Food Properties HandbookSecond Edition 2008 by Taylor &
Francis Group, LLC. 2008 by Taylor & Francis Group, LLC.Food
Properties HandbookEdited byM. Shaur RahmanSecond Edition 2008 by
Taylor & Francis Group, LLC.CRC PressTaylor & Francis
Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL
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/ edited by M. Shafiur Rahman. -- 2nd ed.p. cm.Previous edition has
main entry under Rahman, Shafiur.Includes bibliographical
references and index.ISBN-13: 978-0-8493-5005-4ISBN-10:
0-8493-5005-01. Food--Analysis. 2. Food industry and trade. I.
Rahman, Shafiur. II. Title.TX541.R37 2009664--dc222008019583Visit
the Taylor & Francis Web site
athttp://www.taylorandfrancis.comand the CRC Press Web site
athttp://www.crcpress.com 2008 by Taylor & Francis Group,
LLC.ContentsPrefaceAcknowledgmentsEditorContributorsChapter 1Food
Properties: An OverviewMohammad Shaur RahmanChapter 2Water Activity
Measurement Methods of FoodsMohammad Shaur Rahman and Shyam S.
SablaniChapter 3Data and Models of Water Activity. I: Solutions and
Liquid FoodsPiotr P. LewickiChapter 4Data and Models of Water
Activity. II: Solid FoodsPiotr P. LewickiChapter 5Freezing Point:
Measurement, Data, and PredictionMohammad Shaur Rahman, K.M.
Machado-Velasco, M.E. Sosa-Morales,and Jorge F. Velez-RuizChapter
6Prediction of Ice Content in Frozen FoodsMohammad Shaur
RahmanChapter 7Glass Transitions in Foodstuffs and Biomaterials:
Theory and MeasurementsStefan KasapisChapter 8Glass Transition Data
and Models of FoodsMohammad Shaur Rahman 2008 by Taylor &
Francis Group, LLC.Chapter 9Gelatinization of StarchShyam S.
SablaniChapter 10Crystallization: Measurements, Data, and
PredictionKirsi Jouppila and Yrj H. RoosChapter 11Sticky and
Collapse Temperature: Measurements, Data, and PredictionsBenu P.
Adhikari and Bhesh R. BhandariChapter 12State Diagrams of
FoodsDidem Z. Icoz and Jozef L. KokiniChapter 13Measurement of
Density, Shrinkage, and PorosityPanagiotis A. Michailidis,
Magdalini K. Krokida, G.I. Bisharat,Dimitris Marinos-Kouris, and
Mohammad Shaur RahmanChapter 14Data and Models of Density,
Shrinkage, and PorosityPanagiotis A. Michailidis, Magdalini K.
Krokida, and Mohammad Shaur RahmanChapter 15Shape, Volume, and
Surface AreaMohammad Shaur RahmanChapter 16Specic Heat and Enthalpy
of FoodsR. Paul Singh, Ferruh Erdogdu, and Mohammad Shaur
RahmanChapter 17Thermal Conductivity Measurement of FoodsJasim
Ahmed and Mohammad Shaur RahmanChapter 18Thermal Conductivity Data
of FoodsJasim Ahmed and Mohammad Shaur Rahman 2008 by Taylor &
Francis Group, LLC.Chapter 19Thermal Conductivity Prediction of
FoodsMohammad Shaur Rahman and Ghalib Said Al-SaidiChapter
20Thermal Diffusivity of Foods: Measurement, Data, and
PredictionMohammad Shaur Rahman and Ghalib Said Al-SaidiChapter
21Measurement of Surface Heat Transfer CoefcientShyam S.
SablaniChapter 22Surface Heat Transfer Coefcients with and without
Phase ChangeLiyun Zheng, Adriana Delgado, and Da-Wen SunChapter
23Surface Heat Transfer Coefcient in Food ProcessingPanagiotis A.
Michailidis, Magdalini K. Krokida, and Mohammad Shaur RahmanChapter
24Acoustic Properties of FoodsPiotr P. Lewicki, Agata Marzec, and
Zbigniew RanachowskiAppendix AAppendix BAppendix C 2008 by Taylor
& Francis Group, LLC. 2008 by Taylor & Francis Group,
LLC.PrefaceAfoodpropertyisaparticularmeasureofafoodsbehaviorasamatteroritsbehaviorwithrespect
to energy, or its interaction with the human senses, or its efcacy
in promoting human healthand well-being. An understandingof
foodpropertiesis essentialforscientists and engineers whohave to
solve the problems in food preservation, processing, storage,
marketing, consumption, andeven after consumption. Current methods
of food processing and preservation require accurate
dataonfoodproperties; simple, accurate, andlow-cost measurement
techniques; predictionmodelsbased on fundamentals; and links
between different properties. Therst edition was a well
receivedbestseller, and it received an award. Appreciation from
scientists, academics, and industry
profes-sionalsaroundtheglobeencouragedmetoproduceanupdatedversion.
Thiseditionhasbeenexpanded with the addition of some new chapters
and by updating the contents of therst edition.The seven chapters
in therst edition have now been expanded to 24 chapters.In this
edition, the denition of the terminology and measurement techniques
are clearly presented.The theory behind the measurement techniques
is described with the applications and limitations ofthe methods.
Also, the sources of errors in measurement techniques are compiled.
A compilation ofthe experimental data from the literature is
presented in graphical or tabular form, which should bevery useful
for food engineers and scientists. Models can reduce the number of
experiments,
therebyreducingtimeandexpensesofmeasurements.Theempiricalandtheoreticalpredictionmodelsarecompiled
for different foods with processing conditions. The applications of
the properties are alsodescribed, mentioning where and how to use
the data and models in food processing.Chapter 1 provides an
overview of food properties, including their denition,
classication, andpredictions.
Chapters2through4presentwateractivityandsorption
isothermandincludetermin-ology, measurement techniques, data for
different foods, and prediction models. Chapters 5 through12present
thermodynamicandstructural characteristicsincludingfreezingpoint,
glasstransition,gelatinization, crystallization, collapse,
stickiness, ice content, and state diagram. Chapters 13 through15
discuss the density, porosity, shrinkage, size, and shape of foods.
