Physical chemistry of foods dekker

Food Emulsions: Second Edition, Revised and Expanded, edited by K å re Larsson and Stig E.. Engineering Properties of Foods: Second Edition, Revised and Expanded, edited by M... Handbook

Marcel Dekker, Inc New York•BaselTM

Pieter Walstra

Wageningen University Wageningen, The Netherlands

Physical Chemistry

of Foods

ISBN: 0-8247-9355-2

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PRINTED IN THE UNITED STATES OF AMERICA

FOOD SCIENCE AND TECHNOLOGY

A Series of Monographs, Textbooks, and Reference Books

EDITORIAL BOARD

Senior Editors

Owen R Fennema University of Wisconsin–Madison

Y.H Hui Science Technology System

Marcus Karel Rutgers University (emeritus)

Pieter Walstra Wageningen University

John R Whitaker University of California–Davis

Additives P Michael Davidson University of Tennessee–Knoxville

Dairy science James L Steele University of Wisconsin–Madison

Flavor chemistry and sensory analysis John H Thorngate III University

of California–Davis

Food engineering Daryl B Lund University of Wisconsin–Madison

Food proteins/food chemistry Rickey Y Yada University of Guelph

Health and disease Seppo Salminen University of Turku, Finland

Nutrition and nutraceuticals Mark Dreher Mead Johnson Nutritionals

Phase transition/food microstructure Richard W Hartel University of

Wisconsin–Madison

Processing and preservation Gustavo V Barbosa-C á novas Washington

State University–Pullman

Safety and toxicology Sanford Miller University of Texas–Austin

1 Flavor Research: Principles and Techniques, R Teranishi, I stein, P Issenberg, and E L Wick

Horn-2 Principles of Enzymology for the Food Sciences, John R Whitaker

3 Low-Temperature Preservation of Foods and Living Matter, Owen R Fennema, William D Powrie, and Elmer H Marth

4 Principles of Food Science

Part I: Food Chemistry, edited by Owen R Fennema

Part II: Physical Methods of Food Preservation, Marcus Karel, Owen

R Fennema, and Daryl B Lund

5 Food Emulsions, edited by Stig E Friberg

6 Nutritional and Safety Aspects of Food Processing, edited by Steven

9 Handbook of Tropical Foods, edited by Harvey T Chan

10 Antimicrobials in Foods, edited by Alfred Larry Branen and P Michael Davidson

11 Food Constituents and Food Residues: Their Chromatographic

Determination, edited by James F Lawrence

12 Aspartame: Physiology and Biochemistry, edited by Lewis D Stegink and L J Filer, Jr.

13 Handbook of Vitamins: Nutritional, Biochemical, and Clinical Aspects,

edited by Lawrence J Machlin

14 Starch Conversion Technology, edited by G M A van Beynum and J.

21 Food Biotechnology, edited by Dietrich Knorr

22 Food Texture: Instrumental and Sensory Measurement, edited by Howard R Moskowitz

23 Seafoods and Fish Oils in Human Health and Disease, John E Kinsella

24 Postharvest Physiology of Vegetables, edited by J Weichmann

25 Handbook of Dietary Fiber: An Applied Approach, Mark L Dreher

26 Food Toxicology, Parts A and B, Jose M Concon

27 Modern Carbohydrate Chemistry, Roger W Binkley

28 Trace Minerals in Foods, edited by Kenneth T Smith

29 Protein Quality and the Effects of Processing, edited by R Dixon Phillips and John W Finley

30 Adulteration of Fruit Juice Beverages, edited by Steven Nagy, John A Attaway, and Martha E Rhodes

31 Foodborne Bacterial Pathogens, edited by Michael P Doyle

32 Legumes: Chemistry, Technology, and Human Nutrition, edited by Ruth H Matthews

33 Industrialization of Indigenous Fermented Foods, edited by Keith H Steinkraus

34 International Food Regulation Handbook: Policy · Science · Law,

edited by Roger D Middlekauff and Philippe Shubik

35 Food Additives, edited by A Larry Branen, P Michael Davidson, and Seppo Salminen

36 Safety of Irradiated Foods, J F Diehl

37 Omega-3 Fatty Acids in Health and Disease, edited by Robert S Lees and Marcus Karel

38 Food Emulsions: Second Edition, Revised and Expanded, edited by

K å re Larsson and Stig E Friberg

39 Seafood: Effects of Technology on Nutrition, George M Pigott and Barbee W Tucker

40 Handbook of Vitamins: Second Edition, Revised and Expanded,

edited by Lawrence J Machlin

41 Handbook of Cereal Science and Technology, Klaus J Lorenz and Karel Kulp

42 Food Processing Operations and Scale-Up, Kenneth J Valentas, Leon Levine, and J Peter Clark

43 Fish Quality Control by Computer Vision, edited by L F Pau and R Olafsson

44 Volatile Compounds in Foods and Beverages, edited by Henk Maarse

45 Instrumental Methods for Quality Assurance in Foods, edited by Daniel Y C Fung and Richard F Matthews

46 Listeria, Listeriosis, and Food Safety, Elliot T Ryser and Elmer H Marth

47 Acesulfame-K, edited by D G Mayer and F H Kemper

48 Alternative Sweeteners: Second Edition, Revised and Expanded, ited by Lyn O'Brien Nabors and Robert C Gelardi

ed-49 Food Extrusion Science and Technology, edited by Jozef L Kokini, Chi-Tang Ho, and Mukund V Karwe

