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 33487-2742 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa businessNo claim to original U.S. Government worksPrinted in the United States of America on acid-free paper10 9 8 7 6 5 4 3 2 1International Standard Book Number-13: 978-0-8493-5005-4 (Hardcover)This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the valid-ity of all materials or the consequences of their use. 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CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For orga-nizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.Library of Congress Cataloging-in-Publication DataFood properties handbook / 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--dc22 2008019583Visit 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.PrefaceA food property is a particular measure of a foods behavior as a matter or its behavior withrespect to energy, or its interaction with the human senses, or its efcacy in promoting human healthand well-being. An understanding of food properties is essential for scientists 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 dataon food properties; simple, accurate, and low-cost measurement techniques; prediction modelsbased on fundamentals; and links between different properties. The rst edition was a well receivedbestseller, and it received an award. Appreciation from scientists, academics, and industry profes-sionals around the globe encouraged me to produce an updated version. This edition has beenexpanded with the addition of some new chapters and by updating the contents of the rst edition.The seven chapters in the rst 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, therebyreducing time and expenses of measurements. The empirical and theoretical prediction models arecompiled 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. Chapters 2 through 4 present water activity and sorption isotherm and include termin-ology, measurement techniques, data for different foods, and prediction models. Chapters 5 through12 present thermodynamic and structural characteristics including freezing point, glass transition,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=biological science, and engineering. Writing such a book is a challenge, and anycomments to assist in future compilations will be appreciated. Any errors that remain are entirelymine. 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.AcknowledgmentsI would like to thank Almighty Allah for giving me life and blessing to gain knowledge toupdate this book. I wish to express my sincere gratitude to the Sultan Qaboos University (SQU) forgiving me the opportunity and facilities to initiate such an 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 expertisethrough their encouragement, support, and resources. I wish to express my appreciation to theUNSW, SQU, and HortResearch library staffs, who assisted me patiently with online literaturesearches and interlibrary loans.I sincerely acknowledge the sacrices made by my parents, Asadullah Mondal and SalehaKhatun, during my early education. Appreciation is due to all my teachers, especially ProfessorsNooruddin 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. Saud Al-Jufaily, Yasen Al-Mula, Nadya A-Saadi, and S. Prathapar for theirsupport toward my teaching, research, and extension activities at the SQU. Special thanks to mycolleagues Dr. Conrad Perera, Professor Dong Chen, Drs. Nejib Guizani, Ahmed Al-Alawi, ShyamSablani, Bhesh Bhandar, and Mushtaque Ahmed, and my other research team members, especiallyMohd Hamad Al-Ruzeiki, Rashid Hamed Al-Belushi, Salha Al-Maskari, Mohd Khalfan Al-Khu-saibi, Nasser Abdulla Al-Habsi, Insaaf Mohd Al-Marhubi, Intisar Mohd Al-Zakwani, and ZahraSulaiman Al-Kharousi. I owe many thanks to my graduate students for their hard work in theirprojects related to food properties and building my knowledge base. Special thanks for thecontributing authors; it was a great pleasure working with them. I would also like to appreciatethe enthusiasm, patience, and support provided by the publisher.I wish to thank my relatives and friends, especially Professor Md. Mohar Ali and Dr. Md.Moazzem Hossain, Dr. Iqbal Mujtaba, and Arshadul Haque for their continued inspiration. I amgrateful 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.EditorMohammad Shaur Rahman is an associate professor at the Sultan Qaboos University,Sultanate of Oman. He has authored or coauthored more than 200 technical articles including 81refereed 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. Rahman is a professional member of the New Zealand Institute of Food Science andTechnology and the Institute of Food Technologists; a member of the American Society ofAgricultural Engineers and the American Institute of Chemical Engineers; and a member 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; his MSc (1985) in food engineering from Leeds University, England; and hisPhD (1992) in food engineering from the University of New South Wales, Sydney, Australia.Dr. Rahman has received numerous awards and fellowships in recognition of research=teachingachievements, including the HortResearch Chairmans Award, the Bilateral Research ActivitiesProgram (BRAP) Award, CAMS Outstanding Researcher Award 2003, SQU Distinction inResearch Award 2008, and the British Council Fellowship. The Organization of Islamic Countrieshas named Rahman as the fourth ranked agroscientist in a survey of the leading scientists andengineers 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.CHAPTER 1Food 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 ......................................................................................................................................... 