Chapters 15 through 23 presentthe thermophysical properties
including specic heat, enthalpy, thermal conductivity,
thermaldiffusivity, and heat transfer coefcient. Chapter 24
provides the acoustic properties of foods.This second edition will
be an invaluable resource for practicing and research food
technologists,engineers, and scientists, and a valuable text for
upper-level undergraduate and graduate students
infood,agriculture=biologicalscience, andengineering. Writingsuch
abook isachallenge, andanycommentstoassistinfuturecompilationswill
beappreciated. Anyerrorsthat remainareentirelymine. I am condent
that this edition will prove to be interesting, informative, and
enlightening.Mohammad Shaur RahmanSultan Qaboos UniversityMuscat,
Sultanate of Oman 2008 by Taylor & Francis Group, LLC. 2008 by
Taylor & Francis Group,
LLC.AcknowledgmentsIwouldliketothankAlmightyAllahforgivingmelifeandblessingtogainknowledgetoupdate
this book. I wish to express my sincere gratitude to the Sultan
Qaboos University (SQU) forgivingme the opportunity and facilities
to initiate suchan exciting project to develop the secondedition,
and supporting me toward my research and other intellectual
activities. I would also like tothank all my earlier employers,
Bangladesh University of Engineering and Technology, Universityof
New South Wales (UNSW), and HortResearch, from whom I built my
knowledge and expertisethroughtheir encouragement, support,
andresources. I wishtoexpressmyappreciationtotheUNSW, SQU,
andHortResearchlibrarystaffs,
whoassistedmepatientlywithonlineliteraturesearches and interlibrary
loans.I sincerelyacknowledgethesacrices madebymyparents,
AsadullahMondal
andSalehaKhatun,duringmyearlyeducation.Appreciationisduetoallmyteachers,especiallyProfessorsNooruddin
Ahmed, Iqbal Mahmud, Khaliqur Rahman, Jasim Zaman, Ken Buckle, Drs.
Prakash LalPotluri and Robert Driscoll, and Habibur Rahman, for
their encouragement and help in all aspects ofpursuing higher
education and research. I would like to express my appreciation to
Professor AntonMcLachlan, Drs. SaudAl-Jufaily, YasenAl-Mula,
NadyaA-Saadi, andS. Prathapar for
theirsupporttowardmyteaching,research,andextensionactivitiesattheSQU.Specialthankstomycolleagues
Dr. Conrad Perera, Professor Dong Chen, Drs. Nejib Guizani, Ahmed
Al-Alawi, ShyamSablani, Bhesh Bhandar, and Mushtaque Ahmed, and my
other research team members,
especiallyMohdHamadAl-Ruzeiki,RashidHamedAl-Belushi,SalhaAl-Maskari,MohdKhalfanAl-Khu-saibi,
NasserAbdullaAl-Habsi, InsaafMohdAl-Marhubi, IntisarMohdAl-Zakwani,
andZahraSulaimanAl-Kharousi.
Iowemanythankstomygraduatestudentsfortheirhardworkintheirprojects
related to food properties and building my knowledge base. Special
thanks for thecontributingauthors; it wasagreat
pleasureworkingwiththem. Iwouldalsoliketoappreciatethe enthusiasm,
patience, and support provided by the publisher.I
wishtothankmyrelativesandfriends, especiallyProfessor Md. Mohar Ali
andDr. Md.MoazzemHossain, Dr. IqbalMujtaba,
andArshadulHaquefortheircontinuedinspiration. Iamgrateful to my
wife, Sabina Akhter (Shilpi), for her patience and support during
this work, and to mydaughter, Rubaba Rahman (Deya), and my son,
Salman Rahman (Radhin), for allowing me to workat home. It would
have been very hard for me to write this book without my familys
cooperationand support. 2008 by Taylor & Francis Group, LLC.
2008 by Taylor & Francis Group, LLC.EditorMohammadShaur
Rahmanis anassociate professor at the SultanQaboos
University,SultanateofOman.Hehasauthored orcoauthoredmorethan200
technicalarticles including81refereed journal papers, 71 conference
papers, 40 book chapters, 33 reports, 8 popular articles, and
4books. He is the editor of the internationally acclaimed 2003
bestseller, Handbook of FoodPreservation published by CRC Press. He
was invited to serve as one of the associate editors forthe
Handbook of Food Science, Engineering and Technology, and he is one
of the editors for theHandbook of Food and Bioprocess Modeling
Techniques, also published by CRC Press.Dr. Rahman has initiated
the International Journal of Food Properties (Marcel Dekker, Inc.)
andhas served as its founding editor for more than 10 years. He is
a member of the Food EngineeringSeries editorial board of Springer
Science, New York. Presently, he is serving as a section editor
forthe Sultan Qaboos University journal, Agricultural Sciences. In
1998, he was invited to serve as afood science adviser for the
International Foundation for Science (IFS) in Sweden.Dr. Rahmanis
aprofessional member of theNewZealandInstituteof FoodScience
andTechnology and the Institute of Food Technologists; a member of
the American Society ofAgricultural
EngineersandtheAmericanInstituteof Chemical Engineers; andamember
of theexecutive committee for International Society of Food
Engineering (ISFE). He received his BSc Eng(chemical) (1983) and
MSc Eng (chemical) (1984) from Bangladesh University of Engineering
andTechnology, Dhaka; hisMSc(1985) in food engineering from Leeds
University, England; andhisPhD(1992) infoodengineeringfromthe
Universityof NewSouthWales, Sydney, Australia.Dr. Rahmanhas
receivednumerous awards andfellowships inrecognitionof
research=teachingachievements, including the HortResearchChairmans
Award, the Bilateral Research ActivitiesProgram(BRAP) Award, CAMS
Outstanding Researcher Award 2003, SQUDistinction inResearch Award
2008, and the British Council Fellowship. The Organization of
Islamic CountrieshasnamedRahmanasthefourthrankedagroscientist
inasurveyof theleadingscientistsandengineers in its 57 member
states. 2008 by Taylor & Francis Group, LLC. 2008 by Taylor
& Francis Group, LLC.ContributorsBenu P. AdhikariSchool of
Science and EngineeringThe University of BallaratMount Helen,
Victoria, AustraliaJasim AhmedPolymer Source, Inc.Dorval, Quebec,
CanadaGhalib Said Al-SaidiDepartment of Food Science and
NutritionSultan Qaboos UniversityMuscat, Sultanate of OmanBhesh R.
BhandariSchool of Land, Crop and Food SciencesThe University of
QueenslandBrisbane, Queensland, AustraliaG.I. BisharatDepartment of
Chemical EngineeringNational Technical University of AthensAthens,
GreeceAdriana DelgadoSchool of Agriculture, Food Science
andVeterinary MedicineUniversity College DublinDublin,
IrelandFerruh ErdogduDepartment of Food EngineeringUniversity of
MersinMersin, TurkeyDidem Z. IcozDepartment of Food ScienceRutgers,
The State University of New JerseyNew Brunswick, New JerseyKirsi
JouppilaDepartment of Food TechnologyUniversity of
HelsinkiHelsinki, FinlandStefan KasapisDepartment of
ChemistryNational University of SingaporeSingaporeJozef L.
