk density: 628±2 kg/m3.
Twenty kilograms of pomegranate were frozen at -50 °C for one month. Before
osmotic dehydration process, pomegranates were thawed at room temperature for one hour. A
digital thermometer BT20 (Bresso, Italy) was placed in the pomegranate core to measure the
temperature elevation during one hour of thawing at room temperature. Temperature of
pomegranate core reached -7.5 °C, after thawing. Seeds were immediately separated manually
prior to the osmotic dehydration process.
Osmotic dehydration treatment
About 100 g of frozen seeds were osmodehydrated in sucrose solution (55°Brix) for
20 min at 50 °C using a shaking water bath (GFL instrument D 3006, Germany; oscillation
rate 160 rpm). The time and temperature combination was selected on the basis of our
previous findings which showed that osmotic dehydration of pomegranate seeds for 20 min
using sucrose solution at 50 °C gives higher mass transfer rate (Bchir et al. 2009; Bchir et al.
2010a). Sucrose solution was heated at 50 °C before adding the seeds to the bottles (Schott,
Saint- Gallen, Switzerland) of 500 mL. The volume ratio between the seeds and the sugar
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solution was kept at one part of seeds and four parts of solution (1:4) (Bchir et al. 2009). After
osmotic dehydration process, seeds were removed from the solution, quickly rinsed (with
distilled water, 20min) and the excess of solution at the surface was removed with absorbent
paper.
Air-drying experiment
Approximately 200 g of osmodehydrated seeds were uniformly spread on perforated
stainless trays and dried at three temperatures 40, 50 and 60 °C for 240 min. These
temperatures have been selected according to those mostly used for fruit and vegetable in the
literature (Kingsly and Singh 2007; Erbay and Icier 2009).
Dried seeds were taken out from dryer at different time intervals (30, 60, 120, 180 and
240 min). Drying experiments were carried out with a laboratory scale drier by operating it at
a air velocity of 2.0 ± 0.1 m/s. The drying cabinet (Memmert tcp 800, Schutzart, Germany)
was equipped with an electrical heater, a fan, and temperature indicators.
All analytical determinations were performed in triplicate. Values were expressed as the
mean± standard deviation.
Physico-chemical analysis
Dry matter, moisture contents and water activity
The dry matter (DM) was calculated according to AOAC method 934.01 (1990). For
the different time intervals, approximately 5 g of seeds were oven dried at 103 °C ± 2 °C until
constant weight. Moisture content was estimated by difference of mean values, 100 % - % of
DM (Chenlo et al. 2007). Water activity (aw) was measured using an aqualab (Switzerland)
instrument at 20 °C.
Surface colour measurement
The CIELAB coordinates (L*, a*, b*) were directly read with a
spectrophotocolorimeter Mini Scan XE (Germany) with a lamp (D 65). In this coordinate
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system, L* value is a measure of lightness, ranging from 0 (black) to +100 (white), a* value
ranges from -100 (greenness) to +100 (redness) and b* value ranges from -100 (blueness) to
+100 (yellowness). The total colour difference (∆E*) was determined by using the following
equation (Romano et al. 2008):
Where L*, a*, b* and L0*, a0
*, b0* are the current and the initial CIELAB coordinates,
respectively.
The Hue angle (h*ab) and chroma or intensity (C*) were calculated according to the following
equations:
(2) (3) C* = (a*2+b*2)1/2 h* = arctan (b*/a*)
(1) ( ) ( ) ( )2*0
*2*0
*2*0
** bbaaLLE −+−+−=Δ
For Hue colour index, 0° or 360° represents red, and 90°, 180° and 270° represent
yellow, green and blue, respectively.
Browning index
The methodology applied for determination of browning index was that proposed by
Vega-Galvez et al. (2009). Pomegranate seeds were placed in distilled water at 40 °C for 6 h,
using a solid to liquid ratio of 1:50. Then, water was first clarified by centrifugation
(Beckman coulter J-E, Indianapolis, USA) at 3200×g for 10 min. The supernatant was diluted
with an equal volume of ethanol at 95% and centrifuged again at 3200×g for 10 min. The
browning index (absorbance at 420 nm) of the clear extracts was determined in quartz
cuvettes using a spectrophotometer (Shimadzu UV 240, Cambridge, USA).
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Polyphenol oxidase extraction and activity measurement
A portion of pulp (10 g) was dipped in a McIlvaine buffer solution (1:1) at pH = 6.5.
The buffer contained NaCl 1 M and 5% polyvinylpolypyrrolidone. The homogenate was
centrifuged (8,000 rpm, 30 min) at 4 °C. The solid residue was discarded and the supernatant
was filtered through a Whatman # 1 paper. The resulting liquid constituted the crude enzyme
extract.
Polyphenol oxidase activity was determined by placing 3 ml of 0.05 M cathechol and
75 μl enzyme extract in a 1 cm path cuvette. Assays were carried out at 410 nm using a
shimadzu UV 240 spectrophotometer (Cambridge, USA) (Robert et al., 2002). A change in
absorbance at 410 nm per minute and millilitre of enzymatic extract correspond to one unit of
PPO activity. The initial rate of the reaction was computed from the linear portion of the
plotted curve. Results were expressed as relative activity (RA, %) calculated by Eq. (4)
0
100AARA = (4)
Where A and A0 are the current and the initial PPO activity, respectively (Robert et al., 2002).
Hydroxylmethylfurfural analysis
The analysis of hydroxymethylfurfural (HMF) was carried out by High Pressure
Liquid Chromatography (HPLC). Approximately, 1 g of pulp was placed in 25 ml flask; 2 ml
each of Carrez I and II reagents were added with stirring for 30 min and the volume made up
with Milli-Q water. After standing for 30 min, the supernatant was filtered through a filter of
0.45 μm and then injected in to the chromatograph (Rada-Mendoza et al. 2002).
HPLC determination was carried out, following the method of Vinas et al. (1992),
using a ZorBax 300SB-C18 column (4.6×150 mm; waters) at 30 °C. Mobil phase was a
mixture of methanol/water (10/90, v/v) with isocratic elution with 1mL min-1 flow rate and 20
μl injection volume. The UV detector PDA was set at 280 nm. Quantification was carried out
by the external standard method, using a commercial standard of HMF (Sigma, New Jersey,
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USA). A standard curve was obtained by using HMF standard dissolved in distilled water at
various concentrations (ranging from 0 to 104 μg/ml).
Antioxidant activity
Antioxidant activity was determined using pomegranate seeds extract solution.
Approximately 5 g of pomegranate seeds were crushed and mixed with 15 ml methanol -
water solution (4:1, v/v) at room temperature (20 °C) for 5 h under stirring. The extracts were
then filtered and centrifuged (Beckman coulter J-E, Indianapolis, USA) at 4,000g for 10 min
and the supernatant was concentrated under reduced pressure at 40 °C for 1 h using a rotary
evaporator (Buchi B-461 water Batch, Switzerland) to obtain the crude extract. The crude
extract was kept in dark glass bottles inside the freezer until use (Biglari et al. 2008).
Antioxidant activity of pomegranate seeds was determined using the 2,2,-diphenyl-2-
picryl-hydrazyl (DPPH) method (Vega-Galvez et al. 2009). Two (2) ml of DPPH radical
(0.15 mM in ethanol) was added to a test tube with 1 ml of the crude extract. The reaction
mixture was vortex-mixed for 30 s and left to stand at room temperature in the dark for 20
min. The absorbance was measured at 517 nm using a spectrophotometer (Shimadzu UV 240,
Cambridge, USA). The spectrophotometer was equilibrated with 80% (v/v) ethanol. Control
sample was prepared without adding extract. Total antioxidant activity (TAA) was expressed
as the percentage inhibition of the DPPH radical and was determined by the following
equation:
1001[%] ×⎟⎟⎠
⎞⎜⎜⎝
⎛−=
control
sample
AbsAbs
TAA (5)
Where TAA is the total antioxidant activity and Abs is the absorbance.
Phenolic content
Total phenolic were determined using Folin-Ciocalteau reagents. Crude extract (40 μl)
or gallic acid standard were mixed with 1.8 ml Folin-Ciocalteu reagent (predilued 10-fold
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with distilled water) and allowed to stand at room temperature for 5 min, and then 1.2 ml of
sodium bicarbonate (7.5%) was added to the mixture. After standing for 60 min in darkness at
room temperature, absorbance was measured at 765 nm.
A standard curve was obtained by using gallic acid standard solution at various
concentrations (ranging from 0 to 2 mg/100g). The absorbance of the reaction samples was
compared to that of the gallic acid standard and the results were expressed as mg gallic acid
equivalents (GAE)/100 g sample (Biglari et al. 2008).
Anthocyanin content
Anthocyanin content was determined using the pH-differential method described by
Kirca et al. (2007). Aliquot (1 g) of crush pulp was mixed with 80 ml of distilled water. The
mixture was sonicated (15 min) and centrifuges (1500×g for 10 min) and the upper phase was
used for assay. Two samples of 1 ml were taken from the upper phase and each one was
placed in 25 ml flask. The first flask was diluted with buffer solution pH 1 (1.49 g KCl/100
ml and 0.2N HCl) and the second one with buffer solution pH 4.5 (1.64 g sodium acetate /100
ml). After standing for 30 min at room temperature, absorbance was measured at 510 and 700
nm, using spectrophotometer (Shimadzu UV 240, Cambridge, USA). Pigment content was
calculated, based on cyanidin-3-glucoside using the following equation (Kirca et al. 2007):
(6)
100××××GVDM
eLAbs
wAnthocyanin (cyanidin-3-glucoside equivalents, mg/100 g) =
Where Abs (absorbance) = (Abs510nm-Abs700nm) pH1 - (Abs510nm-Abs700nm) pH4,5;
Mw (molecular weight) = 449.2 g/mol, for cyanidin-3-glucoside ; D = dilution factor ; L = path
length in cm; e = 26 900 molar extinction coefficient of cyanidin-3-glucoside [L × mol-1 ×
cm-1]; V = final volume [ml]; G = sample weight [mg].
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Texture analysis
Texture analysis were carried out using a texture profile analyzer (TA.XT2; Stable
Micro Systems, UK), with 75 mm compression probe as described by Bchir et al. (2010a).
The operating conditions of the instrument were as follows: 1.5 mm/s pre-test speed, 0.5
mm/s test speed, 10.0 mm/s post-test speed, 0.10 N trigger force and 85% sample
deformation. The hardness and toughness of seeds were the means of 20 single seed
measurements. Hardness [N] of seed was taken as the force in compression which
corresponded to the breakage of samples, while the toughness [N mm] is the energy required
to crush the sample completely.
Differential scanning calorimetry
Differential scanning calorimetry (DSC) was performed on the pulp previously
separated from pip. A 2920 TA Instruments (New Castle, Delaware, USA) with a
Refrigerated Cooling Assessory and modulated capability was used. The cell was purged with
70 ml min−1 of dry nitrogen and calibrated for baseline using an empty oven and for
temperature and enthalpy using two standards (indium, Tonset: 156.6 °C, ΔH: 28.7 J g−1;
eicosane, Tonset: 36.8 °C, ΔH: 247.4 J g−1). Specific heat capacity (Cp) was calibrated using a
sapphire. The empty sample and reference pans were of equal mass to within ±0.10 mg. DSC
curves were recorded during heating from -50 to 40 °C at a scan rate of 5 °C/min. All these
DSC experiments were made using hermetic aluminum pans. The analyzed sample mass was
about 3.50 ±0.25 mg.