50 Surimi Technology, edited by Tyre C Lanier and Chong M Lee

51 Handbook of Food Engineering, edited by Dennis R Heldman and Daryl B Lund

52 Food Analysis by HPLC, edited by Leo M L Nollet

53 Fatty Acids in Foods and Their Health Implications, edited by Ching Kuang Chow

54 Clostridium botulinum: Ecology and Control in Foods, edited by Andreas H W Hauschild and Karen L Dodds

Ann-Charlotte Eliasson and K å re Larsson

56 Low-Calorie Foods Handbook, edited by Aaron M Altschul

57 Antimicrobials in Foods: Second Edition, Revised and Expanded,

edited by P Michael Davidson and Alfred Larry Branen

58 Lactic Acid Bacteria, edited by Seppo Salminen and Atte von Wright

59 Rice Science and Technology, edited by Wayne E Marshall and James I Wadsworth

60 Food Biosensor Analysis, edited by Gabriele Wagner and George G Guilbault

61 Principles of Enzymology for the Food Sciences: Second Edition, John

R Whitaker

62 Carbohydrate Polyesters as Fat Substitutes, edited by Casimir C Akoh and Barry G Swanson

63 Engineering Properties of Foods: Second Edition, Revised and

Expanded, edited by M A Rao and S S H Rizvi

64 Handbook of Brewing, edited by William A Hardwick

65 Analyzing Food for Nutrition Labeling and Hazardous Contaminants,

edited by Ike J Jeon and William G Ikins

66 Ingredient Interactions: Effects on Food Quality, edited by Anilkumar

69 Nutrition Labeling Handbook, edited by Ralph Shapiro

70 Handbook of Fruit Science and Technology: Production, Composition,

Storage, and Processing, edited by D K Salunkhe and S S Kadam

71 Food Antioxidants: Technological, Toxicological, and Health

Perspec-tives, edited by D L Madhavi, S S Deshpande, and D K Salunkhe

72 Freezing Effects on Food Quality, edited by Lester E Jeremiah

73 Handbook of Indigenous Fermented Foods: Second Edition, Revised

and Expanded, edited by Keith H Steinkraus

74 Carbohydrates in Food, edited by Ann-Charlotte Eliasson

75 Baked Goods Freshness: Technology, Evaluation, and Inhibition of

Staling, edited by Ronald E Hebeda and Henry F Zobel

76 Food Chemistry: Third Edition, edited by Owen R Fennema

77 Handbook of Food Analysis: Volumes 1 and 2, edited by Leo M L Nollet

78 Computerized Control Systems in the Food Industry, edited by Gauri

S Mittal

79 Techniques for Analyzing Food Aroma, edited by Ray Marsili

80 Food Proteins and Their Applications, edited by Srinivasan daran and Alain Paraf

Damo-81 Food Emulsions: Third Edition, Revised and Expanded, edited by Stig

E Friberg and K å re Larsson

82 Nonthermal Preservation of Foods, Gustavo V Barbosa-C á novas, Usha R Pothakamury, Enrique Palou, and Barry G Swanson

83 Milk and Dairy Product Technology, Edgar Spreer

84 Applied Dairy Microbiology, edited by Elmer H Marth and James L Steele

85 Lactic Acid Bacteria: Microbiology and Functional Aspects: Second

Edition, Revised and Expanded, edited by Seppo Salminen and Atte von Wright

86 Handbook of Vegetable Science and Technology: Production,

Composition, Storage, and Processing, edited by D K Salunkhe and

90 Dairy Technology: Principles of Milk Properties and Processes, P Walstra, T J Geurts, A Noomen, A Jellema, and M A J S van Boekel

91 Coloring of Food, Drugs, and Cosmetics, Gisbert Otterst ä tter