KokiniDepartment of Food ScienceRutgers, The State University of
New JerseyNew Brunswick, New JerseyMagdalini K. KrokidaDepartment
of Chemical EngineeringNational Technical University of
AthensAthens, GreecePiotr P. LewickiDepartment of Food Engineering
and ProcessManagementWarsaw University of Life SciencesWarsaw,
PolandK.M. Machado-VelascoChemical and Food Engineering
DepartmentUniversity of the AmericasPueblaCholula, Puebla,
MexicoDimitris Marinos-KourisDepartment of Chemical
EngineeringNational Technical University of AthensAthens,
GreeceAgata MarzecDepartment of Food Engineering and
ProcessManagementWarsaw University of Life SciencesWarsaw,
PolandPanagiotis A. MichailidisDepartment of Chemical
EngineeringNational Technical University of AthensAthens, Greece
2008 by Taylor & Francis Group, LLC.Mohammad Shaur
RahmanDepartment of Food Science and NutritionSultan Qaboos
UniversityMuscat, Sultanate of OmanZbigniew RanachowskiInstitute of
Fundamental TechnologicalResearchPolish Academy of SciencesWarsaw,
PolandYrj H. RoosDepartment of Food Science and
TechnologyUniversity College CorkCork, IrelandShyam S.
SablaniDepartment of Biological Systems EngineeringWashington State
UniversityPullman, WashingtonR. Paul SinghDepartment of Biological
and AgriculturalEngineeringUniversity of California, DavisDavis,
CaliforniaM.E. Sosa-MoralesChemical and Food Engineering
DepartmentUniversity of the AmericasPueblaCholula, Puebla,
MexicoDa-Wen SunSchool of Agriculture, Food Science andVeterinary
MedicineUniversity College DublinDublin, IrelandJorge F.
Velez-RuizChemical and Food EngineeringDepartmentUniversity of the
AmericasPueblaCholula, Puebla, MexicoLiyun ZhengSchool of
Agriculture, Food Science andVeterinary MedicineUniversity College
DublinDublin, Ireland 2008 by Taylor & Francis Group,
LLC.CHAPTER1Food Properties: An OverviewMohammad Shaur
RahmanCONTENTS1.1 Denition of Food
Property.....................................................................................................
11.2 Classication of Food Property
...............................................................................................
21.2.1 Physical and Physicochemical Properties
....................................................................
31.2.2 Kinetic
Properties.........................................................................................................
41.2.3 Sensory
Properties........................................................................................................
41.2.4 Health
Properties..........................................................................................................
51.3 Applications of Food Properties
..............................................................................................
51.3.1 Process Design and
Simulation....................................................................................
51.3.1.1 Process Design
..............................................................................................
61.3.1.2 Process
Simulation........................................................................................
61.3.1.3 Continuous
Need...........................................................................................
61.3.2 Quality and Safety
.......................................................................................................
61.3.3 Packaging
Design.........................................................................................................
71.4 Prediction of Food Properties
..................................................................................................
71.5 Conclusion
...............................................................................................................................
7References
.........................................................................................................................................
81.1DEFINITION OF FOOD PROPERTYA property of a system or material
is any observable attribute or characteristic of that system
ormaterial. The state of a system or material can be dened by
listing its properties (ASHRAE, 1993).Afoodpropertyis aparticular
measureof thefoods behavior as amatter, its behavior withrespect to
energy, its interaction with the human senses, or its efcacy in
promoting human healthand well-being (McCarthy, 1997; Rahman and
McCarthy, 1999). It is always attempted to preserveproduct
characteristics at a desirable level for as long as possible. Food
properties, in turn, dene thefunctionality of foods (Karel, 1999).
Food functionality, as dened by Karel, refers to the control
offoodpropertiesthat providesadesiredset oforganolepticproperties,
wholesomeness(includinghealth-related functions), as well as
properties related to processing and engineering, in
particular,ease of processing, storage stability, and minimum
environmental impact. 2008 by Taylor & Francis Group,
LLC.Ingeneral,
foodpreservationandprocessingaffectsthepropertiesoffoodsinapositiveornegativemanner.
Duringfoodprocessing,
attemptstoachievethedesiredcharacteristicscanbegrouped as (1)
controlling food characteristics by adding of
ingredients=preservatives or removingcomponents detrimental
toquality, (2) applyingdifferent forms of energy, suchasheat,
light,electricity, and physical forces, and (3) controlling or
avoiding recontamination. In the investigationof foods in the
temperature range between 808C and 3508C many effects can be
observed duringprocessing, preservation, and storage. These
phenomena may either be endothermic (such as melting,denaturation,
gelatinization, and evaporation) or exothermic processes (such as
freezing, crystalliza-tion, and oxidation). Through precise
knowledge of such phase transitions, optimum conditions
forsafestorageor
processingoffoodscanbedened.Inadditiontothermalenergy,otherformsofenergy,
such as electricity, light, electromagnetism, and pressure are also
used in food processing.1.2CLASSIFICATION OF FOOD
PROPERTYClassifying food properties is a difcult task, and any
attempt to do so is likely to becontroversial. However, it is
necessary to develop a well-dened terminology and classication
offoodproperties(Rahman, 1998). RahmanandMcCarthy(1999)
attemptedtodevelopawidelyaccepted classication terminology for food
properties. A good classication could facilitate
soundinterdisciplinary approaches to the understanding of food
properties, and of the measurement anduse of food property data,
leading to better process design and food product characterization.
Jowitt(1974) proposed a classication of foodstuffs and their
physical properties. Rahman (IJFP, 1998)settledonthelist that
appearsat theendoftherst issueoftheInternational Journal of
FoodProperties, after several revisions based on discussions with
many academics and scientists
aroundtheworld(Table1.1).Theclassicationnowproposed
containsfourmajor classes(RahmanandTable 1.1 Food Properties
Grouped in the FirstIssue of International Journal of Food
PropertiesAcoustical propertiesColorimetric propertiesElectrical
propertiesFunctional propertiesMass transfer
propertiesMassvolumearea-related propertiesMechanical
propertiesMedical propertiesMicrobial deathgrowth-related
propertiesMorphometric propertiesOptical propertiesPhysico-chemical
constantsRadiative propertiesRespiratory propertiesRheological
propertiesSensory propertiesSurface propertiesThermodynamic
propertiesTextural propertiesThermal propertiesQuality kinetics
parametersSource: From Int. J. Food Prop., 1, 78, 1998. 2008 by
Taylor & Francis Group, LLC.McCarthy, 1999): (1)physical and
physicochemicalproperties,(2) kinetic properties, (3)
sensoryproperties, and (4) health properties (Table 1.2).1.2.1
Physical and Physicochemical PropertiesPhysical andphysicochemical
propertiesarepropertiesdened, measured,
andexpressedinphysicalandphysicochemical ways. However,
thereisnocleardividinglinebetweenthesetwotypes of properties.
Paulus (1989) classied physical properties as mechanical, thermal,
transport,and other electrical and optical properties. It is
considered misleading to use transport as a subclass ofphysical
properties, since many mechanical, thermal, and electrical
properties are considered transportproperties, e.g., electrical
conductivityandthermal conductivity. Moreover, amongthermal
properties,specic heat isa constitutiveproperty, whereas thermal
conductivityand diffusivity aretransportTable 1.2 List of Four
Classes of Food PropertiesPhysical and physicochemical propertiesa.
Mechanical properties1. Acoustic properties2.
Massvolumearea-related properties3. Morphometric properties4.