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Drying rate and effective diffusion coefficients
The drying rate (DR) was calculated using the equation (Shi et al. 2008);
tMM
DR t−= 0 (7)
Where DR is overall drying rate [g water/g dry matter min-1]; M0 is moisture content of seeds
at time 0 [g water/g dry matter]; and Mt is moisture content of seeds at time t [g water/g dry
solid].
Diffusion coefficients (Deff) were calculated using Fick’s second law equation applied
to a sphere, by modifying the Fourier number using shape factor, due to an
ellipsoids shape of pomegranate seeds as has been reported in our previous investigation
(Bchir et al. 2009).
2/0 RtDFffe=
Statistical Analyses
Statistical analyses were carried out using a statistical software program (SPSS for
windows version 11.0). The data sets were subjected to analysis of variance using the general
linear model option (Duncan test) in order to investigate differences between samples
(P<0.05).
Results and discussion
Chemical composition of pomegranate seeds and osmodehydrated seeds (Table 1)
showed a predominance of carbohydrate in pomegranate seeds (84.93±0.25 g/100 g DM) and
a high amount of protein (7.79±0.86 g/100 DM), in agreement with previous findings of
Espiard (2002) and Fadavi et al. (2005). Pomegranate seeds were found to contain low levels
of ash (2.87±0.19 g/100 g DM) and lipid (4.55±0.40g/100 g DM). The DM and water activity
were about 16 % and 0.989, respectively.
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After osmotic dehydration process, carbohydrate and total soluble solids in
osmodehydrated seeds increased by 10 % and 62 %, respectively. This increasing is due to the
diffusion of sucrose from osmotic solution (high sucrose content) to the seeds. On the
contrary protein and ash contents, decreased from 7.8 and 2.9 to 0.5 and 1.0 g/100 g DM,
respectively. The amount of lipid was found to vary slightly (i.e. 4.5 to 4.0% DM).
DM of osmodehydrated seeds increased by 27 % and water activity slightly decreased
to 0.954 ±0.002. As consequence complementary treatments such as drying would be required
to reduce water activity to 0.650; to control bacteria, fungi and yeast development (Fabiano et
al. 2008; Pinho et al. 2009; Miranda et al. 2009).
Table 1: Chemical characteristic of pomegranate seeds
Untreated seeds Osmodehydrated seeds
Dry matter [%] 16.00 ± 0.05 42.75±0.33 Protein [g/100g DM] 7.79 ± 0.86 0.51±0.02 Lipid [g/100g DM] 4.55 ± 0.40 4.03±0.81 Ash [g/100g DM] 2.87 ± 0.19 1.04±0.04 Carbohydrate [g/100g DM] 84.93 ± 0.25 94.41±0.97 °Brix 15.50 ± 0.09 41.50±0.50 aw 0.989 ± 0.002 0.954±0.002 DM: Dry matter
Drying kinetics
The effect of drying time on dry matter (DM) water activity (aw), and drying rate (DR)
was studied in osmodehydrated seeds at different temperatures (40, 50 and 60 °C). From Fig.
1, DM increased from 42 % to 78, 80 and 84 % after 240 min of the process time, for drying
temperatures of 40, 50 and 60 °C respectively. Moisture content decreased by 26 % and 64 %
after osmotic dehydration and drying compared to untreated seeds, respectively. Water
activity decreased from 0.954 to 0.700, 0.565, and 0.430 in 180 min for drying temperatures
of 40, 50 and 60 °C respectively. After 180 min a slight decrease was observed (40 °C: 0.630;
50°C: 0.478; 60°C: 0.414). Under the same condition, DR decreased (from 2.21 × 10-2 ;
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2.00× 10-2 ; 1.20 × 10-2 to 0.50 × 10-2 ; 0.35 × 10-2 ; 0.30 × 10-2 g water/g dry matter min-1,
for drying temperatures of 60, 50 and 40 °C, respectively), during the first 180 min, tending to
be stable at the end of the process. Statistical analysis (ANOVA) did not show a significant
difference (P>0.05) between 180 and 240 min for all the investigated parameters. Similar
findings have been previously reported in many works (Kingsly and Singh 2007; Falade and
Onyeoziri 2010; Fathi et al. 2010).
The drying kinetics of seeds could be subdivided in two phases. The first period (until
180 min) corresponds to an easy diffusion of water from the inside to the surface of seeds and
the evaporation of free water on the seeds surface during drying; the second one (from 180 to
240 min) corresponds to a difficult diffusion of water. This could be due to the modifications
in seed surface during the drying. In fact, many authors showed that after a few hours of
drying, the product becomes denser, tougher and often leathery in nature with a case hardened
surface which does not facilitate moisture diffusion (Doymaz 2007; Carlos and De-Michelis
2009). This behaviour could be favoured by the pre-treatment (osmotic dehydration). Indeed,
Mandala et al. (2005) showed that sugar surface impregnation during osmosis favours sugar
crystallization, in some parts of the outer layers of apple tissue, forming a barrier to the
movement of water during drying.
From the results showed in Fig. 1, it can be concluded that increasing temperature of
drying from 40 to 60 °C resulted in quicker removal of water and shorter drying times to
reach aw of 0.650. In fact, using a temperature less than 60 °C resulted in a higher water
activity and a lower drying rate. The increase of temperature at 50 °C induced the same
evolution of aw and DM as with 40 °C. At 60 °C significant difference was observed after 60
min and 30 min for aw and DM, respectively. Moreover, using a drying temperature of 60 °C
caused a reduction in the drying time by four times, in order to reach a water activity (aw) of
0.650 as compared to that at 40 °C. These findings are in agreement with previous studies
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reported for various dried fruits and vegetables (Miranda et al. 2009; Gokhale and Lele 2010).
Park et al. (2002) and Shi et al. (2008) found that the increase of air-drying temperature (from
40 to 80 °C) induced an increase of heat energy which speeded up the movement of water
molecules and resulted in higher moisture diffusivity.
The calculated values of effective diffusivity (Deff) at different temperatures are presented
in table 2. It can be seen that the values of Deff greatly increased with the increasing air-drying
temperature. Effective diffusivity values for dried pomegranate seeds are similar to those
estimated by different authors for other vegetables (Madamba et al. 1996; Ahrné et al. 2003;
Doymaz, 2007). Table 2 showed that effective diffusivity values and experimental data of
Peleg’s equation parameters (K1 and K2) presented a good fit, showing an average correlation
coefficients (R2) close to 0.99.
Table 2: Effective diffusivities calculated by Fick’s model and values of Peleg’s equation
parameters (K1 and K2).
Drying temperature
Deff [m2 s-1] R2 [%] K1 K2 R2 [%]
40 °C 2.85 × 10-10 97.57 5.94 × 103 2.00 99.54 50 °C 3.74 × 10-10 99.67 9.43 × 103 1.86 99.31 60 °C 4.49 × 10-10 98.92 17.82 × 103 1.48 99.78
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Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
(a)
0
0,2
0,4
0,6
0,8
1
1,2
0 50 100 150 200 250
Time [min]
Wat
er a
ctiv
ity
60 °C 50 °C 40 °C
1.2
0.8
1
0.6
0.4
0.2
(b)
0
10
20
30
40
50
60
70
80
90
0 50 100 150 200 250Time [min]
Dry
mat
ter
[%]
60 °C 50 °C 40 °C
158
Figure 1: Variation of water activity (a) and dry matter (b) of pomegranate seeds as a
function with time and temperature (40, 50, 60 °C).
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
The investigation of the effect of air-drying temperature on the mobility changes of
water in dried seeds by differential scanning calorimetry (DSC) confirmed the previous
results regarding aw and Deff. From the results obtained, it was possible to determine a
significant decrease in water mobility after OD and drying process. Indeed, DSC results
showed that after 20 min of OD, the % of frozen water (free water) decreased from 70 % to
28 % (determined by dividing the enthalpy of fusion of sample by the enthalpy of fusion of
pure water). After 240 min of drying, free water in seeds was eliminated. In fact, Fig.2
showed a disappearance of the endothermic peak after 240 min of drying at different air-
drying temperatures, compared to osmodehydrated seeds. This is due to the loss of total free
water fraction in seeds. In fact, endothermic peak could be attributed to the melting point of
crystallized water. During the cooling only free water was crystallized to give ice while
during heating, frozen water undergoes a fusion of ice.
On other hand, DSC thermograms (Fig. 2) showed a considerable increase in glass
transition temperature (Tg’) as the air-drying temperature increased. In fact, Tg’ of seeds
dried at 60 °C (Tg’= -21 °C) was higher than those dried at 50 °C (Tg’= -28 °C) or 40 °C
(Tg’= -31 °C) and to the osmodehydrated seeds (Tg’= -34 °C). The increasing of Tg’ could be
induced by a progressive loss of non-freezing water (tightly bound water) of seeds during the
drying process. Sá et al. (1999) found that Tg’ for polysaccharides water systems reach to a
maximum with decreasing water content, inducing the decreased mobility of the polymer
chains. Glass transition temperature was determined from the change in heat capacity (∆Cp).
∆Cp can be related to the glass transition temperature (Tg’) due to the presence of sucrose,
protein, fibre (pectin, lignin, hemicellulose and cellulose), and water in the sample. As
reported in such products, carbohydrates and proteins can be described as amorphous food
polymers constituted by not arranged chains (Roos 1995).
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T°: 40°CT°: 50°CT°: 60°C
DO
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Hea
t Flo
w (W
/g)
-60 -40 -20 0 20 40
Temperature (°C)Exo Up Universal V3.9A TA Instruments
Figure 2: DSC thermograms obtained for osmodehydrated (OD) and dried pomegranate seeds at different temperatures (40, 50, 60 °C).
Hot air-drying temperature is very important for the dehydration, but it is limited by
the heat sensitivity of seeds and the expected quality of the final product (Erbay and Icier
2009; Jaya and Das 2009; Mujumdar and Law 2010). Therefore, the physicochemical
properties of seeds at different air-drying temperature were analysed.
Physico-chemical properties
Antioxidant activity
Antioxidant compounds are considered as an indicator of the quality of food
processing due to its low stability during thermal process (Bilgari et al. 2008; Saxena et al.
2010). Antioxidant activity (AA) was determined in terms of stable free radical DPPH. (2,2,-
diphenyl-2-picryl-hydrazyl) according to the method described by Vega-Galvez et al. (2009).
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Antioxidant activity of pomegranate seeds (84 %), cultivated in Tunisia, was found to be
slightly higher compared to other pomegranate seeds (62-72 %) cultivated in India (Kulkarni
and Aradhya 2005). After osmotic dehydration process, seeds showed a rapid decrease of
antioxidant activity (i.e. until 58 %). This value remained interesting compared to other fruits
and vegetables (Bilgari et al. 2008; Miranda et al. 2009; Kuljarachanan et al. 2009).