92 Listeria, Listeriosis, and Food Safety: Second Edition, Revised and Expanded, edited by Elliot T Ryser and Elmer H Marth

93 Complex Carbohydrates in Foods, edited by Susan Sungsoo Cho, Leon Prosky, and Mark Dreher

94 Handbook of Food Preservation, edited by M Shafiur Rahman

95 International Food Safety Handbook: Science, International

Regula-tion, and Control, edited by Kees van der Heijden, Maged Younes, Lawrence Fishbein, and Sanford Miller

96 Fatty Acids in Foods and Their Health Implications: Second Edition,

Revised and Expanded, edited by Ching Kuang Chow

97 Seafood Enzymes: Utilization and Influence on Postharvest Seafood

Quality, edited by Norman F Haard and Benjamin K Simpson

98 Safe Handling of Foods, edited by Jeffrey M Farber and Ewen C D Todd

99 Handbook of Cereal Science and Technology: Second Edition,

Re-vised and Expanded, edited by Karel Kulp and Joseph G Ponte, Jr.

100 Food Analysis by HPLC: Second Edition, Revised and Expanded,

edited by Leo M L Nollet

101 Surimi and Surimi Seafood, edited by Jae W Park

102 Drug Residues in Foods: Pharmacology, Food Safety, and Analysis,

Nickos A Botsoglou and Dimitrios J Fletouris

103 Seafood and Freshwater Toxins: Pharmacology, Physiology, and

Detection, edited by Luis M Botana

104 Handbook of Nutrition and Diet, Babasaheb B Desai

105 Nondestructive Food Evaluation: Techniques to Analyze Properties

and Quality, edited by Sundaram Gunasekaran

106 Green Tea: Health Benefits and Applications, Yukihiko Hara

107 Food Processing Operations Modeling: Design and Analysis, edited

110 Applied Dairy Microbiology: Second Edition, Revised and Expanded,

edited by Elmer H Marth and James L Steele

111 Transport Properties of Foods, George D Saravacos and Zacharias

B Maroulis

112 Alternative Sweeteners: Third Edition, Revised and Expanded, edited

by Lyn O ’ Brien Nabors

113 Handbook of Dietary Fiber, edited by Susan Sungsoo Cho and Mark

116 Food Additives: Second Edition, Revised and Expanded, edited by A Larry Branen, P Michael Davidson, Seppo Salminen, and John H Thorngate, III

117 Food Lipids: Chemistry, Nutrition, and Biotechnology: Second Edition,

Revised and Expanded, edited by Casimir C Akoh and David B Min

118 Food Protein Analysis: Quantitative Effects on Processing, R K Owusu-Apenten

119 Handbook of Food Toxicology, S S Deshpande

120 Food Plant Sanitation, edited by Y H Hui, Bernard L Bruinsma, J Richard Gorham, Wai-Kit Nip, Phillip S Tong, and Phil Ventresca

121 Physical Chemistry of Foods, Pieter Walstra

122 Handbook of Food Enzymology, edited by John R Whitaker, Alphons

G J Voragen, and Dominic W S Wong

123 Postharvest Physiology and Pathology of Vegetables: Second Edition,

Revised and Expanded, edited by Jerry A Bartz and Jeffrey K Brecht

124 Characterization of Cereals and Flours: Properties, Analysis, and

Ap-plications, edited by G ö n ü l Kaletun ç and Kenneth J Breslauer

125 International Handbook of Foodborne Pathogens, edited by Marianne

D Miliotis and Jeffrey W Bier

Additional Volumes in Preparation

Handbook of Dough Fermentations, edited by Karel Kulp and Klaus Lorenz

Extraction Optimization in Food Engineering, edited by Constantina Tzia and George Liadakis