Rheological properties5. Structural characteristics6. Surface
propertiesb. Thermal propertiesc. Thermodynamic propertiesd. Mass
transfer propertiese. Electromagnetic propertiesf. Physicochemical
constantsKinetic propertiesa. Quality kinetic constantsb. Microbial
growth, decline, and death kinetic constantsSensory propertiesa.
Tactile propertiesb. Textural propertiesc. Color and appearanced.
Tastee. Odorf. SoundHealth propertiesa. Positive health
properties1. Nutritional composition2. Medical properties3.
Functional propertiesb. Negative health properties1. Toxic at any
concentration2. Toxic after critical concentration level3.
Excessive or unbalanced intakeSource: Rahman, M.S. andMcCarthy,
O.J., Int. J. FoodProp., 2, 1, 1999. 2008 by Taylor & Francis
Group, LLC.properties.
Theclassicationproposedhereissimilartotheclassicationofphysical
propertiesproposed by Jowitt (1974):rst, two new subclasses,
thermodynamic and mass transfer properties,replace Jowitts subclass
of diffusion-related properties; most of the properties included in
Jowittssubclass are in fact thermodynamic ones. The new mass
transfer properties subclass now proposedincludes mass transfer by
both diffusion and other mechanisms, and is thus more generic.
Second, anew subclass of physicochemical constants has been
added.Mechanical
propertiesarerelatedtofoodsstructureanditsbehaviorwhenphysical
forceisapplied. Structure is the form of building or construction,
the arrangement of parts or elements ofsomething constructed or of
a natural organism to give an organization of foods. In addition to
naturalstructure, man-made structured foods use assembly or
structuring processes to build product micro-structure. Examples of
essential tools to create microstructure are crystallization, phase
inversion,phase transition, glass transition, emulsication (e.g.,
margarine, ice cream, sauces, and mayonnaise),freeze alignment,
foaming (e.g., whipped cream), extrusion, pufng, drying, kneading
of dough, andbaking. In these products, a complicated multiphase
microstructure is held together by binding forcesbetween the
various phases. This microstructure leads to acceptance of desired
product texture andmouth feel during mastication, which is the key
tonal product quality and is appreciated by theconsumer. Thecontrol
of themicrostructureof man-madestructuredfoodsis
thekeyquality-determining factor apart from requirements on
microbial stability and safety. A detailed review onstructuring
processes since the past 25 years as well as the challenges that
lie ahead has been presentedbyBruinandJongen(2003). Inthepast
25years, substantial progress has beenmadeintheunderstanding and
control of product microstructure, and new ways of achieving them
have beendeveloped. The mechanical properties based on structure
are further classied into six subclasses:acoustic properties,
massvolumearea-related properties, morphometric properties,
rheologicalproperties, structural characteristics, and surface
properties.Thermal properties are related to heat transfer in food,
and thermodynamic properties are relatedto the characteristics
indicating phase or state changes in food. Mass transfer properties
are related tothetransportor
owofcomponentsinfood.Electromagneticpropertiesarerelatedtothefoodsbehavior
with the interaction of electromagnetic energy (e.g., dielectric
constant, dielectric loss, andelectrical resistance).1.2.2 Kinetic
PropertiesKinetic properties are kinetic constants characterizing
the rates of changes in foods. These can bedivided into two groups.
Therst comprises kinetic constants characterizing the rates of
biological,biochemical, chemical, physicochemical, and physical
changes in food. It could include respiratoryconstants, rate
constant, decimal reduction time, half-life, Arrhenius equation
constants, temperaturequotient (Q10), and D and z values. The
second comprises kinetic constants characterizing the ratesof
growth, decline, and death of microorganisms in food. It could
include properties such as specicgrowth rate, the parameters of the
logistic and Gompertz equations (mathematical models of
microbialgrowth), generation time, square root (Ratkowsky) equation
constants, and decimal-reduction time.It shouldbe notedthat these
properties are not actuallyproperties of food, but properties
ofmicroorganisms as moderated by the food they are in (Rahman and
McCarthy, 1999).1.2.3 Sensory
PropertiesAsensorypropertycanbedenedasthehumanphysiologicalpsychologicalperceptionofanumber
of physical and other properties of food and their interactions.
The physiological apparatus(ngers, mouth, eyes,
tasteandaromareceptors, andears)examinesthefoodandreactstothefoods
properties. Signals are sent to the brain, which interprets the
signals and comes to a decisionabout thefoodssensoryquality;
thisisthepsychological bit. Sensorypropertiesaremeasured 2008 by
Taylor & Francis Group, LLC.subjectively using trained and
untrained panels, and individuals or consumers. Sensory
propertiescan be subdivided into tactile properties, textural
properties, color and appearance, taste, odor, andsound.
Tactileproperties areperceivedas touch, i.e., bythe ngers. For
example, thesurfaceroughness and softness of a food can be
evaluated by touch. The main difference between
textureandothersensoryattributesisthattextureisperceivedmainlybybitingandmasticating,i.e.,bythe
mouth. Many of the sensory properties are related to physical and
physicochemical properties asmeasured objectively with instruments.
However, this does not mean that instrumentally
measuredcharacteristics are sensory properties. The following
discussion could help to highlight the differ-ence. The rheological
nature of a food and the foods texture are two different things.
Rheologicalproperties are measured objectively using suitable
instruments that allow controlled deformation
ofthefood.Texture,however,hastobe
measuredsubjectively.Itdependspartly,ofcourse, on thefoods
rheological properties, but also, potentially, on a number of other
properties (e.g., shape, size,porosity, andthermal properties)
andontheexpectationsandprior experienceof
theperson(s)assessingthetexture. Inmanycases,
texturecanbecorrelatedquitewellwithaninstrumentallymeasuredrheological
property(oftenanempirical orimitativeone), but
textureassuchcanbemeasured only by subjective means (Rahman and
McCarthy, 1999). Both subjective and objectivemethods have their
own advantages and limitations. However, food properties measured
bysubjectivemethodscouldbecorrelatedwithpropertiesmeasuredbyobjectivemethodsandthiscould
make the quality control process easy during processing,
preservation, and storage.1.2.4 Health PropertiesHealth properties
relate to the efcacy of foods in promoting human health and
well-being. Notall foods consumed are safe; thus foods have
positive or negative impacts on health. Positive effectscanbe
subdividedintonutritional composition(as denedinnutritional
compositiontables),medical properties, andfunctional properties.