Antioxidant activity continues to be reduced during drying, regardless the considered
drying temperature. In fact, AA reached 46, 39, and 31 %, after 240 min for drying
temperatures of 40, 50, and 60 °C respectively (Table 3). In spite of this decrease, AA%
remained higher than those observed in date and close to tea and coffee antioxidant activity
value (Bilgari et al. 2008). As shown the lowest antioxidant activity was recorded using a
higher air-drying temperature (60 °C). ANOVA analysis showed a significant difference
(P<0.05) of AA% as a function of air-drying temperatures. Similar results have been reported
by Miranda et al. (2009) and Kuljarachanan et al. (2009) during the increase of air-drying
temperature from 50 to 90 °C of Aloe Vera and lime. This reduction of AA% could be
explained due to loss of different components (i.e. phenolics acids, flavonoid, and ascorbic
acid), which are responsible for the antioxidant activity of pomegranate seeds, during heating
(Nicoli et al. 1999; Kulkarni and Aradhya 2005). Vega-Galvez et al. (2009) concluded that
although natural antioxidants are lost during heating, the overall antioxidant properties of
foods could be maintained or enhanced by the formation of new antioxidant compounds with
enhanced antioxidant properties. In fact, increase in AA% following thermal treatment has
been reported in many vegetables (Choi et al. 2006; Kang et al. 2006). As consequence, in
this study the destruction rate of antioxidants during heating was higher than the formation
rate of these compounds.
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Table 3: Value of total phenolic anthocyanin and antioxidant activity of untreated, osmotic dehydrated and dried seeds
Total phenolic
[mg/100g] Total anthocyanin
[mg/100g] Antioxidant activity [%]
Untreated seeds 326.68e±1.40 82.30d±1.42 84.23e±0.31 Osmotic dehydrated seeds
184.39d±1.15 68.43c±0.30 57.88d±1.07 40 °C 151.76c±1.93 40.11b±1.53 46.23c±0.56 50 °C 141.14b±1.23 24.03a±0.14 39.04b±0.80
Dried seeds
60 °C 134.58a±1.14 20.10a±0.28 31.17a±1.16 Means in column with different letters are significantly different (P<0.05)
Total phenolic content
Pomegranate seeds’ phenolic content 326.7±1.4 mg gallic acid equivalent/100g fresh
matter (FM) (Table 3). This value is in agreement with previous finding in pomegranate
seeds, which varied between 230 and 510 mg gallic acid equivalent/100g FM (Kulkarni and
Aradhya 2005). During osmotic dehydration treatment, a decrease of 40 % (184 mg/100g)
compared to the initial phenolic content was observed (Table 3). This value was lower to that
found in fruits (apple and cherry: 500 mg/100g, strawberry: 330 mg/100g) and higher
compared to vegetable (25 – 100 mg/100g) (Yang et al. 2006).
Moreover, pomegranate seeds showed a regress in total phenolic during the drying
from 184 mg/100 g FM (osmotic dehydrated seeds) to 151, 141 and 134 mg gallic acid
equivalent/100g FM for drying temperatures of 40, 50 and 60 °C, respectively (Table 3) in
agreement with other findings Nicoli et al. (1999); Erbay and Icier (2009). In spite of this
reduction during drying, values remained higher compared to those observed in vegetables
(Yang et al. 2006).
The reduction of total phenolic compounds after osmotic process was due to the
migration of phenolic compounds from pulp to osmotic solution induced by a large osmotic
driving force. This fact was due to the higher difference in concentration between dilute seeds
sap (15 °Brix) and the surrounding hypertonic medium (55 °Brix) (Raoult-Wack et al. 1991).
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This behaviour has been reported in the osmotic dehydration of pomegranate seeds (Bchir et
al. 2009; Bchir et al. 2010a). During drying, total phenolic compounds significantly decreased
indicating the negative effect of higher temperature on total phenolics compounds. This could
be ascribed to thermal degradation of naturally occurring anti-oxidative compounds present in
pomegranate seeds as flavonoids (anthocyanins), phenolic acid (Madrigal-Carballo et al.
2009; Devic et al. 2010). This result corroborates the findings of Jukunen-Tiitto (1985) and
Harbourne et al. (2009) who reported that an increase in temperature from 40 to 70 °C caused
a decrease of the flavonoid content in willow leaves and meadowsweet. Moreover, enzymatic
and non-enzymatic reaction could be a responsible for the decrease of phenolic compounds in
seeds supported by the increase of the temperature (Jeantet et al. 2006). In fact, phenolic
compounds are the substrate for polyphenol oxidase enzyme. Also, Maillard reaction (non-
enzymatic reaction) use phenolic compounds having, carbonyl functions, like a substrate
(Jeantet et al. 2006).
Total anthocyanin pigments content
Similar to antioxidant activity and total phenolic, anthocyanin pigment decreased from
82 to 68 mg/100g during the first 20 min of osmotic dehydration process (Table 3). This fact
could be due to the migration of anthocyanin pigment from pulp to the osmotic solution
induced by the driving osmotic force. Antioxidant activity was lower to those observed in
strawberry (450 – 700 mg/100g) and in range compared to blueberry (25 – 495 mg/100g),
mulberry (67 – 107 mg/100g) (Cisse et al. 2009).
During heating (from 40 to 60 °C), a decrease in the anthocyanin pigment
concentration was also observed for pomegranate seeds (Table 3). In spite of this decrease of
antioxidant activity, values remained closer to those observed in pulm (30 mg/100g), grapes
(30 -750 mg/100g) and blueberry (25 – 495 mg/100g) (Yang et al. 2006). The highest
concentration of anthocyanin (40 mg/100g FM) was recorded by using the lower temperature
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(40 °C). In fact, after drying anthocyanin content was reduced by 41, 64 and 70% for drying
temperatures of 40, 50 and 60 °C respectively. Similar trends were observed for pomegranate
seeds anthocyanins (Jaiswal et al. 2010) and black carrot anthocyanins (Kirca et al. 2007)
during heating. The degradation of anthocyanins could be due to enzymatic (polyphenol
oxidase) reaction. In fact, Raynal et al. (1989) reported that polyphenol oxidase playing
important role in oxidative degradation of anthocyanins during the drying of plums.
Moreover, Cemeroglu et al. (1994) founded that the degradation rate of anthocyanins in sour
cherry increased with increasing heating temperature (e.g. 60, 80 °C). In fact, the increase in
the temperature enhanced the modification rate of the anthocyanin chemical structure
favouring its degradation (Jackman and Smith 1996).
The decrease of anthocyanin content contributes to the decline of the colourful
appearance of seeds (Jaiswal et al. 2010).
Relation between antioxidant activity, total phenolic and total anthocyanin pigments
Phenolic compounds, including anthocyanins, display strong antioxidant activity;
contributing significantly to the antioxidant capacity of fruits (Nicoli et al. 1999; Jeantet et al.
2006; Fathi et al. 2009). In fact, the decrease of phenolic compound by 17 % involved a
decrease of the antioxidant activity by 20 % at 40 °C. The percentage of loss in antioxidant
activity remained slightly higher than that observed with total phenolic at different air-drying
temperature. Contrary to others studies, these results showed that the production of new
antioxidant compounds during drying was very weak (Shi et al. 2008; Vega-Galvez et al.
2009).
Colour
The effect of osmotic dehydration and air-drying temperature on seeds colour was
illustrated in table 4. Five chromatics coordinates was used to characterise the changes of
seeds colour during these processes. Seeds colour was found to be dependent on air-drying
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temperature and osmotic process. After osmotic dehydration hue angle (h°) and lightness (L*)
values increased, while an opposite trend was observed for chroma (C*) values. Furthermore,
a* and b* colour parameters showed a slight decrease during osmotic process. These
variations indicated that seeds become less dark during OD. This could be explained due to
the migration of pigment from pulp to the osmotic solution inducing by osmotic driving force.
During drying, hue angle and lightness values decreased with the increase of air-drying
temperature from 84° and 29 to 69° and 23, respectively. This changes indicated the reduction
of colour from a more green (when Hue > 90°) to an orange-red (when Hue <90°) and seeds
turning darker (decreasing of L*). Chroma values, increased with the increase of air-drying
temperature showing that seeds colour became more saturated. Moreover, a* and b* colour
parameters showed a slight increase during drying (Table 4). This modification in seeds
colour is mainly due to the effect of temperature on heat-sensitive compounds such as
carbohydrates, proteins and vitamins, which cause colour degradation during drying process.
According to Mandala et al. (2005) the decrease of “L*” values and the increase of “a*”
values correspond to the increase of fruit browning. To better understand the effect of air-
drying on seeds colour, browning index and total colour difference were determined. It can be
observed that an increase of temperature led to a significant formation of brown products.
Indeed, the maximum browning index was occurred at 60 °C (0.075) as compared to 50 °C
(0.064) and 40 °C (0.051). Similar observations were reported by Miranda et al. (2009) and
Vega-Galvez et al. (2009) using Aleo vera and red pepper, respectively. The total colour
difference (∆E*) value increased slightly with the increase of air-drying temperature (40 °C:
3.0± 0.5; 50 °C: 5.1± 0.2 and 60 °C: 9.8± 0.8). This indicated that seeds became brownish
(Romano et al. 2008). However, ∆E* was lower to those observed in many dried fruits
(Pereira et al. 2007; Chong et al. 2008). In addition, browning index was very low indicating
that air-drying temperature does not have a great influence on the browning of seeds. This
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could be due to the osmotic dehydration pre-treatment. Indeed, Ponting (1973) and Krokida et
al. (2001) showed that dehydration of foodstuffs (e.g. potato) by immersion in osmotic
solutions before convective air-drying, improves the quality of the final product since, it
prevents oxidative browning.
The formation of brown compounds in seeds may be related to both enzymatic and
essentially non-enzymatic (Maillard reaction) reactions (Miranda et al. 2009).
Table 4: Effect of air-drying temperature on chromatics coordinates and on textural properties of seeds.
Dried seeds
Untreated
seeds Osmotic
dehydrated seeds 40 °C 50 °C 60 °C
h° 63.50±2.30 84.60±3.40 77.44±3.10 73.33±3.00 69.47±1.00 C* 15.29±0.10 11.71±0.50 14.30±0.06 15.54±0.48 19.08±0.17 L* 26.31±1.10 28.91±0.17 27.79±1.10 25.90±0.33 22.95±0.01 a* 12.44±0.14 7.80±0.77 10.20±0.41 11.50±0.70 14.65±0.25
Chromatics coordinates
b* 8.90±0.03 8.60±0.02 9.91±0.11 10.40±0.06 12.21±0.01
Hardness [N] 46.73±2.47 63.46±3.04 101.54±4.06 118.61±3.47 123.63±4.91
Toughness [N mm]
54.55±3.96 67.21±5.55 87.01±4.52 92.33±3.24 103.38±4.12
Pip crush [mm] 4.51±0.34 3.70±0.34 2.10±0.17 1.80±0.12 1.65±0.11
Enzymatic browning
Browning colour could be induced by polyphenol oxidase (PPO) present in
pomegranate seeds. PPO was extracted from the pulp and the relative activity was measured
as a function of air drying temperature. Results showed a relative activity of 27 % for PPO in
osmotically dehydration seeds.