Physical Principles of Food Preservation: Second Edition, Revised

and Expanded, Marcus Karel and Daryl B Lund

Handbook of Vegetable Preservation and Processing, edited by Y H Hui, Sue Ghazala, Dee M Graham, K D Murrell, and Wai-Kit Nip Food Process Design, Zacharias B Maroulis and George D Saravacos

Knowledge of physical chemistry is of great importance to anyone who isinterested in understanding the properties of food, improving its quality andstorage stability, and controlling its behavior during handling Yet, curricula

in food science often do not contain a course in food physical chemistry,especially at the undergraduate level, and failure to acquire skills in thisimportant area can hinder a food scientist’s success in scientific endeavors.Some believe that an introductory course in physical chemistry offered by adepartment of chemistry will fill this void, but I disagree If possible, anintroductory course in physical chemistry should be a prerequisite for acourse in food physical chemistry—the former providing a sound back-ground in the principles of physical chemistry and the latter focusing onapplication of the principles most relevant to food

Failure of many food science departments to offer a course on foodphysical chemistry is attributable mainly to the lack of an appropriatetextbook Whereas instructors in food science can select from several goodtextbooks on food microbiology, food engineering, food chemistry, andother more specialized topics, choices in food physical chemistry are severelylimited The publication of Pieter Walstra’s excellent textbook on foodphysical chemistry is therefore an event of major importance to the field offood science This book will stimulate universities that do not offer this

subject to do so, and will improve the quality of instruction in universitiesthat do It will also be of great value to food researchers.

Professor Walstra is eminently qualified to write a book on foodphysical chemistry because of his in-depth knowledge of the subject and hisunderstanding of, expertise in, and dedication to food science education.This book provides comprehensive coverage of food physical chemistry at adepth suitable for students in food science, and will serve as an excellentreference source for food researchers I congratulate Professor Walstra forthis fine accomplishment

Owen FennemaProfessorDepartment of Food ScienceUniversity of Wisconsin–Madison

Madison, Wisconsin

The scientific basis needed to understand and predict food properties andchanges occurring in foods during processing, storage and use has beenenormously expanded over the past half-century This has caused arevolutionary change in the teaching of food science and technology,especially by application of the disciplines of organic chemistry, biochem-istry, and microbiology With the possible exception of rheology, applica-tion of the more physical disciplines has lagged behind Many years’experience in food research and teaching has convinced me—although I amnot a physical chemist—of the importance of physical chemistry and relatedtheories for food science and technology Moreover, great progress has beenmade during the past two decades in the study of physicochemicalphenomena in foods; yet these aspects often remain greatly underexposed.The main reason for this deficiency is, in my opinion, that the teaching

of physical chemistry for food science majors is often inadequate In mostuniversities, students have one introductory course in basic physicalchemistry; this is unsatisfactory for the following reasons:

1 Many of the subjects are of little or no importance for foods or aretreated in too much theoretical detail (e.g., quantum mechanics,statistical thermodynamics, much of spectroscopy)

2 Many subjects of great importance to foods are not included Inorder to keep the theory simple, the treatment is often restricted togases and crystals and dilute, homogeneous, ideal solutions,whereas most foods are concentrated, inhomogeneous, highlynonideal systems In particular, coverage of colloid and surfacescience is insufficient.