Functional propertiesarethosethat impact onanindividualsgeneral
health, physical well-being, andmental health,
andslowtheagingprocess;medical propertiesarethosethat prevent
andtreat diseases. It isnot easytomakeaclear-cutdistinction between
functional and medical properties. For example, the antioxidant
character of afood has effects both in controlling heart disease (a
medical effect) and in slowing down the agingprocess (a functional
effect). Some components of foods, such as pesticides and
fungicides are toxicat any level, or when some critical level is
exceeded. It is not safe to consume unlimited quantities ofsome
foods. Some components (e.g., sugar, salt, fat, fat soluble
vitamins, and alcohol) have negativeeffects if intake is excessive,
or if the diet as a whole is unbalanced. Thus negative health
propertiesare grouped as toxic at any concentration, toxic above a
critical concentration level, and excessive orunbalanced
intake.1.3APPLICATIONS OF FOOD PROPERTIESAnunderstandingof
foodproperties is essential for scientists andengineers tosolve
theproblemsinfoodpreservation, processing, storage,
marketing,consumption, andevenaftercon-sumption. It would be very
difcult tond a branch of food science and engineering that does
notneedtheknowledgeof foodproperties. Theapplicationof
foodpropertiesarediscussedinthefollowing sections.1.3.1 Process
Design and SimulationProcessing causes many changes in the
biological, chemical, and physical properties of foods.A basic
understanding of these properties of food ingredients, products,
processes, and packages is 2008 by Taylor & Francis Group,
LLC.essential for thedesignof efcient processes andminimizationof
undesirablechanges duetoprocessing. The present lack of sufcient
data on physical properties (such as rheological, thermal,mass, and
surface properties) of basic components under real conditions has
limited the applicationof many well-established engineering
principles (IFT, 1993).1.3.1.1 Process DesignFood properties are
used in the engineering design, installation, optimization, and
operation offood processing equipments including a complete plant.
For example, during canning, foods need tobe heated for
sterilization. The duration and the temperature at which heating
needs to be carriedout can be based on quality, safety, nutrition
content, and process efciency. In this case, thermalproperties such
as thermal conductivity and diffusivity as well as microbial
lethality and nutritionloss are required for all heat transfer
calculations and to predict the end point of heating
process.1.3.1.2 Process SimulationProcess simulation is an
important tool for food engineers to develop concept, design,
operation,and improvement of food processes. For example,
owmodeling can investigate more alternatives ofbetter products in
less time at a lower cost. Dhanasekharan et al. (2004) described
examples in whichow modeling was used to overcome challenging
design problems in extrusion, mixing, and foodsafety by
incorporating HACCP. Schad (1998) warned not to gamble with
physical properties whenmakingthemost of processsimulationbenets.
Physical propertiesarecritical insimulatingaprocess. Thus, it
isimportant toknowfromwherepure-component propertieshavecome,
whatbasic property models are being used, and fromwhere the basic
equation has originated. It is importantto be careful in
interpreting the results of the simulation based on the quality and
source of
criticalphysicalpropertydata.Themissingorinadequatephysicalpropertiesunderminetheaccuracyofsimulation.Theproblemisthatthesimulationsoftwareisnotlikelytotelluswhetheransweriserroneous.
The results may appear to be correct, but they may be totally
wrong. It is our responsibilityalone to ensure that we are using
the right property models and have inputted or accessed correct
andsufcient data to describe our physical properties. There are no
shortcuts (Schad, 1998).1.3.1.3 Continuous NeedWe may think that
food properties are important only for the initial design of a
plant or process,thus only those who are building new equipment
need food properties. It is misleading to think thatafter the plant
has been commissioned, food properties are not required for process
design, processoperation, and product development. In many
instances, the existing equipment need to be updatedfor newproduct
linesor whensomeunitsarenot operatingefciently. Inthiscase, it
isveryexpensive to replace whole processing lines or equipment.
Some modications in the process needthe applications of process
design. Thus, the use of food properties in process design is
necessaryduring the entire life of a processing plant.1.3.2 Quality
and SafetyQuality is an illusive, ever-changing concept. It is a
relative perception and is always pegged toexpectations based on
past experiences. It may have different dimensions or attributes,
which couldbe rotated based on the types of users. Several
authorities have dened quality in various ways, butthe term
generally appears to be associated with the degree oftness for use
or the satisfaction levelof consumers (ITC, 1993); the absence of
defects or a degree of excellence (Shewfelt, 1999); thedegree of
conformance to the desired functionality (Karel, 1999); or the
degree of acceptability of a 2008 by Taylor & Francis Group,
LLC.product to users. Every food product has characteristics
measurable by sensory evaluation methodsor physicochemical tests.
Some characteristics or properties are physical and are easily
perceived;others are unseen. The applications of food properties
can also be described as characterizing thedefects in a food
product.Understanding these quality characteristics and emotional
factors and familiaritywith
theappropriatemeasuringtoolsarevitaltothequalitycontroloffoodproducts.Qualitylosscanbeminimized
at any stage thus quality retaining depends on the overall control
of the processing chain.Whenpreservationfails,
theconsequencesrangebroadlyfrombeingextremelyhazardoustotheloss of
color. Automatic control of food-processing systems helps to
improvenal product quality,increase process efciency, andreduce
waste of rawmaterials. Foodprocesses are generallymultiple-input,
multiple-output systems involvingcomplexinteractions betweenprocess
inputsand outputs (Figure 1.1).1.3.3 Packaging DesignIt is
important to know product and packaging characteristics,
food-packaging interaction, andstability of packaging during
storage and distribution (Petersen et al., 1999). Information on
foodproperties is needed in the selection of packaging materials,
and in the design of packages, packagingoperations, and packaging
machines (McCarthy, 1997). It is important to know how food
materialsinteract with packaging materials, and deterioration
kinetics of food during storage and distribution.1.4PREDICTION OF
FOOD PROPERTIESThe experimental measurement is verycostly, labor
intensive, andmayrequire specialistknowledge. Computer models can
be run very quickly, and in many cases do not require a lot
ofdetailedtechnical knowledge. Theycanbeusedtopredict what might
happenintheprocess,handling, storage, andconsumption. Oneof thebest
featuresof computer models is that theycan be used to explore any
number ofwhat if scenarios. In many instances simulation refers
towhat if scenarios and optimization refers to best way to do it.
This can be useful as tools, sincethey can be used to investigate
the possible effects before undertaking detailed and
time-consumingexperimental work. There is a need for models that
can predict complicated phenomena such as tastedevelopment or the
effect of complex food processing events on product
properties.1.5CONCLUSIONAclear denitionof foodproperties is
presentedfollowedbywell-denedclassications.The needs of
understanding food properties are clearly identied with the
different applications infood processing, preservation, storage,
and quality control. Foodproperties canbe measuredProcessing
conditionsProcessingPropertiesof raw materials(input)Propertiesof
finished product(output)Figure 1.1 Interaction of processing
variables with input and output materials variables. 2008 by Taylor
& Francis Group, LLC.experimentally when needed. This task
could be achieved by developing prediction models, whichwould save
money for costly instruments or methods, reduce labor costs, and
avoid hiring skilledoperators for complex methods. However,
prediction models will not be able to replace the needs
ofdeveloping measurement techniques.REFERENCESASHRAE. 1993. ASHRAE
Fundamentals. American Society of Heating, Refrigerating and
Air-ConditioningEngineers, New York.Bruin, S. andJongen, T.R.G.