81.1 DEFINITION 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).A food property is a particular measure of the foods behavior as a matter, 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 offood properties that provides a desired set of organoleptic properties, 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.In general, food preservation and processing affects the properties of foods in a positive ornegative manner. During food processing, attempts to achieve the desired characteristics can begrouped as (1) controlling food characteristics by adding of ingredients=preservatives or removingcomponents detrimental to quality, (2) applying different forms of energy, such as heat, 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 forsafe storage or processing of foods can be dened. In addition to thermal energy, other forms ofenergy, such as electricity, light, electromagnetism, and pressure are also used in food processing.1.2 CLASSIFICATION 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 offood properties (Rahman, 1998). Rahman and McCarthy (1999) attempted to develop a widelyaccepted 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)settled on the list that appears at the end of the rst issue of the International Journal of FoodProperties, after several revisions based on discussions with many academics and scientists aroundthe world (Table 1.1). The classication now proposed contains four major classes (Rahman andTable 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 physicochemical properties, (2) kinetic properties, (3) sensoryproperties, and (4) health properties (Table 1.2).1.2.1 Physical and Physicochemical PropertiesPhysical and physicochemical properties are properties dened, measured, and expressed inphysical and physicochemical ways. However, there is no clear dividing line between these twotypes 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 is a constitutive property, whereas thermal conductivity and diffusivity are transportTable 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. and McCarthy, O.J., Int. J. FoodProp., 2, 1, 1999. 2008 by Taylor & Francis Group, LLC.properties. The classication proposed here is similar to the classication of physical 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 properties are related to foods structure and its behavior when physical force isapplied. 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 to nal product quality and is appreciated by theconsumer. The control of the microstructure of man-made structured foods is the key quality-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 presentedby Bruin and Jongen (2003). In the past 25 years, substantial progress has been made in theunderstanding 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 tothe transport or ow of components in food. Electromagnetic properties are related to the foodsbehavior 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. The rst 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 should be noted that these properties are not actually properties of food, but properties ofmicroorganisms as moderated by the food they are in (Rahman and McCarthy, 1999).1.2.3 Sensory PropertiesA sensory property can be dened as the human physiologicalpsychological perception of anumber of physical and other properties of food and their interactions. The physiological apparatus(ngers, mouth, eyes, taste and aroma receptors, and ears) examines the food and reacts to thefoods properties. Signals are sent to the brain, which interprets the signals and comes to a decisionabout the foods sensory quality; this is the psychological bit. Sensory properties are measured 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. Tactile properties are perceived as touch, i.e., by the ngers. For example, the surfaceroughness and softness of a food can be evaluated by touch. The main difference between textureand other sensory attributes is that texture is perceived mainly by biting and masticating, 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 ofthe food. Texture, however, has to be measured subjectively. It depends partly, of course, on thefoods rheological properties, but also, potentially, on a number of other properties (e.g., shape, size,porosity, and thermal properties) and on the expectations and prior experience of the person(s)assessing the texture. In many cases, texture can be correlated quite well with an instrumentallymeasured rheological property (often an empirical or imitative one), but texture as such can bemeasured only by subjective means (Rahman and McCarthy, 1999). Both subjective and objectivemethods have their own advantages and limitations. However, food properties measured bysubjective methods could be correlated with properties measured by objective methods and thiscould 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 effectscan be subdivided into nutritional composition (as dened in nutritional composition tables),medical properties, and functional properties. Functional properties are those that impact on anindividuals general health, physical well-being, and mental health, and slow the aging process;medical properties are those that prevent and treat diseases. It is not easy to make a clear-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.1.3 APPLICATIONS OF FOOD PROPERTIESAn understanding of food properties is essential for scientists and engineers to solve theproblems in food preservation, processing, storage, marketing, consumption, and even after con-sumption. It would be very difcult to nd a branch of food science and engineering that does notneed the knowledge of food properties. The application of food properties are discussed in thefollowing 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 the design of efcient processes and minimization of undesirable changes due toprocessing. 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 whenmaking the most of process simulation benets. Physical properties are critical in simulating aprocess. Thus, it is important to know from where pure-component properties have come, 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 criticalphysical property data. The missing or inadequate physical properties undermine the accuracy ofsimulation. The problem is that the simulation software is not likely to tell us whether answer iserroneous. 