3 In most cases, the course is taken at the wrong time, that is, beforethe student is able to see the relevance of the various subjects forfoods

The primary aim of this book is to help in remedying this situation Itcan be used as the basis of a course for food science majors at a not-too-early stage in the curriculum (some universities may prefer a graduatecourse) Hence it is written as a textbook A second aim is its use as areference book, since basic aspects of physical chemistry are not alwaystaken into account in food research or process development Therefore, Ihave tried to cover all subjects of importance that are not treated in mostcourses in food chemistry or food processing/engineering

The selection of topics—including theories, phenomena, systems, andexamples—is naturally colored by my experience and opinions This meansthat the treatment is to some extent biased In my view this is unavoidable,but I would like to hear from readers who feel that a topic has been omitted

or overemphasized Remarks about errors and unclear explanations are alsowelcome

ACKNOWLEDGMENTS

The idea for this book was born when I was asked to give a course on thephysical chemistry of foods in the Department of Food Science at theUniversity of Guelph, Ontario, Canada, in 1993 This course was basedlargely on courses in food physics given at Wageningen University,developed in cooperation with my colleagues Dr Ton van Vliet and Dr.Albert Prins I also want to mention that Dr Owen Fennema of theUniversity of Wisconsin has greatly encouraged me in writing this text.Several colleagues have made valuable suggestions, which have greatlyimproved the quality of this book I am especially indebted to Dr EricDickinson, Professor of Food Colloids at the University of Leeds, England,with whom I discussed plans and who has read and commented on virtuallyall my drafts His contributions have been invaluable Furthermore, severalcolleagues have scrutinized one or more draft chapters: Dr C van den Berg,

Dr B H Bijsterbosch, Dr M A J S van Boekel, Dr O R Fennema,

Dr G J Fleer, Dr G Frens, Dr H D Goff, Dr H H J de Jongh,

Dr J Lucassen, Dr E H Lucassen-Reynders, Dr J Lyklema, Dr E R.Morris, Dr W Norde, Dr J H J van Opheusden, Dr L van der Plas,

Dr M J W Povey, Dr J Verhagen, and Dr T van Vliet I express mygratitude to all these friends and colleagues

Pieter Walstra

10.6 Contact Angles and Wetting

10.7 Interfacial Tension Gradients

11.2 Foam Formation and Properties

11.3 Breakup of Drops and Bubbles

16 GLASS TRANSITIONS AND FREEZING

16.1 The Glassy State

16.2 The Special Glass Transition

APPENDIX D: SI Rules for Notation

APPENDIX E: The SI Units System

APPENDIX F: Some Conversion Factors

APPENDIX G: Recalculation of Concentrations

APPENDIX H: Physical Properties of Water at 0–1008C

APPENDIX I: Thermodynamic and Physical Properties of

Water and IceAPPENDIX J: Some Values of the Error Function

second virial coefficient (mol ? m3?kg
2)

b length of statistical chain element (m)

bch distance between charged groups along chain (m)

C constant

c concentration

c* chain overlap concentration

csat solubility

cp specific heat at constant pressure (J ? kg ?K )

cn relative standard deviation of order n (–)

G Gibbs (free) energy

g acceleration due to gravity (9:807 m2?s
1)

H enthalpy

h distance from a surface; interparticle distance (m)

hP Planck’s constant (6:626 ? 10
34J ? s)

i as subscript: indicates class number (–)

J aggregation rate; particle flux

nucleation rate (m
3?s
1)

K constant

stability constant; equilibrium constant

k reaction rate constant

number per unit cross-sectional area (m
2)

NAV Avogadro’s number (6:022 ? 1023mol
1)

pv vapor pressure (Pa)

R gas constant (8:315 J ? K
1?mol
1)

radius; radius of curvature; radial coordinate (m)

W retardation factor in aggregation, etc (–)

(specific) work

w0 kg water per kg dry matter (–)

z valence; net number of charges

g+ free ion activity coefficient (–)

D root-mean-square displacement in diffusion (m)

phase angle (tan d¼ G00 / G0) (rad)

e power density (energy dissipation rate) (W ? m )

relative deformation or strain (–)relative dielectric constant (–)

e0 dielectric permittivity of vacuum (8:854 ? 10
12C ? V
1?m
1)

eH natural or Hencky strain (–)

[Z] intrinsic viscosity

surface fraction (covered) (–)

k reciprocal Debye length (m
1)

L stress concentration factor (–)

x relative surface expansion rate (s
1)

t characteristic time (scale) (s)

F hydrophobicity

w solvent-segment interaction parameter (–)

C velocity gradient, strain rate (s
1)

c0 electrostatic surface potential (V)

O number of degrees of freedom (–)

o revolution rate; (angular) frequency ðrad ? s
1;s
1ÞOther

[A] molar concentration of substance A

pI isoelectric pH

pK log(stability constant)

pKa log(association constant)