2003. Foodprocess engineering: Thelast 25years andchallenges
ahead.Comprehensive Reviews in Food Science and Food Safety, 2:
4280.Dhanasekharan, K.M., Grald, E.W., and Mathur, R. 2004. Howow
modeling benets the food industry. FoodTechnology, 58(3): 3235.IFT.
1993. IFT special report: Americas food research needs into the
21st century. Food Technology, 47(3):1S39S.IJFP. 1998. Instructions
for preparation of manuscript. International Journal of Food
Properties, 1: 9599.ITC. 1993. QualityControl fortheFoodIndustry:
AnIntroductoryHandbook. International TradeCentreUNCTAD=GATT,
Geneva.Jowitt, R. 1974. Classication of foodstuffs and physical
properties.Lebensmittel-WissenschaftundTechno-logie, 7(6):
358378.Karel,M.1999.FoodresearchtasksatthebeginningofthenewMillenniumapersonalvision.In:WaterManagement
in the Design and Distribution of Quality of Foods, Roos, Y.H.,
Leslie, R.B., and Lillford,P.J. (eds.). Technomic Publishing,
Lancaster, Pennsylvania, pp. 535559.McCarthy,
O.J.1997.Physicalpropertiesoffoodsandpackagingmaterialsanintroduction.
In:FoodandPackaging Engineering I Course Material. Department of
Food Technology, Massey University,Plmerston North.Paulus, K. 1989.
Nutritionaland sensoryproperties of processedfoods.
In:FoodPropertiesandComputer-Aided Engineering of Food Processing
Systems, Singh, R.P. and Medina, A.G. (eds.). Kluwer
AcademicPublishers, New York, pp. 177200.Petersen, K., Nielsen, P.,
Bertelsen, G., Lawther, M., Olsen, M.B., Nilsson, N.H., and
Morthensen, G. 1999.Potential biobased materials for food
packaging. Trends in Food Science and Technology, 10: 5268.Rahman,
M.S. 1998. Editorial. International Journal of Food Properties,
1(1): vvi.Rahman, M.S. andMcCarthy, O.J. 1999. Classicationof
foodproperties. International Journal of FoodProperties, 2(2):
16.Schad, R.C. 1998. Make the most of process simulation. Chemical
Engineering Progress, 94(1): 2127.Shewfelt, R.L. 1999. What is
quality? Postharvest Biology and Technology, 15: 197200. 2008 by
Taylor & Francis Group, LLC.CHAPTER2Water Activity Measurement
Methods of FoodsMohammad Shaur Rahman and Shyam S.
SablaniCONTENTS2.1
Introduction..............................................................................................................................
92.2 Water Activity Measurement
.................................................................................................
102.2.1 Colligative Properties
Methods..................................................................................
102.2.1.1 Vapor Pressure Measurement
.....................................................................
102.2.1.2 Water Activity above
Boiling.....................................................................
132.2.1.3 Water Activity by Freezing Point
Measurements....................................... 142.2.2
Gavimetric Methods Based on Equilibrium Sorption
Rate....................................... 142.2.2.1 Discontinuous
Registration of Mass Changes
............................................ 152.2.2.2 Methods with
Continuous Registration of Mass Changes .........................
202.2.3 Hygrometric Methods
................................................................................................
252.2.3.1 Mechanical
Hygrometer..............................................................................
252.2.3.2 Wet and Dry Bulb Hygrometer
..................................................................
252.2.3.3 Dew Point Hygrometer
...............................................................................
262.2.3.4 Hygroscopicity of
Salts...............................................................................
262.2.3.5 Electronic Sensor Hygrometer
....................................................................
272.2.4 Other Methods
...........................................................................................................
292.3 Selection of a Suitable
Method..............................................................................................
292.4 Conclusion
.............................................................................................................................
29References
.......................................................................................................................................
302.1INTRODUCTIONWater is an important constituent of all foods. In
the middle of the twentieth century,
scientistsbegantodiscovertheexistenceofarelationshipbetweenthewatercontainedinafoodanditsrelative
tendency to spoil. They also began to realize that the chemical
potential of water is related
toitsvaporpressurerelativetothatofpurewaterwasmoreimportant.Thisrelativevaporpressure(RVP)istermedaswateractivityoraw.
Scott (1957)clearlystatedthat
thewateractivityofamediumcorrelatedwiththedeteriorationoffoodstabilityduetothegrowthofmicroorganisms.Thus,
it ispossibletodevelopgeneralizedrulesorlimitsfor thestabilityof
foodsusingwater 2008 by Taylor & Francis Group, LLC.activity.
This was the main reason why food scientists started to emphasize
water activity along withwatercontent. Sincethen,
thescienticcommunityhasexploredthegreat signicanceofwateractivity
in determining the physical characteristics, processes, shelf life,
and sensory properties offoods. Recently,
RahmanandLabuza(2007)havepresentedadetailedreviewonthisaspect
ofwater activity. Details of the various measurement techniques are
presented by Labuza et al. (1976),Rizvi (1995), Rahman (1995), and
Bell and Labuza (2000).Water activity, a thermodynamic property, is
dened as the ratio of the vapor pressure of waterin a system to the
vapor pressure of pure water at the same temperature, or the
equilibrium relativehumidity (ERH) of the air surrounding the
system at the same temperature. Thus, water activity canbe
expressed as follows:aw Pvw syPvw ERH (2:1)whereaw is the water
activity (fraction) at t (8C)(Pvw)sy and Pvware the vapor pressures
of water in the system and pure water, respectively, att 8C (Pa)ERH
is the equilibrium relative humidity of air at t 8C2.2WATER
ACTIVITY MEASUREMENTWiederhold (1987), Labuza et al. (1976), Rizvi
(1995), Smith (1971), and Stoloff (1978)
studiedtheaccuracyandprecisionof variouswater
activitymeasuringdevicesandfoundconsiderablevariations. The
accuracy of most of the methods lies in the range of 0.010.02 water
activity units(Rizvi, 1995). Thechoiceof onetechniqueover another
dependsontherange, accuracy, cost,responsetime(speed), suitability,
portability, simplicity, precision,
maintenanceandcalibrationrequirements,
andtypesoffoodstobemeasured(Wiederhold, 1987; Rizvi, 1995;
RahmanandAl-Belushi, 2006). The required accuracy of the routine
and reference methods is given in Table 2.1.More details of the
measurement techniques are presented by Rizvi (1995), Wiederhold
(1987), Gal(1981), andSmith(1971). Thewater activitymeasurement
methodscanbeclassiedasgivenin Table 2.2.2.2.1 Colligative
Properties Methods2.2.1.1 Vapor Pressure MeasurementThe water
activity of food samples can be estimated by direct measurement of
vapor pressureusing a manometer (Sood and Heldman, 1974; Lewicki et
al., 1978; Lewicki, 1987, 1989). A simpleTable 2.1 Precision
Requirements for Temperature, Water Activity,and Moisture
Measurement EquipmentVariable Routine Method Reference
MethodTemperature (8C) 0.2 0.02Relative humidity (%) 1.0 0.10Xwe
(%) 0.1 0.01Source:
Spiess,W.E.L.andWolf,W.inWaterActivity:TheoryandApplicationstoFood,
Rockland, L.B. andBeuchat, L.R. (eds.), Marcel Dekker, Inc.,New
York, 1987. 2008 by Taylor & Francis Group, LLC.schematic
diagram is shown in Figure 2.1. A sample of mass 1050 g of unknown
water activity isplaced in the sampleask and sealed on to the
apparatus. The airspace in the apparatus is evacuatedwith the
sample ask excluded fromthe system. The sample ask is connected
with theevacuated airspaceand the spacein thesampleaskis evacuated
to lessthan 200mmHg, whichis followedbytheevacuationof samplefor
12min. After isolatingthevacuumsourceandequilibrationfor
3050minthepressureexertedbythesampleisrecorded(Dh1). Thesampleask
is subsequently excluded from the system, and the desiccant ask is
opened. Water vapor isremoved by sorption onto CaSO4, and the
pressures exerted by volatiles and gases are indicated byDh2 after
equilibrium. The water activity of the sample is calculated as
(Labuza et al., 1976):aw [h1h2]rgPvw(2:2)wherePvwis the vapor
pressure of pure water at t 8C (Pa)r is the density of
manometricuid (kg=m3)h1 and h2 are the manometer readings (m)Rizvi
(1995) mentioned that for precise results it is necessary to
maintain the following conditions:(1) the whole system should be
maintained at a constant temperature, (2) ratio of the sample
volumeto vapor space volume should be large enough to minimize
changes in water activity due to loss ofwater byvaporization,
and(3) alow-densityandlow-vapor-pressureoil
shouldbeusedasthemanometricuid. Apiezon B manometric oil (density
866 kg=m3) is generally used as manomet-ricuid. If Tsa (sample
temperature) and Tme (medium temperature) are different, then water
activityis corrected as (Rizvi, 1995):Table 2.2 Methods for the
Determination of Sorption IsothermColligative properties methods1.