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 new product lines or when some units are not operating efciently. In this case, it is veryexpensive 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 of tness 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 familiarity with theappropriate measuring tools are vital to the quality control of food products. Quality loss can beminimized at any stage thus quality retaining depends on the overall control of the processing chain.When preservation fails, the consequences range broadly from being extremely hazardous to theloss of color. Automatic control of food-processing systems helps to improve nal product quality,increase process efciency, and reduce waste of raw materials. Food processes are generallymultiple-input, multiple-output systems involving complex interactions between process 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.1.4 PREDICTION OF FOOD PROPERTIESThe experimental measurement is very costly, labor intensive, and may require specialistknowledge. Computer models can be run very quickly, and in many cases do not require a lot ofdetailed technical knowledge. They can be used to predict what might happen in the process,handling, storage, and consumption. One of the best features of computer models is that theycan be used to explore any number of what 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.1.5 CONCLUSIONA clear denition of food properties is presented followed by well-dened classications.The needs of understanding food properties are clearly identied with the different applications infood processing, preservation, storage, and quality control. Food properties can be 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. and Jongen, T.R.G. 2003. Food process engineering: The last 25 years and challenges ahead.Comprehensive Reviews in Food Science and Food Safety, 2: 4280.Dhanasekharan, K.M., Grald, E.W., and Mathur, R. 2004. How ow 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. Quality Control for the Food Industry: An Introductory Handbook. International Trade CentreUNCTAD=GATT, Geneva.Jowitt, R. 1974. Classication of foodstuffs and physical properties. Lebensmittel-Wissenschaft und Techno-logie, 7(6): 358378.Karel, M. 1999. Food research tasks at the beginning of the new Millenniuma personal vision. 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. Physical properties of foods and packaging materialsan introduction. In: Food andPackaging Engineering I Course Material. Department of Food Technology, Massey University,Plmerston North.Paulus, K. 1989. Nutritional and sensory properties of processed foods. In: Food Properties and Computer-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. and McCarthy, O.J. 1999. Classication of food properties. 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.CHAPTER 2Water 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 ....................................................................................................................................... 302.1 INTRODUCTIONWater is an important constituent of all foods. In the middle of the twentieth century, scientistsbegan to discover the existence of a relationship between the water contained in a food and itsrelative tendency to spoil. They also began to realize that the chemical potential of water is related toits vapor pressure relative to that of pure water was more important. This relative vapor pressure(RVP) is termed as water activity or aw. Scott (1957) clearly stated that the water activity of amedium correlated with the deterioration of food stability due to the growth of microorganisms.Thus, it is possible to develop generalized rules or limits for the stability of foods using water 2008 by Taylor & Francis Group, LLC.activity. This was the main reason why food scientists started to emphasize water activity along withwater content. Since then, the scientic community has explored the great signicance of wateractivity in determining the physical characteristics, processes, shelf life, and sensory properties offoods. Recently, Rahman and Labuza (2007) have presented a detailed review on this aspect 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 Pvw are 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 8C2.2 WATER ACTIVITY MEASUREMENTWiederhold (1987), Labuza et al. (1976), Rizvi (1995), Smith (1971), and Stoloff (1978) studiedthe accuracy and precision of various water activity measuring devices and found considerablevariations. The accuracy of most of the methods lies in the range of 0.010.02 water activity units(Rizvi, 1995). The choice of one technique over another depends on the range, accuracy, cost,response time (speed), suitability, portability, simplicity, precision, maintenance and calibrationrequirements, and types of foods to be measured (Wiederhold, 1987; Rizvi, 1995; Rahman andAl-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), and Smith (1971). The water activity measurement methods can be classied as givenin 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. and Wolf, W. in Water Activity: Theory and Applicationsto Food, Rockland, L.B. and Beuchat, 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 sample ask and sealed on to the apparatus. The airspace in the apparatus is evacuatedwith the sample ask excluded from the system. The sample ask is connected with theevacuated airspace and the space in the sample ask is evacuated to less than 200 mmHg, whichis followed by the evacuation of sample for 12 min. After isolating the vacuum source andequilibration for 3050 min the pressure exerted by the sample is recorded (Dh1). The sampleask 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)wherePvw is the vapor pressure of pure water at t 8C (Pa)r is the density of manometric uid (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 by vaporization, and (3) a low-density and low-vapor-pressure oil should be used as themanometric uid. Apiezon B manometric oil (density 866 kg=m3) is generally used as manomet-ric uid. 