LJFP.book(LJFP_A_174372.fm)
International Journal of Food Properties, 9: 877888, 2006
Copyright Taylor & Francis Group, LLC ISSN: 1094-2912 print
/ 1532-2386 online DOI: 10.1080/10942910600744098
SECADO CON AIRE FRESCO Y COMPORTAMIENTO DE LAS REBANADAS DE
CEBOLLA DESHIDRATAS OSMTICAMENTE (Allium cepa) Y los frutos de
tomate (Lycopersicon esculentum) Claudine Valrie Passo Tsamo,
Anny-Flore Bilame andRobert NdjouenkeuDepartment of Food Science
and Nutrition, University of Ngaoundr, Ngaoundr, Cameroon.Rebanadas
de la cebolla ( 0,5 cm de grosor) y frutos de tomate , fresco y
osmticamente deshidratadas en azcar( 600 g / l ) , sal ( 300 g / l)
o sal mixta y el azcar ( 45/15 , w / w ) soluciones , se secaron al
aire a 60 C y de secado constantes determinadas . El secado al aire
se produjo en dos tasas de periodos de cada, caracterizado cada uno
por una constante de secado. La velocidad de secado y la
difusividad de humedad de osmotizada muestras fueron mayores que la
de las muestras frescas. El comportamiento general de secado de
rodajas de cebolla osmotizada no fue influenciada por las
condiciones osmticas (solucin y tiempo), mientras que el
comportamiento de secado de muestras de tomate osmotizada dependa
de solucin osmtica y el tiempo de pre - tratamiento. 15 minutos y
20 horas de osmosis pre parecan ms conveniente antes del secado de
rodajas de cebolla y tomate , respectivamente . A este respecto, la
modificacin estructural de las clulas de la membrana de tomate
durante el tratamiento previo prolongado pareca explicar la
variacin en el comportamiento de secado.Palabras clave:
deshidratacin osmtica , rodajas de cebolla, tomate, secado
constante de velocidad , difusividad aparente .INTRODUCCIN
El secado al sol es una de las tcnicas ms asequibles que usan
los agricultores para extender la vida til de los alimentos. El
principio de la tcnica se basa en la eliminacin de agua libre de
los alimentos , con el fin de evitar tanto la degradacin enzimtica
y microbiana . Puesto que los componentes biolgicos de los
alimentos son susceptibles a la degradacin de calor , muchos
alimentos secos presente desnaturalizacin fsica y organolptica , lo
que resulta en la reduccin de la de su valor de mercado . A este
respecto, diferentes tcnicas de secado se han desarrollado hacia la
eliminacin de agua con cuidado para preservar la integridad qumica
y fsica de los alimentos.Muchos trabajos cientficos han demostrado
el inters de la deshidratacin osmtica de alimentos antes de secar
[1,2,3,4,5]El tratamiento osmtico tiene la ventaja de mantener
organolpticas - tic (color , textura , aroma ) y nutricionales
(vitaminas, minerales ) caractersticas de los alimentos . El
tratamiento consiste en sumergir el producto en una solucin
concentrada , que absorbe el agua del producto por smosis . La
transferencia de agua va acompaada de intercambio de solubles entre
el producto y la solucin . Despus de este paso osmtica inicial , el
producto osmotizada por lo tanto, puede ser secado utilizando un
mtodo de secado convencional, tal como secado por aire caliente ,
para producir un estante estable , comida seca .Proceso osmtica es
un pre-tratamiento muy adecuado antes de su secado al aire de
frutas y verduras, ya que inhibe la polifenol oxidasa y prevenir el
pardeamiento oxidativo..[6] El intercambio material simultnea entre
el producto alimenticio y la solucin osmtica est acompaada por una
importante reduccin, la deformacin y la interaccin de flujo.