¼ pH of 50% dissociation

Appendix B

TABLESome Frequently Used Abbreviations

CMC critical micellization concentration

DLVODeryagin–Landau–Verwey–Overbeek (colloidal interaction)DSC differential scanning calorimetry

HLB hydrophile–lipophile balance

PIT phase inversion temperature

r.p.m number of revolutions per minute

SDS sodium dodecyl sulfate

WLF Williams–Landel–Ferry (viscosity equation)

Appendix D

SI Rules for Notation

Symbols for (physical) quantities, be they variables or constants, are given

by a single character (generally Latin or Greek letters) and are printed initalics, e.g., F (force), p (pressure), m (chemical potential), k (Boltzmannconstant) Further differentiation is achieved by the use of subscripts and/orsuperscripts; these are printed in italics if it concerns the symbol of aquantity, otherwise in roman type, e.g., cp (specific heat at constantpressure), hP^ (Planck’s constant), ESD (surface dilational modulus) Forclarity, symbols are generally separated by a (thin) space, e.g., F¼ m a, not

ma Some generally accepted exceptions occur, such as pH, as well assymbols (or two letter abbreviations, rather) for the dimensionless ratiosfrequently used in process engineering, like Re for Reynolds number and Trfor Trouton ratio (in roman type)

Symbols for operators are given in roman (upright) characters, e.g.,log, ln,4, d (differential), sin Preferably, they are separated by a thin spacefrom the quantity symbol, e.g., ln j

Symbols for unitsare also given in roman type, e.g., m, Pa Separation

of units is (preferably) by an elevated dot, e.g., Pa ? s and kg ? m
3—or by aspace: Pa s and kg m
3—(but not Pas, etc.) Division is indicated by anegative sign in the exponent, such as m ? s
1; m/s is also allowed, but not m/kg/s, or m/kg s, since these notations are equivocal Numerical prefixes areput directly before the main symbol, e.g., mm, kPa, GV For compound

quantities, only the first symbol can contain a prefix—e.g., kPa ? m , not

Appendix E

The SI Units System

SI stands for ‘‘Syste`me International,’’ and the SI System of Units concernsthe internationally accepted and standardized rules for the values and thenotation of units for physical quantities One basic rule is that

Quantity¼ number6unitFurthermore, it is a metric system, all units for the same quantity differing

by one or more factors of 10

Base Units

These concerns the units for some measurable, dimensionally independentquantities from which all other units can be derived The magnitude of eachbase unit has been unequivocally and precisely defined

Quantity Name Symbol

Amount of substance mole mol

Luminous intensity candela cd

Quantity Name Symbol SI base units

Amount of electricity coulomb C A ? s

Electric capacity farad F A2?s4?kg
1?m
2 (C/V)Electric potential volt V kg ? m2?s
3?A
1 (W/A)Electric resistance ohm O kg ? m2?A
2?s
3 (V/A)Energy (work, amount

of heat)

joule J kg ? m2?s
2 ðN ? m; C ? VÞForce newton N kg ? m ? s
2

The most important numerical prefixes are

Multiplication factor Name Symbol

Multiplication factor Name Symbol

a Only to be used for some volume and area units.

Some Other Units

The % symbol is often used and, unless stated otherwise, it is often meant tosignify kg/100 kg

For the temperature scale, SI rules allow the use of degrees Celsius,symbol 8C, but not for temperature intervals; nor can 8C it be used with aprefix For example: cool the liquid to
158C at a rate of 10 mK ? s
1.For angles, often the ‘‘degree’’ is used, symbol 8

For volumes, the unit liter can be used (10
3m3), symbol l However,the symbol L often is used for liter without a prefix, to avoid confusion withthe numeral 1; however, use ml rather than mL, etc

Confusion can readily arise for concentrations of chemical substances.The unit ‘‘molar’’ (often used symbol M) means mol/L or kmol ? m
3, notmol/m,3which may be considered a violation of SI rules

Appendix G

Recalculation of Concentrations

Assume a binary mixture of components 1 (solvent) and 2 (solute) The massfraction of the solute is given by c Molar mass M is given in daltons, r isthe density of the solution We now have