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 can be used for more compactness of the large setup and bettertemperature control (Troller, 1983). In order to incorporate the change in volume that occurswhen water vapor is eliminated from the airwater mixture during desiccation, 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, respectivelyThe additional step performed for the correction of volume requires initially placing 1 g of P2O5in both sample and desiccant asks. With stopcocks 1, 3, and 5 in open position and stopcock 4 inFanTsTm5 4321DesiccantflaskSampleflaskTemp. control Vacuum gauge VoltmeterBaffleManometerSensorSensorHeatersLiquid N2trapVacuumFigure 2.1 Schematic diagram of a thermostatized vapor pressure manometer apparatus. Numbers 15indicate the locations of the stopcocks used for performing an experiment. (From Rizvi, S.S.H.,in Engineering Properties of Foods, 2nd edn., Rao, M.A., Rizvi, S.S.H., and Datta, A. (eds.),CRC Press, Boca Raton, FL, 1995.) 2008 by Taylor & Francis Group, LLC.close position the sample ask 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,respectively are also measured. The details were provided by Rahman et 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. This method can be used only in the laboratory and is limited for eld applications.Stamp et al. (1984) measured the water activity of salt solutions and foods by several electronicmethods 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 of ve 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)and if the pressure is released in order to reach the atmospheric pressure (1.0133 105Pa), thenwater activity is only a function of temperature (Loncin, 1988). The vapor pressure of water inthe product is 1.0133 105Pa because it is in equilibrium with the atmosphere. The vapor pressureof pure water is 1.43 105Pa at 1108C (Table B.1 in Appendix B). Thus, water activity of theproduct 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 solidsand composition. This fact is very important for extrusion, where water activity at the outlet is afunction 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 for1 h. The test duration depends on the set temperature and air circulation rate in the oven. Atequilibrium, the vapor pressure of the sample must be equal to the steam surrounding it and thewater activity can be written asaw PSSPvST(2:6)where PSS is the pressure of the surrounding steam, and PvST is the vapor pressure of water attemperature TB from steam tables. The moisture content of the equilibrated sample can be deter-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 condensation of water vapor on the sample can be estimated from the knowntemperature and volume. Bassal et al. (1993) used the above procedure for measuring desorptionisotherms of microcrystalline cellulose (MCC) and potato starch at temperatures from 1008C to 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 activity above 0.85 as mentioned by Wodzinski and Frazier (1960), Strong et al. (1970),Fontan and Chirife (1981), Rey and Labuza (1981), and Lerici et al. (1983). This method isapplicable only to liquid foods and provides the water activity at freezing point instead of 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 freshseafood independently, and found that water activity prediction from freezing point data was0.020.03 units higher than the actual water activity data. This method has advantages at 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.In a two-phase system (ice and solution) at equilibrium, the vapor pressure of solid water asice 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 Vapor pressure of solid water (ice)Vapor pressure of liquid water (2:8)At 108C, water activity in an aqueous system at equilibrium containing ice crystals is equal to260.0=286.6 0.907 (data Table A.3) and is independent of nature and initial concentration ofsolutes, 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. In this method, it is important to achieve both hygroscopic and thermal 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 a xed part ofthe apparatus and samples must be conditioned to different ERH values and conditioning can becarried out in a static or dynamic way. With these methods it is possible to visually examine thesamples to detect immediate physical changes, like caking, shrinkage, discoloration, and loss offree-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.This method is also known as isopiestic method. In this method, a weighted sample of knownmass (around 23 g) is stored in an enclosure and allowed to reach equilibrium with anatmosphere of known ERH (or aw), for example, by a saturated salt solution, and reweighed atregular intervals until constant weight is established. The condition of equilibrium is 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 desiccatoris commonly used as a chamber to generate controlled atmosphere (Figure 2.2). The details ofmeasuring water activity using isopiestic methods are presented in Rahman and Al-Belushi (2006),Lewicki and Pomaranska-Lazuka (2003), and Sablani et al. (2001). Several days, or even weeks,may be required to establish equilibrium under static air conditions, but results can be obtained forall relative humidity values simultaneously with little effort if the apparatus is replaced withdifferent salt solutions (Smith, 1971).The main advantages of this method are its simplicity, low cost, ability to handle manysamples simultaneously, and easy operability (Lewicki and Pomaranska-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 thymol in the chamber for slowing the microbial growth, there is no option available to ensurephysical 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.The loss of conditioned atmosphere each time the chamber is opened to remove the sample forweighing 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 tothe balance for checking mass. It was shown that opening the desiccator, taking the sample, andclosing it again caused the most disturbance. The error depends on the water activity and number oftimes the desiccator was opened. At low water activity (aw