The increase in air-drying temperature involved a decline in PPO relative activity. The
relative activity decreased by 4 and 5 % at air drying temperatures of 50 and 60 °C
respectively as compared to that at 40 °C. Our results were in agreement with previous
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findings showing that PPO is a heat-labile compound (Mandala et al. 2005; El-Aouar et al.
2003; Jaiswal et al. 2010).
The presence of PPO in seeds could be responsible for phenolic compounds
(flavonoids, tannins, lignins, phenolic acids) degradation involving colour modification
(Jaiswal et al. 2010). In fact, Saxena et al. (2010) showed that tissue browning is mainly due
to the oxidation of phenolic compounds into quinine compounds under aerobic conditions by
PPO, then the quinine compounds undergoes polymerization forming brown polymeric
pigments, leading to browning. However, Lenart (1996) found that the presence of sugar on
the surface of the osmodehydrated sample, is a barrier for the contact with oxygen thus
reducing the oxidative reactions and the resultant discolouring of the fruit.
The inactivation of PPO by drying prevents the browning reaction in seeds. However
in precedent paragraph, we found that the browning colours increased slightly as function of
air-drying temperature. Therefore, there is another reaction that induced browning reaction.
Many authors found that during drying non-enzymatic browning (Maillard reaction,
caramelisation) was responsible for browning of fruits (Maskan 2001; Lewicki 2006; Miranda
et al. 2009).
Non-enzymatic browning
Maillard reaction, also called sugar-amino browning reaction, which is a form of non-
enzymatic browning, is a chemical reaction between an amino acid and reducing sugar under
heating conditions (Rada-Mendoza et al. 2002). The reactive carbonyl group of the sugar
interacts with the nucleophilic amino group of the amino acid to create hundreds of different
compounds. 5-Hydrorymethylfurfural (HMF) is one of the major intermediate products in the
Maillard reaction (Rada-Mendoza et al. 2002).
It was observed that increasing the air-drying temperature leads to enhanced HMF
content (40 °C: 0.017 mg/100 g FM; 50 °C: 0.019 mg/100 g FM and 60 °C: 0.024 mg/100 g
167
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FM) compared to osmodehydrated seeds (0.011 mg/100 g FM). However, HMF values of
different seeds were very low showing that air-drying temperatures do not have a great
influence on the formation of HMF. This could be due to the low content of protein, in
osmodehydrated seeds. In fact, protein is a necessary substrate for the Maillard reaction
(Rada-Mendoza et al. 2002). That is confirming the lower browning index and the decrease of
AA% as function of temperature. In fact, low HMF content and the decrease of AA% induced
through the enhancing of the temperature, show that the rate of destruction of antioxidant
compounds was higher than the rate of formation of these compounds. Indeed many authors
found that Maillard reaction let the formation of many antioxidant compounds (e.g.
melanoidins) (Shi et al. 2008; Vega-Galvez et al. 2009).
Texture analysis
Texture analysis of osmodehydrated and dried pomegranate seeds were studied over
time periods of up to 20 min and 4h, respectively (Table 4). Two textural parameters
(Hardness and Toughness) were used to characterize seeds texture modification. Based on the
results, hardness and toughness were affected by osmotic process and air-drying temperature.
In fact, after OD, seeds hardness and toughness increased by 17 % and 13 %, respectively
compared to untreated seeds. During drying, hardness increased by 38, 55 and 60 N, while
toughness also increased by 20, 25 and 36 N mm at drying temperatures of 40, 50 and 60 °C
respectively. This behaviour could be explained by the structural collapse of seeds induced by
the increased water loss during osmotic and drying process (Mandala et al. 2005). As a
consequence of this exchange, the products will more or less lose weight and will shrink
eventually (Aversa et al. 2009). Indeed, the peaks of pip crushing decreased after osmotic
process and drying (Table 4). Thus, compared to untreated seeds, those that were only
osmodehydrated reduced thickness by 18 % and those that were also dried at 40, 50 and 60 °C
lost 43, 51 and 55 % thickness, respectively. Similar results have been reported by Mandala et
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al. (2005), and Bchir et al. (2010a) using the textural changes during the drying of apple and
chempedack and osmotic dehydration of pomegranate seeds, respectively.
Conclusion
Osmotic dehydration (OD) and drying process could be used for the conservation of
pomegranate seeds. Indeed, OD followed by drying allowed to reduce water activity until a
value less than 0.650 after 60, 120 and 240 min at drying temperatures of 60, 50, and 40 °C,
respectively. To reduce energy consumption and improve food quality, it would be interesting
that drying stopped after these times. From the obtained results, it is recommended to use 40
°C since the low influence on the quality parameters of seeds was observed.
The determination of PPO activity and HMF content after drying showed that
enzymatic and non-enzymatic reactions (Maillard reaction) have not a market effect on
browning index, showing the benefit effect of pre-treatment (osmotic dehydration) on colour
stability.
During drying not only the composition of the tissue is changed but also the textures
since seeds reduce their thickness to maximum 55 % using 60 °C. Differential scanning
calorimetry data showed a relation between Tg’ and texture parameters. In fact, water loss of
seeds induced an increase of hardness and toughness and also an increase of Tg’.
These processes permit a microbiological stability but also a degradation of the
nutritional quality of the fruit that remained slightly lower compared to other fruits and
vegetables. It should be interesting to use seeds as ingredients in food formulations.
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References
Ahrné, L., Prothon, F., & Funebo, T. (2003). Comparison of drying kinetics and texture
effects of two calcium pre-treatment before microwave-assisted dehydration of apple
and potato. International Journal of Food Science and technology, 38, 411-420.
AOAC. (1990). Official Methods of Analysis of the Association of Official Analytical 436
Chemists, 15th Ed., (K. Helrich, ed.), Association of Official Analytical Chemists, Inc.,
437 Arlington, Virginia, VA.
Arabhosseini, A., Padhye, S., Huisman, W., Boxtel, A., & Müller, J. (2009). Effect of Drying
on the Color of Tarragon (Artemisia dracunculus L.) Leaves. Food and Bioprocess
Technology, DOI: 10.1007/s11947-009-0305-9, in press.
Aversa, M., Curcio, S., Calabrò, V., & Iorio, G. (2009). Experimental Evaluation of Quality
Parameters During Drying of Carrot Samples. Food and Bioprocess Technology,
DOI: 10.1007/s11947-009-0280-1, in press.
Bchir, B., Besbes, S., Attia, H., & Blecker C. (2009). Osmotic dehydration of pomegranate
seeds: Mass transfer kinetics and DSC characterization. International Journal of Food
Science and Technology, 44, 2208-2217.
Bchir B., Besbes S., Attia H., & Blecker C. (2010a). Osmotic dehydration of pomegranate
seeds (Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process
Engineering, DOI: 10.1111/j.1745-4530.2010.00591.x, in press.
Bchir B., Besbes S., Karoui R., Paquot M., Attia H., & Blecker C. (2010b). Osmotic
Dehydration Kinetics of Pomegranate Seeds Using Date Juice as an Immersion
Solution Base, Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0442-1,
in press.
Bchir, B., Besbes, S., Giet, J., Attia, H., & Blecker C. Synthèse des connaissances sur la
déshydratation osmotique. Biotechnologie Agronomie Société et Environnement, in
press.
Biglari, F., Abbas. F., & Easa, A. (2008). Antioxidant activity and phenolic content of various
date palm (Phoenix dactelifera) fruits from Iran. Food Chemistry, 107, 1636-1641.
170
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
Carlos, A., & De-Michelis, A. (2009). Comparison of Drying Kinetics for Small Fruits with
and without Particle Shrinkage Considerations. Food and Bioprocess Technology,
DOI: 10.1007/s11947-009-0218-7, in press.
Cemeroglu, B., Velioglu, S., & Isik, S. (1994). Degradation kinetics of anthocyanins in sour
cherry juice and concentrate. Journal of Food Science, 59, 1216-1217.
Chenlo, F., Moreira, R., Herrero, F., & Vazquer, G. (2007). Osmotic dehydration of chestnut
with sucrose: Mass transfer processes and global Kinetics modelling. Journal of Food
Engineering, 78, 765-774.
Chong, C., Law, C., Cloke, M., Hii, C., & Abdullah, L. (2008). Drying kinetics and product
quality of dried Chempedak. Journal of Food Engineering, 88, 522-527.
Choi, Y., Lee, S., Chun, J., Lee, H., & Lee, J. (2006). Influence of heat treatment on the
antioxidant activities and polyphenolic compounds of Shiitake mushroom, Food
Chemistry, 99, 381-387.
Cisse, M., Dornier, M., Sakho, M., Ndiaye, A., Reynes, M., & Sock, O. (2009). Le bissap
(Hibiscus sabdariffa L.): composition et principales utilisations. Fruits, 179-193.
Defilippi, G., Whitaker, B., Hess-Pierce, B., & Kader, A. (2006). Development and control of
scald on wonderful pomegranates during long-term storage. Postharvest Biology and
Technology, 41, 234-243.
Devic, E., Guyot, S., Daudin, J., & Bonazzi, C. (2010). Kinetics of Polyphenol Losses During
Soaking and Drying of Cider Apples. Food and Bioprocess Technology,
DOI: 10.1007/s11947-010-0361-1, in press.
Doymaz, I. (2007). Air-drying characteristics of tomatoes. Journal of Food Engineering, 78,
1291-1297.
El-Aouar, A., Azoubel, P., & Murr, F. (2003). Drying kinetics of fresh and osmotically pre-
treated papaya (Carica papaya L.). Journal of Food Engineering, 59, 85-91.
Erbay, Z., & Icier, F. (2009). Optimization of hot air drying of olive leaves using response
surface methodology. Journal of Food Engineering, 91, 533-541.
171
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC and DOC-
Lavoisier, pp. 181–182.
Fabiano, A.N., Oliveira, F., & Rodrigues, S. (2008). Use of Ultrasoud for Dehydration of
Papayas. Food and Bioprocess Technology, 1, 339-345.
Fadavi, A., Barzegar, M., Azizi, H., & Bayat, M. (2005). Physicochemical Composition of
Ten Pomegranate Cultivars (Punica granatum L.) Grow in Iran. Journal of Food
Science Technology, 11, 113 - 119.
Falade, O., & Onyeoziri, N. (2010). Effects of Cultivar and Drying Method on Color, Pasting
and Sensory Attributes of Instant Yam (Dioscorea rotundata) Flours. Food and
Bioprocess Technology, DOI: 10.1007/s11947-010-0383-8, in press.
Fathi, M., Mohebbi, B., & Razavi, S. (2009). Application of Image Analysis and Artificial
Neural Network to Predict Mass Transfer Kinetics and Color Changes of Osmotically
Dehydrated Kiwifruit. Food and Bioprocess Technology, DOI: 10.1007/s11947-009-
0222-y, in press.
Fathi, M., Mohebbat, M., & Razavi, S. (2010). Effect of Osmotic Dehydration and Air Drying
on Physicochemical Properties of Dried Kiwifruit and Modeling of Dehydration
Process Using Neutral Network and Genetic Algorithm. Food and Bioprocess
Technology, DOI: 10.1007/s11947-010-0452-z, in press.