Vapor pressure measurement2. Freezing point measurement3. Boiling
point measurementGravimetric methods1. Methods with discontinuous
registration of mass changesa. Static systems (isopiestic method)b.
Evacuated systemsc. Dynamic systems2. Methods with continuous
registration of mass changesa. Static chamberb. Dynamic systemsc.
Evacuated systemHygrometric systems1. Mechanical hygrometers2. Wet
and dry bulb hygrometers3. Dew point hygrometers4. Hygroscopicity
of salts5. Electronic sensor hygrometersOther methods 2008 by
Taylor & Francis Group, LLC.aw Dh1Dh2Pvw !TsaTme !rg (2:3)The
capacitance manometer canbe usedfor more compactness of the large
setupandbettertemperature control (Troller, 1983). Inorder
toincorporatethechange involumethat occurswhen water vapor is
eliminated fromthe airwater mixture duringdesiccation, Nunes et
al.(1985) presented the following corrections:aw [h1 Ch2]rgPvwand C
1 VdVs !(2:4)whereC is the correction factorVd and Vs are the
volumes of vapor space and sample,
respectivelyTheadditionalstepperformedforthecorrectionofvolumerequiresinitiallyplacing1
gofP2O5in both sample and desiccantasks. With stopcocks 1, 3, and 5
in open position and stopcock 4 inFanTsTm5
4321DesiccantflaskSampleflaskTemp. control Vacuum gauge
VoltmeterBaffleManometerSensorSensorHeatersLiquid
N2trapVacuumFigure 2.1 Schematic diagramof a thermostatized vapor
pressure manometer apparatus. Numbers 15indicatethelocationsof
thestopcocksusedfor performinganexperiment. (FromRizvi,
S.S.H.,inEngineeringPropertiesof Foods, 2ndedn., Rao, M.A., Rizvi,
S.S.H., andDatta, A. (eds.),CRC Press, Boca Raton, FL, 1995.) 2008
by Taylor & Francis Group, LLC.close position the sampleask is
evacuated. Manometric reading (h1) is taken by closing stopcock3,
and the manometric reading (h2) is obtained with stopcock 5 in
close position and stopcock 4 inopen position. The void volumes Vs
and Vd corresponding to the sample and the desiccant
asks,respectivelyarealsomeasured. Thedetails wereprovidedbyRahmanet
al. (2001). Althoughvapour pressure manometer (VPM) is considered a
standard method, it is not suitable for materialseither containing
large amounts of volatiles and bacteria or mold, or undergoing
respirationprocesses.
Thismethodcanbeusedonlyinthelaboratoryandislimitedfor
eldapplications.Stampetal.
(1984)measuredthewateractivityofsaltsolutionsandfoodsbyseveralelectronicmethods
as compared to direct vapor pressure measurement. An error of
approximately 0.01 wateractivity units was found using the 1 h data
but no signicantly better regression line was found usingthe 24 h
data. Measurement of the aw ofve foods, however, gave values
differing by an average of0.051 aw units as compared to the VPM
readings. Their study demonstrates justication of the foodand drug
administration (FDA) cutoff aw values of 0.85 for low-acid foods as
a margin of safety.2.2.1.2 Water Activity above BoilingLoncin
(1988) proposed a method to determine the water activity above
1008C. If any substanceinitially containing free water is heated in
a closed vessel at a temperature above 1008C (say
1108C)andifthepressureisreleasedinordertoreachtheatmosphericpressure(1.0133
105Pa),thenwateractivityisonlyafunctionoftemperature(Loncin, 1988).
Thevaporpressureofwaterinthe product is 1.0133 105Pa because it is
in equilibrium with the atmosphere. The vapor
pressureofpurewateris1.43 105Paat 1108C(TableB.1inAppendixB). Thus,
wateractivityoftheproduct at 1108C isaw 1:0133 1051:43 105 0:70
(2:5)In this case, water activity does not depend on the binding
forces between water and solutes or
solidsandcomposition.Thisfactisveryimportantforextrusion,wherewateractivityattheoutletisafunction
of the temperature only (Loncin, 1988). Bassal et al. (1993)
proposed a method based onthe equilibration of food samples with an
atmosphere of pure water vapor at constant pressure.
Theequilibration cell consisted of 100 mL glass bottle with a Teon
stopper through which a capillarytube (1.3 mm diameter and 20 cm
long) was inserted. The bottle with the sample was placed in
atemperature-regulated oven (air circulated) and the total pressure
inside the bottle was measured bya barometer. An in situ
weight-measuring device was also attached to the system. Boiling
equilib-rium (TB) was assumed to be reached when the variation of
the sample mass was less than 0.01 g for1h. Thetest
durationdependsontheset temperatureandair circulationrateintheoven.
Atequilibrium,thevaporpressureofthesamplemustbeequaltothesteamsurroundingitandthewater
activity can be written asaw PSSPvST(2:6)wherePSSisthepressureof
thesurroundingsteam, andPvSTisthevapor pressureof water
attemperatureTBfromsteamtables.Themoisturecontentoftheequilibratedsamplecanbedeter-mined
by air drying of the equilibrated sample in a conventional dryer.