Molar concentration (mol/L) m¼ r c

M2Molality (mol per kg water) m*¼ c

Appendix I

Thermodynamic and Physical

Properties of Water and Ice

Properties at 273.15 K and 100 kPa

Enthalpy of fusion
DL?SH ¼ 6012 J ? mol
1

¼ 334 kJ ? kg
1Entropy of fusion
DL?SS ¼ 22.01 J ? mol
1?K
1Expansion upon solidification DL?SV ¼ 90.6 ml ? kg
1(Enthalpy of sublimation ¼ 50.9 kJ ? mol
1)Specific heat, water cp ¼ 4218 J ? kg
1?K
1ice cp ¼ 2101 J ? kg
1?K
1Thermal conductivity, water l ¼ 0.56 W ? m
1?K
1

Thermal diffusivity, water DH ¼ 1.3 6 10
7 m2?s
1

ice DH ¼ 11.7 6 10
7 m2?s
1Volume compressibility,awater ¼ 5.0 6 10
10 Pa
1

Z y 0

expð
z2Þ dzwhere z is an integration variable

y erf y y erf y0.1 0.112 0.8 0.7420.2 0.223 1.0 0.8430.3 0.329 1.2 0.9110.4 0.428 1.4 0.9520.5 0.520 1.6 0.9760.6 0.604 2.0 0.9950.7 0.678 2.5 0.9997

FUNDAMENTAL CONSTANTS

Avogadro’s number NAV 6.022 ? 1023mol
1Planck’s constant hP 6.626 ? 10
34J ? s

Boltzmann constant kB 1.381 ? 10
23J ? K
1Gas constant R¼ kB? NAV 8.315 J ? K
1?mol
1Elementary charge e 1.602 ? 10
19C

Faraday constant F¼ e ? NAV 96485 C ? mol
1

Permittivity of vacuum e0 8.854 ? 10
12C ? V
1?m
1Zero of the Celsius scale 273.15 K

Standard acceleration of gravity g 9.807 m ? s
2

ATOMIC MASS OF SOME ELEMENTS

Name Symbol

Atomicnumber

Strontium Sr 38 87.6

Introduction

AND TECHNOLOGYFood science and technology are concerned with a wide variety of problemsand questions, and some will be exemplified below For instance, foodscientists want to understand and predict changes occurring in a food duringprocessing, storage, and handling, since such changes affect food quality.Examples are

The rates of chemical reactions in a food can depend on manyvariables, notably on temperature and water content However, therelations between reaction rates and the magnitude of thesevariables vary widely Moreover, the composition of the mixture

of reaction products may change significantly with temperature.How is this explained and how can this knowledge be exploited?How is it possible that of two nonsterilized intermediate-moisture foods

of about the same type, of the same water activity, and at the sametemperature, one shows bacterial spoilage and the other does not?Two plastic fats are stored at room temperature The firmness of theone increases, that of the other decreases during storage How is thispossible?

Bread tends to stale—i.e., obtain a harder and shorter texture—duringstorage at room temperature Keeping the bread in a refrigeratorenhances staling rate, but storage in a freezer greatly reduces staling.How is this explained?

The physical stability of a certain oil-in-water emulsion is observed todepend greatly on temperature At 408C it remains stable, aftercooling to 258C also, but after cooling to 108C and then warming to258C small clumps are formed; stirring greatly enhances clumpformation What are the mechanisms involved and how is thedependence on temperature history explained?

Another emulsion shows undesirable creaming To reduce creamingrate a small amount of a thickener, i.e., a polysaccharide, is added.However, it increases the creaming rate How?