Friant, N., Marks, B., & Barkker-Arkema, F. (2004). Drying rate of corn. Transaction of the
ASAE, 47, 1605-1610.
Gokhale, S., & Lele, S. (2010). Optimization of Convective Dehydration of Beta vulgaris for
Color Retention. Food and Bioprocess Technology,DOI: 10.1007/s11947-010-0359-8,
in press.
Harbourne, N., Marete, E., & Jacquier. J., & O’Riordan, D. (2009). Effect of drying methods
on the phenolic constituents of meadowsweet (Filipendula ulmaria) and willow (Salix
alba). LWT-Food Science and Technology, 42, 1468-1473.
172
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
Hernandez, F., Melgarejo, P., Tomas-Barberan, F.A., & Artes, F. (1999). Evolution of juice
anthocyanins Turing ripening of new selected pomegranate (Punica granatum) clones.
European Food Research and Technology, 210, 39-42.
Jackman, R.L., & Smith, J. (1996). Anthocyanins and betalains. Natural food colorants,
Blackie Acadenamic and professional, 244-280.
Jaiswal, V., DerMarderosian, A., & Porter, J. (2010). Anthocyanins and polyphenol oxidase
from dried arils of pomegranate (Punica granatum L.). Food Chemistry, 118, 11-16.
Jaya, S., & Das, H. (2009). Glass Transition and Sticky Point Temperatures and
Stability/Mobility Diagram of Fruit Powders. Food and Bioprocess Technology, 2, 89-
95.
Jeantet, R., Croguennec, T., Schuch, P., & Brulé, G. (2006). Science des aliments, TEC and
DOC-Lavoisier, pp. 121–161.
Jukunen-Tiitto, R. (1985). Phenolic constituents in the leaves of northern willows: methods
for the analysis of certain phenolics. Journal of Agricultural and Food Chemistry, 33,
213-217.
Kang, K., Kim, H., Pyo, J., & Yokozawa, T. (2006). Increase in free radical scavenging
activity of ginseng by heat-processing. Biological and Pharmaceutical Bulletin, 29, 750-
754.
Kingsly, A., & Singh, D. (2007). Drying kinetics of pomegranate arils. Journal of Food
Engineering, 79,741-744.
Kirca, A., Ozkan, M., & Cemerglu, B. (2007). Effects of temperature, solid content and pH on
the stability of black carrot anthocyanins. Food Chemistry, 101, 212-218.
Krokida, M.K., Oreopoulou, V., Maroulis, Z.B., & Marinoskouris, D. (2001). Effect of
osmotic dehydration pre-treatment on quality of French fries. J. Food Engin., 49, 339–
345.
Kuljarachanan, T., Devahastin, S., & Chiewchan, N. (2009). Avolution of antioxidant
compound in lime residues during drying. Food Chemistry, 113, 944-949.
173
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
Kulkarni, P., & Aradhya, S. (2005). Chemical changes and antioxidant activity in
pomegranate arils during fruit development. Food Chemistry, 93, 319-324.
Lenart, A. (1996). Osmo-convective drying of fruits and vegetables: Technology and
application. Drying Technology, 14, 391-413.
Lewicki, P. (2006). Design of hot air drying for better foods. Food Science and Technology,
17, 153-163.
Madamba, P., Driscoll, R., & Buckle, K. (1996). The thin-layer drying characteristics of
garlic slices. Journal of Food Engineering, 29, 75-97.
Madrigal-Carballo, S., Rodriguez, G., Krueger, C., & Dreher, M. (2009). Reed, J.
Pomegranate (Punica granatum) supplements: Authenticity, antioxidant and polyphenol
composition. Journal of Functional Food, In press, DOI: 10.1016/j.jff.2009.02.005, in
press.
Mandala, I., Anagnostaras, E., & Oikonomou, C. (2005). Influence of osmotic dehydration
conditions on apple air-drying kinetics and their quality characteristics. Journal of Food
Engineering, 69, 307-316.
Maskan, M. (2001). Kinetics of colour change of kiwifruits during hot air and microwave
drying. Journal of Food Engineering, 48, 169-175.
Miranda, M., Maureira, H., Rodriguez, K., & Vega-Galvez, A. (2009). Influence of
temperature on the drying kinetics, physicochemical properties, and antioxidant capacity
of Aloe Vera (Aloe Barbadensis Miller) gel. Journal of Food Engineering, 91, 297-304.
Mujumdar, A., & Law, C. (2010). Drying Technology: Trends and Applications in
Postharvest Processing. Food and Bioprocess Technology, DOI: 10.1007/s11947-010-
0353-1, in press.
Nicoli, M., Anese, M., & Parpinel, M. (1999). Influence of processing on the antioxidant
properties of fruit and vegetables. Trends in Food Science and Technology, 10, 94-100.
Park, J., Bin, A., & Brod, F. (2002). Drying of pear d’Anjou with and without osmotic
dehydration. Journal of Food Engineering, 56, 97-103.
174
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
Pereira, N., Antonia, M., & Ahrne, L. (2007). Effect of microwave power air velocity and
temperature on final drying of osmotically dehydrated bananas. Journal of Food
Engineering, 81, 79-87.
Pinho, R., Guiné, F., Henrriques, F., & Barroca, M. (2009). Mass Transfer Coefficients for the
Drying of Pumpkin (Cucurbita moschata) and Dried Product Quality. Food and
Bioprocess Technology, DOI: 10.1007/s11947-009-0275-y, in press.
Pokharkar, S. M., Prasad, S., & Das, H. (1997). A model for osmotic concentration of banana
slices. Journal of Food Science and Technology, 34, 230- 232.
Ponting, J.D. (1973). Osmotic dehydration of fruits. Resents modifications and applications.
Process Biochemistry, 8, 18-20.
Rada-Mendoza, M., Olano, A., & Villamiel, M. (2002). Determination of
hydroxymethylfurfural in commercial jams and in fruit-based infant foods. Food
Chemistry, 79, 513-516.
Raoult-Wack, A.L., Guilbert, S., Le Maguer, M., & Rios, G. (1991). Simultaneous water and
solute transport in shrinking media-part 1: application to dewatering and impregnation
soaking process analysis (osmotic dehydration). Drying Technnol., 589–612.
Raynal, J., Moutounet, M., & Souquet, J.M. (1989). Intervention of phenolic compound in
plum technology. Changes during drying. J. Agric. Food Chem. 37, 1046-1050.
Robert, C., Soliva, F., Martinez, P., Sebastian, M., & Martin, O. (2002). Kinetics of
polyphenol oxidase activity inhibition of avocado preserved by combined methods.
Journal of Food Engineering, 55, 131-137.
Romano, R., Baranyai, L., Gottschalk, K., & Zude, M. (2008). An approach for monitoring
the moisture content changes of drying banana slices with laser light backscattering
imaging. Food and Bioprocess Technology, 4, 410-414.
Roos, Y. (1995). Characterisation of food polymers using state diagrams. Journal of Food
Engineering, 24, 339–360.
175
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
Rosenblat, M., & Hayek, T. (2006). Aviram, M. Anti-oxidative effects of pomegranate juice
(PJ) consumption by diabetic patients on serum and macrophages. Atherosclerosis, 187,
363-371.
Ruiz-Lopez, I., Castillo-Zamudio, R., Salgado-Cervantes, M.A., Rodriguez-Jimenes, G.C., &
Garcia-Alvarado, M.A. (2010). Mass Transfer Modeling During Osmotic Dehydration
of Hexahedral Pineapple Silices in Limited Volume Solutions. Food and Bioprocess
Technology, 3, 427-433.
Sá, M.M., Figueiredo, A.M., & Sereno, A.M. (1999). Glass transitions and state diagrams for
fresh and processed apple. Thermochimica Act, 329, 31–38.
Saxena, A., Maity, T., Raju, P., & Bawa, A. (2010). Degradation Kinetics of Colour and Total
Carotenoids in Jackfruit (Artocarpus heterophyllus) Bulb Slices During Hot Air
Drying. Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0409-2, in
press.
Sharma, G., Prasad, S., & Chahar, V. (2003). Moisture transport in garlic cloves undergoing
microwave-convective drying. Food and Bioproducts Processing, 87, 11-16.
Shi, J., Pan, Z., McHugh, T., Wood, D., & Hirschberg, E., Olson, D. (2008). Drying and
quality characteristics of fresh and sugar-infused blueberries dried with infrared
radiation heating. LWT-Food Science and Technology, 41, 1962-1972.
Singh, H., & Sodhi, N. (2000). Dehydration kinetics of onions. Journal of Food Science and
Technology, 37, 520-522.
Storey, T. (2007). La grenade, le fruit medicament, Magazine NEXUS. Santé, 51, 46-54.
Uribe, E., Miranda, M., Vega-Galvez, A., Quispe, I., Claveria, R., & Di Scala, K. (2010).
Mass Transfer Modelling During Osmotic Dehydration of Jumbo Squid (Dosidicus
gigas): Influence of Temperature on Diffusion Coefficients and Kinetic Parameters.
Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0336-2, in press.
Uribe, E., Vega-Gálvez, A., Scala, K., Oyanadel, R., Torrico, J., & Miranda, M. (2009).
Characteristics of Convective Drying of Pepino Fruit (Solanum muricatumAit.):
176
Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________
177
Application of Weibull Distribution. Food and Bioprocess Technology,
DOI: 10.1007/s11947-009-0230-y, in press.
Vega-Galvez, A., Di Scala, K., Rodriguez, K., Lemus-Mondaca, R., Miranda, M., Lopez, J.,
& Perez-Won, M. (2009). Effect of air-drying temperature on physico-chemical
properties, antioxidant capacity, colour and total phenolic content of red pepper
(Capsicum annuum, L. var. Hungarian). Food Chemistry, 117, 647-653.
Vinas, P., Campillo, N., Hernandez-Cordoba, M., & Candela, M.E. (1992). Simulation liquid
chromatographic análisis of 5-(hydroxymethyl)-2-furaldehyde and metil anthranilate in
honey. Food Chemistry, 44, 67-72.
Yang, R.Y., Tsou, S. C. S., Lee, T. C., Chang, L. C., Kuo, G., & Lai, P. Y. (2006). Moringa, a
novel plant rich in antioxidants, bioavailable iron, and nutrients. In: C. T. Ho (ed)
Challenges in Chemistry and Biology of Herbs.American Chemical Society,
Washington, D.C. pp. 224-239.
Chapitre 6:
Discussion générale, conclusions et perspectives
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6.1. Discussion générale
Les fruits et les légumes, de par leur caractère saisonnier, sont disponibles en abondance
pendant de courtes périodes de l’année. La conservation des végétaux au cours des récoltes
peut prolonger leur période de disponibilité. Elle permet ainsi aux producteurs d’écouler leurs
productions à de meilleurs prix et aux utilisateurs de pouvoir les consommer sur de longues
périodes de l’année à des prix raisonnables. Parmi les méthodes de conservation, la
déshydratation osmotique vise à réduire, à moindre coût, le risque d’altération de la qualité
nutritionnelle et organoleptique du produit traité (Ade-Omowaye et al., 2003). Cependant, si
cette technique est appliquée sur de multiples fruits et légumes (pomme, abricot, banane,
carotte, etc.), elle n’a jamais été étudiée auparavant sur les graines de grenade.