[7,8,9,10] Estos cambios en la composicin y la estructura de la
comida du-rante el paso osmtica puede influir en el comportamiento
de secado de aire ms tarde. Diferentes estudios han informado de
una reduccin de la velocidad de secado despus de la deshidratacin
osmtica de los alimentos utilizando la solucin de sacarosa, y han
atribuido la disminucin de la velocidad de secado a la presencia de
sacarosa infusa. [2,11]Un estudio previo de transferencia de
material durante la deshidratacin osmtica de rodajas de cebolla y
frutos de tomate en diferentes soluciones osmticos (cloruro sdico,
sacarosa y mezcla de cloruro de sodio y sacarosa) mostr un
predominio de ganancia solubles sobre solubles, con un mejor
control de la penetracin de soluto cuando se mezcla osmtica se
utiliz solucin. Adems, el tratamiento extendido, ms all de 15
minutos y 20 horas, respectivamente, para rodajas de cebolla y
tomate, dio lugar a alteracin estructural de las clulas, y la
perturbacin de los intercambios de material..[10]El presente
estudio tiene como objetivo examinar el comportamiento de secado al
aire de las rodajas de cebolla por encima de la presin osmtica, sed
y frutos de tomate. Los agricultores de la regin del Lago Chad en
frica Central han desarrollado la produccin local de las rebanadas
de cebolla seca y polvo de tomate, pero la calidad de mercado de
estos productos desecados se reduce por su color oscuro.[12] Por lo
tanto, la introduccin de un paso osmtico en el proceso de secado
parece una buena oportunidad para mejorar la calidad.MATERIALES Y
METODOSFUENTE Y PREPARACIN DE RODAJAS DE CEBOLLA Y TOMATE Frutos de
cebolla (Allium cepa), comprados en el mercado local (Ngaoundr,
Camern), eran focos de 8 a 12 cm de dimetro longitudinal y 6-8 cm
de dimetro transversal. Se eliminaron los sobres externas de las
bombillas, y la carne se lav con agua destilada, a continuacin, en
rodajas horizontalmente con el fin de obtener anillos de 0,5 cm de
espesor. Frutos de tomate rojo (Lycopersicon esculentum), cultivar
"Roma", se recogieron de una granja local (Ngaoundr, Camern). Todas
las frutas tenan forma ovoide con una longitud de 5,2 0,4 cm, un
dimetro medio de 3,3 0,2 cm, y un peso de 45 2 g. Los frutos
seleccionados fueron se lav en agua destilada y se utiliza para los
ensayos durante la semana despus de la cosecha. Los tomates fueron
tratados sin rebanar.PRE-TRATAMIENTO OSMTICOLas muestras fueron
osmotizada en frascos de vidrio sellados a 25 C para las rebanadas
de cebolla y 60 C para los frutos de tomate, con agitacin
permanente (80 giro / min), utilizando una relacin producto /
solucin de 1/10 (W / W). Los medios de comunicacin osmtico consisti
en soluciones de sacarosa comercial (600 g / l), cloruro de sodio
comercial (300 g / l), o una mezcla de sacarosa y NaCl (45/15, w /
w). Para cada solucin, las muestras se sumergieron durante 5, 10,
15, 30, 60, y 120 minutos de rodajas de cebolla, y 1, 5, 10, 20,
40, 60, 80, y 100 horas para los tomates.[10]TRATAMIENTO DE
SECADOLas muestras no tratadas y osmotizada se colocaron en capas
simples y gaseosas en bandejas de secado previamente pesados, y se
secaron a 60 C en un secador de cabina de flujo transversal
(Heraeus T6), con una tasa de flujo de aire de 0,13 ms-1. Bandejas
de secado se pesaron peridicamente durante el secado proceso. Para
cada muestra, las bandejas se prepararon por triplicado, y a partir
de los tres valores de peso de la bandeja, promedio de humedad de
la muestra se determinaron como una funcin de tiempo. Estos valores
se utilizaron para construir las curvas de secado.APROXIMACIN
TERICASuponiendo que la descripcin del comportamiento de secado de
las muestras por la ecuacin de difusin de Fick y la solucin
analtica dada por manivela, [13] se expres la relacin de humedad
extrable (Hr) de las muestras durante el secado usando la ecuacin
simplificada de Perry et al. [14] y Henderson y Perry: [15]
Dnde: Ht (gH2O.g1 DM) es el contenido de humedad dependiente del
tiempo de la muestra; H0 es el contenido de humedad inicial de la
muestra; He es el contenido de humedad de equilibrio (al final del
proceso de secado); D (m2 h1) es la difusividad de humedad
aparente; T (h) es el tiempo de secado, y, k (h1) es la constante
de secado, obtenido a partir de la trama de lnHrvs. t. Desde los
experimentos de secado se llevan a cabo durante mucho tiempo hasta
que se acerc tasa de equilibrio, se supone que en el equilibrio, la
velocidad de secado es cero; [3] es decir:Therefore, from plots of
near equilibrium, final values of
dHt dt
vs.H0, and through
regression analyses, He values were determined as the point
where the graph cuts the Htaxis.[3] The values obtained were
included in Eq. (1) to determine drying constants.