Food technologists have to design and improve processes to makefoods having specific qualities in an efficient way Examples of problems areMany foods can spoil by enzyme action, and the enzymes involvedshould thus be inactivated, which is generally achieved by heatdenaturation For several enzymes the dependence of the extent ofinactivation on heating time and temperature is simple, but forothers it is intricate Understanding of the effects involved is needed

to optimize processing: there must be sufficient inactivation of theenzymes without causing undesirable heat damage

It is often needed to make liquid foods with specific rheologicalproperties, such as a given viscosity or yield stress, for instance toensure physical stability or a desirable eating quality This can beachieved in several ways, by adding polysaccharides, or proteins, orsmall particles Moreover, processing can greatly affect the result Adetailed understanding of the mechanisms involved and of theinfluence of process variables is needed to optimize formulation andprocessing

Similar remarks can be made about the manufacture of dispersions ofgiven properties, such as particle size and stability This greatlydepends on the type of dispersion (suspension, emulsion, or foam)and on the specific properties desired

How can denaturation and loss of solubility of proteins duringindustrial isolation be prevented? This is of great importance for theretention of the protein’s functional properties and for the economy

of the process

How can one manufacture or modify a powdered food, e.g., dried milk or dry soup, in such a manner that it is readilydispersable in cold water?

spray-How does one make an oil-in-water emulsion that is stable duringstorage but that can be whipped into a topping? The first questionthen is: what happens during a whipping process that results in asuitable topping? Several product and process variables affect theresult.

All of these examples have in common that knowledge of physicalchemistry is needed to understand what happens and to solve the problem.Physical chemistryprovides quantitative relations for a great number

of phenomena encountered in chemistry, based on well-defined andmeasurable properties Its theories are for the most part of a physicalnature and comprise little true chemistry, since electron transfer isgenerally not involved Experience has shown that physicochemical aspectsare also of great importance in foods and food processing This does notmean that all of the phenomena involved are of a physical nature: it isseen from the examples given that food chemistry, engineering, and evenmicrobiology can be involved as well Numerous other examples are given

In the second place, most foods are inhomogeneous systems.Consequently, various components can be in different compartments,greatly enhancing complexity This means that the system is even fartherremoved from thermodynamic equilibrium than are most homogeneoussystems Moreover, several new phenomena come into play, especiallyinvolving colloidal interactions and surface forces These occur on a largerthan molecular scale Fortunately, the study of mesoscopic physics—whichinvolves phenomena occurring on a scale that is larger than that ofmolecules but (far) smaller than can be seen with the naked eye—has madegreat progress in recent times

In the third place, a student of the physical chemistry of foods has tobecome acquainted with theories derived from a range of disciplines, as alook at the table of contents will show Moreover, knowledge of the systemstudied is essential: although basic theory should have universal validity, the

particulars of the system determine the boundary conditions for application

of a theory and thereby the final result

All of this might lead to the opinion that many of the problemsencountered in food science and technology are so intricate that application

of sound physical chemistry would hardly be possible and that quantitativeprediction of results would often be impossible Nevertheless, making use ofthe basic science involved can be quite fruitful, as has been shown for a widevariety of problems Reasons for this are

Understanding of basic principles may in itself be useful A fortunatecharacteristic of human nature is the desire to explain phenomenaobserved and to create a framework that appears to fit theobservations However, if such theorizing is not based on soundprinciples it will often lead to wrong conclusions, which readily lead

to further problems when proceeding on the conceived ideas withresearch or process development Basic knowledge is a great help in(a) identifying and explaining mechanisms involved in a process and(b) establishing (semi-)quantitative relations

Even semiquantitative answers, such as giving the order of magnitude,can be very helpful Mere qualitative reasoning can be quitemisleading For instance, a certain reaction proceeds much faster at

a higher temperature and it is assumed that this is because theviscosity is lower at a higher temperature This may be true, but only

if (a) the reaction rate is diffusion controlled, and (b) the relativeincrease of rate is about equal to the relative decrease in viscosity.When the rate increases by a factor of 50 and the viscosity decreases

by a factor of 2, the assumption is clearly wrong

Foods are intricate systems and also have to meet a great number ofwidely different specifications This means that process and productdevelopment will always involve trial and error However, basicunderstanding and semiquantitative relations may greatly reducethe number of trials that will lead to error

The possibilities for establishing quantitative relations are rapidlyincreasing This is due to further development of theory andespecially to the greatly increased power of computer systems usedfor mathematical modeling of various kinds In other words, severalprocesses occurring in such complex systems as foods—or in modelsystems that contain all the essential elements—can now be modeled

or simulated

Altogether, in the author’s opinion, application of physical chemistryand mesoscopic physics in the realm of food science and technology is oftenneeded—besides food chemistry, food process engineering, and food