At the end of the test, thecapillary tube was damped to prevent any
loss of steam from the bottle during cooling. A correctionfor the
partial condensationof water vapor onthe sample canbe
estimatedfromthe knowntemperatureand volume.
Bassaletal.(1993)usedthe above procedure
formeasuringdesorptionisothermsofmicrocrystallinecellulose(MCC)andpotatostarchat
temperaturesfrom1008Cto 2008 by Taylor & Francis Group,
LLC.1508C at 1 atm and recommended the suitability of this method
to measure water activity of foods athigher pressure or vacuum.
Boiling point elevation of solution can also be used to predict the
wateractivity by the equation given by Fontan and Chirife (1981)
asln aw 1:1195 104(TbsTbw)2 35:127 103(Tbs Tbw) (2:7)where Tbs and
Tbw are the boiling points of sample and pure water,
respectively.2.2.1.3 Water Activity by Freezing Point
MeasurementsThe determination of water activity by cryoscopy or
freezing point depression is very accurate atwater
activityabove0.85asmentionedbyWodzinski andFrazier (1960), Stronget
al. (1970),FontanandChirife (1981), ReyandLabuza(1981), andLerici
et al. (1983). This methodisapplicableonlytoliquidfoods andprovides
thewater activityat freezingpoint insteadof atroom temperature. In
the case of solution, the difference is not larger than 0.01 water
activity unit(Fontan and Chirife, 1981). Rahman (1991) measured the
water activity and freezing point of freshseafoodindependently,
andfoundthat water activitypredictionfromfreezingpoint data
was0.020.03unitshigher thantheactual water activitydata.
Thismethodhasadvantagesat highwater activity and for the materials
having large quantities of volatile substances which may
createerror in vapor pressure measurement and in electric
hygrometer due to contamination of the
sensor.Inatwo-phasesystem(iceandsolution)atequilibrium,
thevaporpressureofsolidwaterasice crystals and the interstitial
concentrated solution are identical; thus water activity depends
onlyon the temperature, and not on the nature and initial
concentration of solutes, present in the third orfourth phase
(i.e., with respective kind of food). This creates a basis to
estimate the water activity offoods below the freezing point using
the equation:aw Vaporpressure ofsolid water(ice)Vaporpressure
ofliquid water(2:8)At
108C,wateractivityinanaqueoussystematequilibriumcontainingicecrystalsisequalto260.0=286.6
0.907(dataTableA.3) andisindependent of natureandinitial
concentrationofsolutes, presence of third or fourth phase as in the
case of ice cream (Loncin, 1988). Fennema (1981)concluded that
changes in properties could occur below freezing point without any
change in wateractivity. These include changes in diffusion
properties, addition of additives or preservatives, anddisruption
of cellular systems. The water activity data of ice from 08C to
508C are correlated withan exponential function as (Rahman and
Labuza, 2007):aw 8:727 exp 595:1T !(2:9)where T is measured in
kelvin. The maximum error in prediction is 0.012 unit water
activity and theaverage is 0.0066. Other colligative properties
such as osmotic pressure and boiling point elevationhave not yet
been used for food systems (Rizvi, 1995).2.2.2 Gavimetric Methods
Based on Equilibrium Sorption RateThe gravimetric method is based
on the equilibration of samples with its atmosphere of
knownhumidity. Inthismethod, it isimportant
toachievebothhygroscopicandthermal equilibrium(Gal, 1981). 2008 by
Taylor & Francis Group, LLC.2.2.2.1 Discontinuous Registration
of Mass ChangesIn this method, the sample in the controlled
atmosphere needs to be taken out for weighing andis then placed
back in the atmosphere chamber for equilibration. The balance is
not axed part
oftheapparatusandsamplesmustbeconditionedtodifferentERHvaluesandconditioningcanbecarriedout
ina staticordynamicway.Withthese methodsit ispossible tovisually
examine thesamplestodetect immediatephysical changes, likecaking,
shrinkage, discoloration, andlossoffree-owing properties (Gal,
1975).2.2.2.1.1 Static Systems (Isopiestic Method)The static method
is the most simple and common method of measuring water activity of
food.Thismethodisalsoknownasisopiesticmethod. Inthismethod,
aweightedsampleof knownmass (around 23 g) is stored in an enclosure
and allowed to reach equilibriumwith anatmosphereofknownERH(oraw),
forexample, byasaturatedsalt solution, andreweighedatregular
intervals until constant weight is established. The condition of
equilibriumis thusdetermined in this manner. The moisture content
of the sample is then determined, either directlyor by calculation
from the original moisture content and the known change in weight.
A desiccatoriscommonlyusedasachamber
togeneratecontrolledatmosphere(Figure2.2). Thedetailsofmeasuring
water activity using isopiestic methods are presented in Rahman and
Al-Belushi (2006),Lewicki and Pomaranska-Lazuka (2003), and
Sablanietal.(2001). Severaldays,orevenweeks,may be required to
establish equilibrium under static air conditions, but results can
be obtained forall relative humidityvalues simultaneouslywithlittle
effort if the apparatus is
replacedwithdifferentsaltsolutions(Smith,1971).The mainadvantages
of this methodare its simplicity, lowcost, abilitytohandle
manysamplessimultaneously, andeasyoperability(Lewicki
andPomaranska-Lazuka, 2003; Rahmanand Al-Belushi, 2006). The main
disadvantages of this simple method areSample A Sample B Sample
CSample containerExcess saltSaturated saltsolutionFigure 2.2
Humidity control chamber using desiccator. 2008 by Taylor &
Francis Group, LLC..Slowness of the equilibrium process, which
usually takes from 3 to 6 weeks. In certain instances, itcould take
a few months to equilibrate. It is therefore doubtful whether the
microbial and physico-chemical stability remain valid in the sample
during long experimental periods, especially at higherwater
activity..At high relative humidity values, the delay in
equilibration can lead to mould or bacterial growth onthe samples
and consequent invalidation of the results. Although it is
recommended to place tolueneor thymolinthe chamberforslowingthe
microbialgrowth,thereis no optionavailable toensurephysical and
chemical stability in the course of the equilibration period. In
addition, care should betaken not to inhale these toxic chemicals
from the desiccator chamber while preparing the sample
andperforming the weighing process..Condition of equilibrium is
determined by reweighing the sample at regular intervals until
constantweight is established. The equilibrium can also be hastened
by evacuating the conditioning chamber.Thelossof
conditionedatmosphereeachtimethechamber
isopenedtoremovethesampleforweighing delays the equilibrium
process. Lewicki and Pomaranska-Lazuka (2003) studied the effectsof
individual operations on this process such as opening the
desiccator, and transferring samples tothebalanceforcheckingmass.
It wasshownthat openingthedesiccator, takingthesample, andclosing
it again caused the most disturbance. The error depends on the
water activity and number oftimes the desiccator was opened. At
lowwater activity(aw