The apparent moisture diffusivities D, during the drying process
were estimated at various moisture contents by the method of Perry
et al.,[14] as described by Sankat et al.[3]2For a given value of
Hr, the theoretical value of Dt / L0
is calculated from Eq. 1, and the
predicted experimental value of t obtained from the linear
regression of Hr vs. t. Then D isobtained from:(D / L2)D = t 0 th
(t / L2)
(3)
where subscripts th and exp refer to theoretical and
experimental values, respectively.
RESULTS AND DISCUSSIONDrying curves of untreated and osmosed
onion slices and tomato fruits are shown respectively on Figs. 1
and 2. The difference in initial moisture content of osmosed prod-
ucts reflects the varying degrees of water loss during osmotic
pre-treatment and indicates reduction of residual water content
with increasing of osmotic treatment time.[9] In this respect,
initial water content is higher in the sugar osmosed samples than
in the salt and mixed osmosed ones. In spite of these initial
differences, all curves show convergence after 20 hours and 60
hours of drying, respectively, for onion slices and tomato.
The predicted values of equilibrium moisture contents (He)
attained at the end of the drying process (Table 1) show that in
onion slices, He decreases with the increase in pre- osmosis time,
whatever the osmotic solution used. When osmotic pre-treatment was
done for more than 15 minutes in a given osmotic solution, the
decrease in He was not signifi-
cantly different. He values of tomato samples are relatively
comparable when pre-treatmenthas been less than 20 hours. Above
this pre-treatment time, He values become irregular.The previously
mentioned behaviour in He values could be related to the structural
alter-ation of cells and disturbance of material exchange
previously observed for extended osmotictreatments. The cell
alteration occurred after 15 minutes and 20 hours of osmosis,
respectively, for onion slices and tomato.[10] It could be
hypothesized that the destruction of the proteins of cells membrane
by osmotic shock, after extended osmotic treatment,[16] disturbed
the water- binding capacity of the macromolecules. For short and
medium time osmosed products, i.e., equal or less than 15 minutes
and 20 hours, respectively, for onion slices and tomatoes, the
equilibrium moisture contents at the end of the drying process are
comparable to the untreated samples; which indicate that the cell
structure of these samples are relatively intact.The drying rate
curves (Figs. 3,4) show two periods of drying. In the early period
of dry- ing, when moisture content is the highest, drying rates
start with the highest values, dependent upon the level of osmotic
pre-treatment, then decline rapidly. After this initial period of
rapid decline, the drying rate curves continue to decline, but more
gradually and in a linear fashion, to equilibrium conditions. In
this second period, the differences in equivalent drying rate
between fresh and osmosed samples are not easily perceptible. The
break point separating thetwo falling rate periods occurs at
moisture contents of about 2g H2O.g1 (DM basis) and 0.75gH2O.g1 (DM
basis), respectively, for onion slices (Fig. 3) and tomato (Fig.
4).From linear regression analyses of the drying rate (lnHr vs. t),
considering transientmoisture content values in each of the two
falling rate periods, drying constants k1 and k2 wereestablished
(Eq. 1) for all drying runs (Tables 2, 3). k1 and k2 represent the
drying constantsrespectively in the first falling rate period, when
drying rates are high, and in the second fallingrate period, when
decline in drying rate is linear and drying curves are about the
same. In all drying runs, k1 is higher than k2, confirming a higher
drying rate in the first falling rate period.In onion slices, for
each falling-rate period, drying constants of osmosed samples
are relatively comparable for all osmotic pre-treatment times
and solutions used. There- fore, k1 and k2 means have been
calculated (Table 2). In addition, it should be noted that the
drying of untreated onion slices does not show two falling-rate
periods (k1 is almost equal to k2), indicating that in this case,
drying occurs in one falling rate. In tomato (Table
3), k1 constants are comparable and higher than k2 for samples
pre-osmosed during 5h and10h. During this drying time, drying rate
is in general higher for salt pre-osmosed samples
than for sucrose pre-osmosed samples and than for mixed
pre-osmosed ones. Above this pre-treatment time, there is a change
in the trend of drying constant values for all pre-osmosis
solutions, with tendency of k1 values to increase for sucrose and
mixed pre-osmosed
Pre-osmosis in Salt solution10 10
Pre-osmosis in Sugar solution10
Pre-osmosis in mixed solution8 8 8
Untreated5 min osmosis10 min osmosis15 min osmosis30 min
osmosis60 min osmosis120 min osmosis6 6 64 4 42 2 200 1 2 3 4
10 20 30 40
050 0 1 2 3 4
10 20
30 40
050 0 1 2 3 4
10 20
30 40 50
Drying time (hours)Figure 1 Drying curves of onion slices from
different osmotic pre-treatments.