Dans ce contexte, le présent travail de thèse s’est attaché à délivrer les bases
scientifiques et techniques pour l’étude des possibilités de conservation des graines de
grenades (variété Tunisienne El-Gabsi) par déshydratation osmotique.
Pendant la DO, les transferts de masse dépendent, d’une part des propriétés intrinsèques
des tissus traités, et d’autre part des conditions opératoires de traitement (pression,
température, composition de la solution d’immersion, durée de traitement). Dans un but
d’optimisation du procédé, nous avons envisagé dans un premier temps d’évaluer l’influence
de la température (30, 40, 50°C), de la composition des solutions osmotiques (saccharose,
glucose, saccharose/glucose) et du temps (0, 20, 60, 120, 180, 240 min) sur la cinétique de
transfert de masse et sur les caractéristiques physico-chimiques de la préparation du fruit
déshydraté osmotiquement.
Deux approches ont été employées afin d’étudier cette cinétique. L’approche
« classique » qui se base sur la détermination de trois paramètres, la perte d’eau (« Water loss
», WL), le gain en solides (« Solids Gain », SG) et la réduction en poids (« weight
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réduction », WR). Ces paramètres sont déterminés par la mesure des solides totaux (Krokida
et al., 2000 ; Riva et al., 2005; Garcia-Segovia et al., 2010). Nous avons aussi eu recours à une
modélisation de la DO au moyen des solutions analytiques de l’équation de Fick, permettant
ainsi de déterminer les coefficients de diffusion de l’eau et du soluté. La deuxième approche,
plus fine, se base sur l’étude des paramètres thermiques (Tg’, ∆H, Tf, UFW) déterminés à
partir de la calorimétrie différentielle à balayage. Cette technique permet de suivre l’évolution
des différentes fractions d’eau (libre et liée) dans le produit au cours du traitement.
Sur base des résultats obtenus, indépendamment de la solution ou de la température, la
cinétique de transfert de matière pouvait se décomposer en deux phases. Une première phase
rapide d'une durée systématiquement voisine de 20 min, durant laquelle l’essentiel des
transferts d’eau et de solutés s’opèrent. Elle était suivie d'une seconde phase marquée par la
forte diminution de l’intensité des échanges. La perte en eau, le gain en solides et la perte de
poids ont subi des variations tout au long du procédé, dont l’amplitude, passées les 20
premières minutes de la phase initiale de transfert, est suffisamment faible pour préconiser
l’arrêt du procédé.
L'augmentation de la température de traitement a eu pour effet de favoriser les transferts
de matière, en augmentant les coefficients de diffusion de l’eau et de solutés. La
détermination des coefficients de diffusion a également montré que la nature du soluté a un
effet significatif sur la cinétique de transfert de masse. En effet, à une même température, le
coefficient de diffusion de l'eau dans une solution de saccharose est supérieur à celui du
glucose, de masse molaire plus important. A l'inverse, le coefficient de diffusion des solutés
est deux fois plus élevé dans le glucose que dans le saccharose. Ainsi on préconise d’utiliser
le saccharose pour une meilleure déshydratation des graines de grenade.
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L’analyse des propriétés thermiques de la pulpe des graines de grenade déshydratées
osmotiquement à 50°C, en utilisant différentes solutions, a confirmé les résultats précédents.
La détermination des fractions d’eau non congelable et congelable a montré une évolution
différente au cours du temps. En effet, le produit final présente une teneur en eau non-
congelable supérieure de 7% à celle de l’eau congelable. Cela favorise une meilleure
stabilisation du produit au cours de l’entreposage. D’autre part, ces résultats montrent qu’il
existe une fraction d’eau fortement liée au produit et qu’il est impossible de l’enlever avec ce
procédé.
On peut conclure à ce stade que pour une meilleure déshydratation des graines de
grenade, les conditions suivantes sont les plus intéressantes: un traitement de 20 min, une
température de 50°C et une solution de saccharose (55°Brix).
Ayant adopté ces conditions opératoires, l’étude a ensuite envisagé une comparaison
entre les graines fraiches et congelées afin de déterminer l’impact du pré-traitement de
congélation sur la cinétique de transfert de masse et sur la qualité organoleptique des graines
de grenade. Pour ce faire, nous avons étudié l’évolution des paramètres physico-chimiques
(MS, °Brix, couleur, conductivité, pH, aw, Deff, etc.) et de transfert de masse (WL, SG, WR).
Afin de mieux élucider l’influence du pré-traitement de congélation sur la qualité
organoleptique des graines de grenade, nous avons mis en œuvre les techniques de
microscopie électronique à balayage et de texturométrie.
Les résultats ont montré que l’utilisation des graines fraîches entraine une
déshydratation plus lente au début du procédé, comparé aux graines congelées. Cependant le
pourcentage de perte en eau le plus élevé a été obtenu à la fin du traitement en utilisant des
graines fraîches. En contrepartie, le pourcentage de gain en solides des graines congelées était
plus important que celui des graines fraîches, même après 360 minutes.
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Cela s’explique, comme le montrent les analyses en microscopie électronique, par une
destructuration cellulaire survenant à la suite de la congélation des graines. En effet, après
congélation les cellules apparaissaient déchirées, irrégulières (en forme) et avec une présence
de zones vides. Ces régions sont probablement engendrées par la croissance des cristaux de
glace au cours de la congélation entrainant ainsi une destruction cellulaire. Cette
décompartimentation provoquée par les cristaux de glace empêche le retour de l'eau au milieu
intracellulaire pendant la décongélation, causant ainsi la perte de sa turgescence. Ceci a des
conséquences pratiques en termes de perte de capacité de la paroi cellulaire d'agir en tant que
barrière semi-perméable ou de diffusion, mais aussi sur la texture du fruit (Kovacs et Meresz
2004).
Il convenait ainsi d’évaluer l’impact de la congélation sur les propriétés sensorielles, en
particulier la texture. Pour cela une méthode d’analyse de la texture a été adaptée en fonction
de la spécificité de la matière première. L’analyse texturale des graines congelées a montré
une diminution des paramètres texturaux (hardness et toughness) traduisant ainsi une perte de
la fermeté. Comparée à la graine fraiche, la graine congelée a perdu 9% de son épaisseur. Cela
confirme les observations précédentes par microscopie électronique.
Dans le contexte de la valorisation d’une deuxième agrofourniture tunisienne, nous
avons substitué une partie du saccharose, utilisé pour la préparation de la solution
d’immersion, par du jus de datte, qui contient naturellement ~19 % de matière sèche
composée essentiellement par du sucre (~ 53 g/100 g MS de saccharose et ~ 35 g/100 g MS
de sucre réducteur). Le jus de datte utilisé permet de valoriser les écarts de triages de datte qui
représentent 30% de la production, ce qui constitue un tonnage énorme avoisinant 30.000
tonnes/an pour la Tunisie et avoisinant 2.000.000 tonnes/an dans le monde (Besbes et al.,
2009). Ces quantités importantes de dattes ne sont pas consommées par l’Homme pour
plusieurs raisons : faible qualité gustative, texture trop dure, contamination par des
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champignons ou des insectes ou tout simplement parce qu’elles sont dévalorisées face à des
dattes plus attractives.
L’étude de la cinétique de DO des graines de grenade dans des solutions à base de jus de
datte montre le même profil que celui observé lors de l’utilisation des solutions de glucose ou
de saccharose à 55°Brix. Cependant, il présente des valeurs de perte en eau et de gain en
solides légèrement plus faibles. Cela pourrait être dû à la présence de plusieurs types de
sucres dans le jus de datte, en comparaison d’une solution qui contient seulement de
saccharose à 55°Brix. Ici encore, les changements les plus cruciaux sont intervenus pendant
les 20 premières minutes du procédé, entraînant une perte en eau à ~ 39%. Après cette période
de déshydratation, le pourcentage de perte d'eau a encore légèrement augmenté, atteignant une
moyenne de ~ 40%. La même tendance a été observée pour la réduction en poids, le gain en
solides.
Après DO, les modifications des caractéristiques physico-chimiques des graines ont été
étudiées. Les résultats ont montré une modification de la qualité intrinsèque des graines de
grenade caractérisée par un gain de sucre et une perte significative de protéines et d’éléments
minéraux. Concernant la solution d’immersion, une amélioration de la qualité sensorielle a été
observée, suite a un gain de solutés issu des graines. Ainsi, la solution osmotique pourrait être
incorporée dans des formulations alimentaires, contribuant ainsi à une meilleure valorisation
du produit fini.
Notre étude a également porté sur l’observation microscopique et l’analyse de texture
des graines de grenade avant et après DO. Les résultats ont montré, après DO, des cellules
déformées, déchirées, avec des formes plus irrégulières. Cela est probablement dû à la
plasmolyse des cellules, causée par la perte en eau par osmose, mais aussi à la solubilisation
des polysaccharides (cellulose, hémicellulose et pectine) qui composent les membranes des
cellules. Ces modifications ont un effet direct sur la texture de la graine. En effet, les résultats
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de l’analyse texturale ont montré une modification de la fermeté des graines, induite par une
réduction de leurs tailles, ce qui vient conforter les résultats des observations microscopiques.
Les modifications texturales (hardness et toughness) sont plus faibles que ceux enregistrées
lors de l’utilisation d’une solution d’immersion à base de saccharose. Ainsi, la teneur en
saccharose est apparue comme le principal facteur influençant la texture de la graine, mais
aussi la viscosité du jus de datte.
La DO seule ne pourrait pas maintenir une stabilité du produit au cours de la
conservation. En effet, l’activité d’eau du produit fini après DO est proche de 0,9. Ainsi dans
un but plus appliqué, un traitement supplémentaire de séchage par entrainement (2 m/s durant
4 heures) a été mis en place afin de réduire l’activité d’eau à une valeur inférieure à 0,65.
L’objectif étant d’améliorer la conservation des graines de grenade.
Avant le procédé de séchage, les graines de grenade ont été soumises à une
déshydratation osmotique en utilisant les conditions précédentes (20 minutes de traitement,
une température de 50°C et une solution de saccharose à 55°Brix). Afin d’optimiser le
traitement de séchage, nous avons étudié au premier lieu l’effet de la température (40, 50,
60°C) et du temps du traitement (0, 30, 60, 120, 180, 240 min) sur l’évolution de la matière
sèche (MS), l’activité d’eau (aw) et le pourcentage de séchage (DR) des graines. Une méthode
plus fine a été adoptée, qui consiste à analyser les propriétés thermiques de la pulpe des
graines de grenade par calorimétrie différentielle à différentes températures, afin de
déterminer l’évolution des différentes fractions d’eau dans la graine après 240 min de
traitement.