Pre-osmosis in salt solution25 25
Pre-osmosis in sugar solution25
Pre-osmosis in mixed solution20 2015 15
20 Untreated5h osmosis10h osmosis20h osmosis40h osmosis60h
osmosis15 80h osmosis10 10 105 5 50010
20 40 60
080 100 0 10
20 40 60
080 100 010
20 40
60 80
100
Drying time (hours)Figure 2 Drying curves of tomato from
different osmotic pre-treatments.
Table 1 Equilibrium moisture contents (g H20.g1 DM) of dried,
fresh, and osmosed onion slices and tomato.Osmotic
solutionOsmotic
pre-treatment
NaCl solutionSucrose solutionMixed solutionOnion
slicesUntreated0.34 0.165 min0.42 0.040.42 0.120.35 0.0510 min0.29
0.050.27 0.040.39 0.0415 min0.18 0.050.20 0.060.31 0.0530 min0.11
0.040.15 0.040.25 0.0760 min0.12 0.070.15 0.070.29 0.08120 min0.18
0.050.23 0.080.24 0.03Tomato
Untreated0.01 0.005 h0.02 0.000.02 0.000.02 0.0010 h0.04
0.010.02 0.010.02 0.0020 h0.12 0.020.13 0.020.10 0.0340 h0.08
0.010.09 0.020.04 0.0060 h0.03 0.010.26 0.010.01 0.0080 h0.03
0.010.20 0.010.01 0.00samples, while for salt pre-osmosed tomato
samples the drying tends to occur in one fall- ing period (k1 K2).
The incrustation of sugar molecules at the surface of cytoplasm
dur-
ing long period of osmotic pre-treatment and the resulting
alteration of cell membrane[10,17] may explain the increase in
drying rate for sucrose and mixed pre-osmo- sed tomato samples. The
presence of salt in mixed solution seems to have contributed to
limit the incrustation of sugar and the physical alteration of cell
membrane during osmotic pre-treatment,[7] which may be the reason
why the increase in drying rate is lower for mixed solution
pre-osmosed tomato samples than in sucrose pre-osmosed ones.
The calculation of apparent moisture diffusivity of onion slices
during drying, as a func- tion of samples moisture content (Fig. 5)
shows that during drying, moisture diffusivityTable 2 Drying
constants k1 and k2, during the air drying of onion slices as a
function of osmotic pre-treatment time and solution.
Osmotic solutionOsmotic
pre-treatment
NaCl solutionSucrose solutionMixed solutionk1 (h1)k2 (h1)k1
(h1)k2 (h1)k1 (h1)k2 (h1)Untreated0.250.250.25
5 min0.520.270.440.080.360.20
10 min0.440.170.400.100.300.16
15 min0.510.110.490.090.330.25
30 min0.430.150.410.080.480.26
60 min0.390.120.330.110.370.20
120 min0.330.170.420.100.360.20
Mean0.440.160.420.090.370.21
Pre-treatment in salt solution10
Pre-treatment in sugar solution10
Pre-treatment in mixed solution108 8 8
Fresh sample5min. osmosis10min. osmosis15min. osmosis30min.
osmosis60min. osmosis120min. osmosis6 6 64 4 42 2 200 1 2 4 6
08 10 0 1 2
4 6 8
010 0 1
2 4 6
8 10Moisture Content (gH O.g-1
DM)
Figure 3 Drying rate curves for onion slices from different
osmotic pre-treatments.