Les résultats obtenus ont montré que, quelle que soit la température de séchage utilisée,
la cinétique de séchage des graines apparaît comme la juxtaposition de deux périodes
distinctes. La première période, durant les premières 180 minutes, correspond à une diffusion
facile de l’eau, de l'intérieur de la graine vers sa surface et ainsi une évaporation de l'eau libre
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pendant le séchage. Cette évaporation est due à une différence de pression partielle d’eau
entre l’air et la surface du produit (Jeantet et al., 2006). La deuxième période, de 180 à 240
minutes, correspond à une diffusion de l'eau rendue difficile à la suite de la modification
structurale de la graine pendant le séchage (Torreggiani et Bertolo, 2001).
La variation de la température de séchage a eu un effet significatif sur le coefficient de
diffusion, l’activité d’eau, la matière sèche, le pourcentage de séchage, et sur les paramètres
thermiques relevés en DSC. L’analyse des propriétés thermiques de la pulpe des graines de
grenade à différentes températures a montré une élimination totale de l’eau libre à partir de
40°C après 240 min de traitement. En considérant le profil thermique obtenu, on peut
considérer qu’il s’agit d'un produit assez stable.
La valeur cible de l’activité d’eau (0,65) a été atteinte après 60, 120, et 240 min
respectivement à 60, 50, et 40°C. Ainsi, on préconise d’arrêter le procédé après ces temps
respectifs afin de minimiser le besoin énergétique du procédé.
Le séchage est de moins en moins performant au regard des exigences croissantes en
matière de qualité des produits finis. Ainsi nous avons jugé utile d’étudier l’effet de la
température (40, 50, 60°C) de séchage sur plusieurs paramètres de qualité des graines de
grenade tels que l’activité antioxydante, la teneur en composés phénoliques, les
anthocyanines, la couleur, et la texture.
Le séchage par entrainement, ainsi que la déshydratation osmotique, ont exercé une
influence significative sur la qualité des graines. En effet, le procédé de DO a entraîné une
réduction de l'activité de scavenging du radical diphénylpicryl-hydrazyl (DPPH). Cette
réduction est suivie par une diminution des teneurs en composés phénoliques et en
anthocyanines. Ce phénomène est accentué par des températures de séchage de plus en plus
élevées. Cependant, ces valeurs restent comparables (voires supérieures) celles observées
chez d’autres fruits (date, raisin etc.) (Yang et al., 2006 ; Bilgari et al., 2008).
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Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
Les composés phénoliques, y compris les anthocyanines, contribuent significativement à
l’activité antioxydante du fruit. En effet, une réduction mesurée de la teneur en composés
phénoliques de 17% a été acompagnée par une réduction de 20% de l’activité antioxydante à
40°C. Le pourcentage de perte de l’activité antioxydante diminue en fonction de
l’augmentation de la température et reste toujours supérieur à celui de la perte des composées
phénoliques, ce qui montre que la production de composés antioxydants au cours du séchage
est très faible.
L’étude de la qualité organoleptique a montré que les paramètres chromatiques
(luminosité : L*, saturation : C* et angle de teinte : h°) ainsi que l’indice de brunissement ont
été affectés par le procédé de séchage, qui a contribué à une modification de la couleur des
graines de grenade. Afin de pouvoir mieux cerner l’origine de cette décoloration, il était
impératif de déterminer l’activité polyphénol oxydase (PPO) et la teneur en
hydroxyméthylfurfural (HMF). Dans cette optique, les résultats ont montré une faible activité
PPO et des teneurs réduites en HMF dans les graines de grenade après séchage. Par
conséquent, les réactions enzymatiques et non-enzymatiques (réaction de Maillard) n’ont pas
une influence significative sur le brunissement de la graine. Cela confirme les faibles valeurs
de l’indice de brunissement. Outre la couleur, la combinaison entre la DO et le séchage a
influencé la forme et la texture, puisque les graines ont perdu jusqu'à 55% de leur épaisseur
après séchage à 60°C.
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Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
6.2. Conclusion générale et perspectives :
Ce travail a permis de mettre en place, pour la première fois, un procédé global de
conservation des graines de grenade (Punica granatum L.). Ce procédé se compose de trois
étapes : un pré-traitement de congélation suivi par une déshydratation osmotique et terminé
par un post-traitement de séchage par entrainement. Plusieurs paramétres d’optimisation du
procédé ont été étudiés, nous permettant de retenir les conditions suivantes : une
déshydratation osmotique pendant 20 min en utilisant des graines congelées, une température
de 50°C et une solution de saccharose avec un extrait sec soluble de 55°Brix. Concernant le
séchage par entrainement, nous préconisons d’utiliser une température de 40°C, afin de
reduire la dégradation de la qualité nutritionelle des graines après 240 min du traitement.
A la fin de ce procédé, l’activité d’eau de la graine est inférieure à 0,65. Un tel niveau
d’activité d’eau assure une bonne stabilité au cours de l’entreposage. L’étude de la qualité
nutritionnelle des graines après DO et séchage révèle des pertes significatives en activité
antioxydante, composés phénoliques, anthocyanines qui laissent néanmoins au produit des
valeurs nutritionnelles intéressantes, et proches de celles d’autres fruits n’ayant subi aucun
traitement.
Cette thèse a également contribué à une valorisation d’une deuxième agrofourniture
tunisienne : les écarts de triage des dattes. A partir de ces dattes, nous avons préparé du jus
contenant naturellement du sucre, qui représente 35% de la quantité totale de sucre des
solutions d’immersion à 55°Brix. Cette substitution permet une amélioration de la qualité
organoleptique des graines, au travers d’un gain de solutés naturels au cours de la DO, mais
aussi une réduction du coût économique du procédé.
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Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
Cette recherche doctorale a conduit à plusieurs développements méthodologiques
adaptés aux caractéristiques des graines de grenade, en ce qui concerne en particulier
l’analyse de la texture et de la structure des graines.
Notons enfin qu’au dela des conclusions qui viennent d’être tirées, ce travail montre des
caractéristiques du produit fini qui semblent pouvoir justifier de nouvelles voies de
transformation et d’exploitation des graines de grenade. Pour consolider ce concept on se
propose dans un futur travail :
D’optimiser et de comparer l’effet des différents méthodes de séchage telles que :
le séchage solaire, la micro-onde sur les propriétés physico-chimique (vitamines, minéraux,
etc.) et antioxydante (composés phénoliques, etc.) des graines de grenade, afin de réduire la
dégradation de leurs qualité organoleptique et nutritionnelle.
D’étudier l’incorporation des graines de grenade séchées dans une matrice
alimentaire tel que le yoghourt ou le pain, et la caractérisation organoleptique (texture,
couleur, etc.) du produit fini. Cela permettra la consommation de ce fruit sous plusieurs
formes tout au long de l’année.
D’étudier la stabilité microbiologique (coliformes, flore totale et fongique) et
physico-chimique (teneur en anthocyanines et en polyphénols etc.) au cours du stockage des
graines séchées et incorporées dans des formulations alimentaires, ce qui permet de suivre le
vieillissement du produit au cours du temps.
D’envisager une étude sensorielle, autre que la texture en particulier, pour
déterminer le niveau de la satisfaction du consommateur envers les graines séchées et
incorporées dans des formulations alimentaires.
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Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
Pour que ce travail de laboratoire concernant la conservation des graines de grenade
soit clos et que l’on puisse passer à son extrapolation à l’échelle industrielle, quelques
éléments restent à réaliser tels que :
- L’exploitation des résultats obtenus à l’échelle laboratoire pour passer à l’échelle
pilote puis par extrapolation à l’échelle industrielle ;
- L’étude des équipements industriels de DO et de séchage ;
- La réalisation d’une étude du marché et du coût économique global du procédé.
189
Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
6.3. Références bibliographiques
Ade-Omowaye B., Rastogi N., Angersbach A. & Knorr D. (2003). Combined effects of
pulsed electric field pre-treatment and partial osmotic dehydration on air drying
behaviour of red bell pepper. Journal of Food Engineering, 60, 89-98.
Adsule, N.R., & Patill, N.B. (1995). Pomegranate. In: Salunkhe, D.K., Kadam, S.S., ed.
Handbook of Fruit Science and Technologies: Production, Composition, Storage and
Processing. New York, USA, pp.455-463.
Aslam, M.N., Lanky, E. P., & Varani, J. (2006). Pomegranate fractions promote proliferation
and procollagen synthesis and inhibit matrix metalloproteinase-1 production in human
skin cells. Journal of Ethno-Pharmacology, 103, 311-318.
Ben-Thabet, I., Attia, H., Besbes, S., Deroanne, C., Francis, F., Drira, N., & Blecker, C.
(2007). Physicochemical and Functional Properties of Typical Tunisian Drink: Date
Palm Sap (Phoenix dactylifera L.). Food Biophysics, 2, 76-82.
Besbes, S., Drira, L., Blecker, C., Deroanne, C., & Attia, H. (2009). Adding value to hard date
(Phoenix dactylifera L.): compositional, functional and sensory characteristics of date
jam. Journal of Food Chemistry, 112, 406-411.
Biglari, F., Abbas. F., & Easa, A. (2008). Antioxidant activity and phenolic content of various
date palm (Phoenix dactelifera) fruits from Iran. Food Chemistry, 107, 1636-1641.
Cheikh-Rouhou, S., Besbes, S., & Blecker, C. (2008). Black cumin (Nigella sativa L.) and
allepo pine (Pinus halepensis Mill.) seeds oils: stability during thermal oxidation at
60°C and 100°C. Microbiol. Hyg. Alim. 19, 12-20.
Curtay, J.P., Jacob, L., Jung, R.R., & Kaplan, M. (2008). Jus de grenade fermenté, la grenade,
“aliment-plus” un nouvel outil puissamment cardiovasculaire et anti-cancer dans
l’arsenal de la nutrithérapie, Ed. Macro pietteur, Paris, pp. 2-73.
Dermesonlouoglou, E.K., Pourgouri, S., & Taoukis, P.S. (2008). Kinetic study of the effect of
the osmotic dehydration pre-treatment to the shelf life of frozen cucumber. Innovative
190
Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
Food Science and Emerging Technologies, 9, 542-549.
Edas, S. (2009). Polyphénols et jus de grenade. Pharmacognosie, 7, 1-5.
Emna, A. (2010). Impulser l’investissement agricole privé. Magazine presse économique
Tunisie, n°3, pp. 15-16.
Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC&DOC-
Lavoisier, Paris, France, pp. 181-182.
FAOSTAT. (2009). Bases de données statistiques de la FAO, Food and Agriculture
Organization of the United Nations, Rome.
Fernandes, F., Rodrigues, S., Gaspareto, O., & Oliveira, E. (2006). Optimization of osmotic
dehydration of papaya followed by air-drying. Food Research International, 39, 492-
498.
Garca, M., Suana, D., Dulce, A., & Acinda, N. (2004). The effect of two methods of
pomegranate (Punica granatum L) juice extraction on quality during storing at 4°C.
Journal of Biomedicine and Biotechnology, 5, 332 - 337.
Garcia-Martinez, E., Martinezmonzo, J., Camacho, M., & Martineznavarrete, N. (2002).
Characterisation of reused osmotic solution as ingredient in new product
formulation. Food Research International, 35, 307-313.
Garcia-Segovia, P., Mognetti, C., André-Bello, A. & Martinez-Monzo, J. (2010). Osmotic
dehydration of Aloe vera (Aloea barbadensis Miller). Journal of Food Engineering,
97, 154-160.
Hernandez, F., Melgarejo, P., Tomas-Barberan, F.A., & Artes, F. (1999). Evolution of juice
anthocyanins Turing ripening of new selected pomegranate (Punica granatum L.)
clones. European Food Research and Technology, 210, 39-42.
Jaiswal, V., DerMarderosian, A., & Porter, J. (2010). Anthocyanins and polyphenol oxidase
from dried arils of pomegranate (Punica granatum L.). Food Chemistry, 118, 11-16.
Jeantet, R., Croguennec, T., Schuch, P., & Brulé, G. (2006). Science des aliments, TEC and
DOC-Lavoisier, Paris, pp. 121–161.
191
Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
Jena, S., & Das, H. (2004). Modelling for moisture variation during osmo-concentration in
apple and pineapple. Journal of Food Engineering, 66, 425-432.
Kovacs, E., & Meresz, P. (2004). The effect of harvesting time on the biochemical and
ultrastructural changes in Idared apple. Acta Alimentaria, 33, 285-296.
Kowalska, H., Lenart, A., & Leszczyk, D. (2008). The effect of blanching and freezing on
osmotic dehydration of pumpkin. Journal of Food Engineering, 86, 30-38.
Krokida, M.K., Karathanos, V.T., & Maroulis, Z.B. (2000). Effect of osmotic dehydration on
colour and sorption characteristics of apple and banana. Drying Technology, 18, 937-
950.
Masmoudi, M., Besbes, S., Blecker, C., & Attia, H. (2007). Preparation and characterization
of osmodehydrated fruits from lemon and date by-products. Journal of Food Science
and Technology international, 13, 405-412.
Mujumdar, A. S. (2006). Handbook Of Industrial Drying, 3rd Edition. Taylor and Francis
Group, LLC, 688-700.
Rastogi, N.K., & Raghavarao, K.S. (2004). Mass transfer during osmotic dehydration of
pineapple: considering Fickian diffusion in cubical configuration. Lebensmittel-
Wissenschaft und-Technologie, 37, 43-47.
Riva, M., Campolongo, S., Leva, A.A., Maestrelli, A., & Torreggiani, D. (2005). Structure
property relationships in osmo-air-dehydrated apricot cubes. Food Research
International, 38, 533-542.
Shi, J., Pan, Z., McHugh, T., Wood, D., Hirschberg, E., & Olson, D. (2008). Drying and
quality characteristics of fresh and sugar-infused blueberries dried with infrared
radiation heating. LWT-Food Science and Technology, 41, 1962-1972.
Storey, T. (2007). La grenade, le fruit médicament, Magazine NEXUS. Santé, France, 51, 46-
54.
Torreggiani, D., & Bertolo, G. (2001). Osmotic pre-treatments in fruit processing: chemical,
physical and structural effects. Journal of Food Engineering, 49, 247-253.
192
Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________
193
Uddin, M., Ainsworth, P., & Ibanoglu, S. (2004). Evaluation of mass exchange during
osmotic dehydration of carrots using response surface methodology, Journal of Food
Engineering, 65, 473-477.
Wang, W.C., & Sastry, S.K. (2000). Effects of thermal and electrothermal pre-treatment on
hot air drying rate of vegetable tissue. Journal of Food Process Engineering, 23, 299-
319.
Yang, R.Y., Tsou, S. C. S., Lee, T. C., Chang, L. C., Kuo, G., Lai, P. Y. (2006). Moringa a
novel plant rich in antioxidants, bioavailable iron, and nutrients. In: C. T. Ho (ed)
Challenges in Chemistry and Biology of Herbs.American Chemical Society,
Washington, USA, D.C. pp. 224-239.
Liste des figures
194
Liste des figures ___________________________________________________________________________
Chapitre 1 : .............................................................................................................................. 7
Figure 1. Les différents systèmes de mise en contact des phases (solution osmotique et
l’aliment) pour une déshydratation osmotique (Marouzé et al., 2001)..................... 32 Chapitre 2 : ............................................................................................................................ 44
Figure 1’. Différentes étapes du procédé de déshydratation osmotique.............................. 46 Figure 1. Variation of water loss (WL) (a) weight reduction (WR) (b) and solids gain (SG)
(c) with time and temperature (30, 40, 50°C) using sucrose solution during osmotic dehydration ............................................................................................................... 61
Figure 2. DSC thermogram obtained for pomegranate seeds soaked in sucrose solution at 50°C .......................................................................................................................... 65
Figure 3. Comparison of water loss (WL) (a) and solids gain (SG) (b) using different osmotic solutions (sucrose, glucose and mixture sucrose & glucose) at 50°C......... 70
Chapitre 3 : ............................................................................................................................ 77
Figure 1’. Différentes étapes du procédé de déshydratation osmotique des fraiches et
congelées................................................................................................................... 79 Figure 1. Comparison of WL and SG using fresh (× WL, ● SG) and frozen (ΔWL, ○SG)
seeds during osmotic dehydration process................................................................ 91 Figure 2. Scanning electron microscopy photographs of fresh (a), frozen (b) and
osmodehydrated fruits prepared with fresh (c) and frozen (d) seeds........................ 97 Figure 3. Characteristic force-distance curve for texture analysis using fresh seeds.......... 98
Chapitre 4 : .......................................................................................................................... 107 Figure 1’. Procédé de déshydratation osmotique des graines de grenade dans du jus de
datte......................................................................................................................... 109 Figure 1. D Water loss (WL: ♦), weight reduction (WR: ◌) and solids gain (SG: ×) from
the osmodehydrated seeds using Deglet Nour as an immersion solution base....... 122 Figure 2. D Scanning electron microscopy photographs of frozen (a) and osmodehydrated
seeds (b) and a characteristic force-distance curve for texture analysis using frozen (•) and osmodehydrated (‒) seeds (c)...................................................................... 128
Figure 3. Flow behaviour of the osmotic solutions after 120 min of the process ............. 131 Chapitre 5 : .......................................................................................................................... 139
Figure 1’. Différentes étapes des procédés de déshydratation osmotique et de séchage des
graines de grenade................................................................................................... 141 Figure 1. Variation of water activity (a) and dry matter (b) of pomegranate seeds as a
function with time and temperature (40, 50, 60 °C) ............................................... 158 Figure 2. DSC thermograms obtained for osmodehydrated (OD) and dried pomegranate
seeds at different temperature (40, 50, 60 °C) ........................................................ 160
195
Liste des tableaux
196
Liste des tableaux ___________________________________________________________________________
Chapitre 1 : .............................................................................................................................. 7 Tableau 1. Application de la déshydratation osmotique sur des fruits (1a) et légumes (1b)
................................................................................................................................... 14 Tableau 2. Les conditions optimales de déshydration osmotique des fruits (2a) et légumes
(2b). ........................................................................................................................... 23 Tableau 3. Différents travaux utilisant une solution ternaire pour le traitement des fruits par
déshydratation osmotique ......................................................................................... 28 Chapitre 2 : ............................................................................................................................ 44
Tableau 1. Chemical characteristic of pomegranate seeds .................................................. 57 Tableau 2. Evolution of osmotic dehydration parameters in sucrose solution at different
temperatures 30, 40, and 50 °C................................................................................. 58 Tableau 3. Water and solids effective diffusivities calculated by Fick’s model ................. 60 Tableau 4. Values of Peleg’s equation parameters for water loss and solids gain .............. 62 Tableau 5. CieLab coordinates of sucrose solution at different temperatures 30, 40, and 50
°C .............................................................................................................................. 64 Tableau 6. Differential scanning calorimetry results for pomegranate seeds over soaking
time in sucrose, glucose and mixture sucrose and glucose solution at 50 °C........... 68 Chapitre 3 : ............................................................................................................................ 77
Tableau 1. Chemical characteristic of pomegranate seeds .................................................. 90 Tableau 2. Evolution of osmotic dehydration parameters in sucrose solution using frozen
and fresh seeds .......................................................................................................... 94 Tableau 3. Water and solids effective diffusivities calculated by Fick’s model and values of
Peleg’s equation parameters...................................................................................... 95 Tableau 4. Textural properties of pomegranate seeds ....................................................... 100
Chapitre 4 : .......................................................................................................................... 107
Tableau 1. Evolution of osmotic dehydration parameters of pomegranate seeds and osmotic
solution (using Deglet Nour date juice) .................................................................. 125 Tableau 2. Physico-chemical properties of pomegranate seeds before and after osmotic
dehydration process ................................................................................................ 126 Chapitre 5 : .......................................................................................................................... 139
Tableau 1. Chemical characteristic of pomegranate seeds ................................................ 155 Tableau 2. Effective diffusivities calculated by Fick’s model and values of Peleg’s
equation parameters (K1 and K2) ............................................................................ 157 Tableau 3. Value of total phenolic, anthocyanin and antioxidant activity of untreated,
osmotic dehydrated and dried seeds........................................................................ 162 Tableau 4. Effect of air-drying temperature on chromatics coordinates and on textural
properties of seeds................................................................................................... 166
197
Productions scientifiques ___________________________________________________________________________
PRODUCTIONS SCIENTIFIQUES
Publications et communications scientifiques réalisées dans le cadre du Doctorat
1. Publications
Bchir B., Besbes S., Giet, J., Attia H., & Blecker C. 2010. Synthèse des connaissances sur la
déshydratation osmotique. Biotechnologie Agronomie Société et Environnement (in
press).
Bchir B., Besbes S., Attia H., & Blecker C. 2009. Osmotic dehydration of pomegranate seeds:
Mass transfer kinetics and DSC characterisation. International Journal of Food
Science and Technology, 44, 2208-2217.
Bchir B., Besbes S., Attia H., & Blecker C. 2010. Osmotic dehydration of pomegranate seeds
(Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process
Engineering, DOI: 10.1111/j.1745-4530.2010.00591.x.
Bchir B., Besbes S., Karoui R., Paquot M., Attia H., & Blecker C. 2010. Osmotic dehydration
kinetics of pomegranate seeds using date juice as an immersion solution base, Food
and Bioprocess Technology, DOI: 10.1007/s11947-010-0442-1.
Bchir B., Besbes S., Karoui R., Attia H., Paquot M., & Blecker C. 2010. Effect of air-drying
conditions on physico-chemical properties of osmotically pre-treated pomegranate
seeds, Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0469-3.
2. Communications
Bchir B., Roiseux O., Attia H., Deroanne C., & Blecker C. Contribution to the valorisation of
pomegranate (Punica granatum L.). 11ème édition du BioFrorum organisé par
l’université de liège, Belgique, 11 octobre 2007, (Poster).
Bchir B., Besbes S., Karoui R., Paquot M., Attia H., & Blecker C. Utilisation du jus de datte
comme milieu d’immersion pour la déshydratation osmotique des graines de grenade
(Punica granatum L.). Journée scientifique de la Société Royale de Chimie sur le
thème « Chimie verte » organisé par l’Université de Liège Gembloux Agro-bio tech,
Belgique, 14 octobre 2010, (Poster).
198