0,70,60,50,4
Pre-treatment in salt solution
0,70,60,50,4
Pre-treatment in sugar solution
0,70,60,50,4
Pre-treatment in mixed solutionFresh sample
5h osmosis
10h osmosis
20h osmosis
40h osmosis
60h osmosis
80h osmosis
0,3
0,3
0,30,2
0,2
0,20,1
0,1
0,10,0
0,0
0,5
4,0
8,0
12,0
0,0
0,0
0,5
4,0
8,0
12,0
0,0
0,0
0,5
4,0
8,0
12,0Moisture content (g H2O.g
DM)
Figure 4 Drying rate curves for tomato from different osmotic
pre-treatements.
Table 3 Drying constants k1 and k2, during the air drying of
tomato fruits as a function of osmotic pre-treatment time and
solution.
Osmotic solutionNaCl solutionSucrose solutionMixed
solutiondecreases with the decrease of moisture content. This
behaviour is normal since moisture migration becomes increasingly
difficult as the consequence of toughness and hardness of sample
structure during drying. Meanwhile, though residual water is higher
in untreated onion than in pre-osmosed one, the moisture
diffusivity during drying is higher in the later; the mini-mum
predicted diffusivity value in this case is around 1.5 108 m2h1
when moisture contenttends towards zero, while in unosmosed onion
slice, the minimum diffusivity is around 0 when the moisture
content is still around 8 gH2O g1 DM (Fig. 5). This observation
indicates thatpre-osmosed dry sample contains less residual
moisture than untreated dry sample.During the decrease of apparent
diffusivity, for a given moisture content of pre- osmosed onion, D
is higher as the pre-treatment time increases, particularly between
5 minutes to 60 minutes of pre-osmosis. Above 60 minutes, D
decreases. In addition, from the trend of moisture diffusivity
(Fig. 5), pre-osmosed samples can be divided in 2 homogenous groups
characterised by their pre-treatment time (5 to 15 minutes and
above 15 minutes), and their comparable diffusivity towards
equilibrium, particularly for sugar and mixed pre osmo- sed
samples. These observations are related to the trend of equilibrium
moisture content (Table1); He decreased as pre-osmosis time
increased up to 15 minutes, and remained constant for sam- ples pre
treated above 15 minutes. From 60 minutes of pre-treatment, change
in trend of He and D values may be attributed to alteration of
cells membrane due to long osmotic pre-treatment.
CONCLUSIONThis study has shown that extension of osmotic
pre-treatment results in reduction of equilibrium moisture content
at the end of air drying. This reduction in He value seems to be
related to the alteration of the cells membrane during long osmotic
pre-treatment. In this respect, and considering the mastering of
the drying process, 15 minutes of osmosis seems an acceptable
pre-treatment time before drying of onion slices, while for tomato,
pre-osmosis should end before 20 hours.
Air drying occurs in two falling rate periods, whose constants
depend on the nature of the food material. For onion slices,
osmotic pre-treatment increases the drying con- stants; but since
the cells membrane do not seem to be significantly affected by
osmotic pre-treatment, the drying constants are not significantly
influenced neither by the osmotic solution, nor by the time of
pre-treatment. On the other hand, for tomato, the structure of
Pre-Osmosis in salt solution
Pre-Osmosis in sugar solution
Pre-Osmosis in mixed solution20 2015 15
Untreated20 5 min10 min15 min30 min15 60 min120 min10 10 105 5
500 2 4 6
08 10 0 2 4
6 810
00 2 4 6
8 10
Moisture content (g H2O g-1 DM)Figure 5 Changes in apparent
moisture diffusivity in onion slices as a function of residual
moisture content during the drying process.
cells membrane are more affected by osmotic conditions (time and
solution) and results thus in variation of drying constant
values.With respect to the structural effect of osmotic
pre-treatment on the drying behav- iour of food, the study of the
ultra structure of cells both during osmosis and air drying
represents a research opportunity for better understanding of the
behaviour of food materi- als during these treatments.
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0 exp
Water content (g/100g DW)
881
Water content (g/100g DW)
882
2
Drying rate (gH2O.g DM.h )
-1-1
884
885
Osmotic
pre treatment
k1 (h1)
k2 (h1)
k1 (h1)
k2 (h1)
k1 (h1)
k2 (h1)Untreated0.150.030.150.030.150.035
h0.240.050.160.040.100.0410 h0.320.090.140.030.070.0120
h0.100.060.100.040.060.0140 h0.140.050.200.040.090.0260
h0.080.050.440.060.130.0180 h0.040.430.060.24
Moisture diffusivity, Dx10 (m2h-1)
887
