water as the determinant of food engineering properties. a review

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Water as the determinant of food engineering properties. A review

Piotr P. Lewicki *

Department of Food Engineering and Process Management, Warsaw Agricultural University (SGGW),

ul. Nowoursynowska 159c, 02-787 Warsaw, Poland

Received 1 August 2002

Abstract

Water affects safety, stability, quality and physical properties of food. The influence of water on physical properties of food is

dependent on the state of water in food. The state, expressed as water activity, is briefly discussed in the paper. Further, the influence

of water on such physical properties as rheological, thermal, mass transfer, electrical, optical and acoustic is presented in details.Ó 2003 Elsevier Ltd. All rights reserved.

Keywords: Physical properties of food; Water

1. State of water in food

Water is a constituent of food, which affects food

safety, stability, quality and physical properties. Range

of water concentrations in food is very broad and begins

with a fraction of a per cent and reaches even more than

98%. Fresh products and liquid food contain usuallylarge amounts of water while baked and dry products

are poor in water. In many products the water is con-

stitutive, but there are foods which contain water added

during processing.

State of water in a solution or a solid is expressed by

the activity coefficient––a thermodynamic measure of

chemical potential of water in the system. Scott (1953,

1957) proposed to express water activity as the ratio of

vapour pressure of water in food ( p ) to the vapour

pressure of pure water ( p 0) at the same temperature and

total pressure:

aw ¼p

p 0

P;T

The above description of water activity assumes that

food is in equilibrium with the surrounding atmosphere.

Since, most of food processing is done under isobaric

conditions, and at moderate temperatures the deviation

of water vapour from the ideal gas is small, and water

activity calculated from the above equation differs less

than 0.5% from the thermodynamic value.

The state of water in food results from the structure

of the water molecule and its interactions with remain-

ing food constituents. Spatial configuration of the water

molecule is well known. It was proposed by Bjerrum in

1951 (Siniukow, 1994) and is pictured as a regular tetra-

hedron (Fig. 1). Inside of the solid there is an atom of

oxygen, and in corners there are partial charges distant0.099 nm from the centre of the tetrahedron. The van

der Waals diameter of the water molecule is 0.282 nm.

According to this model hydrogen atoms are in corners

with positive charge and two orbitals of paired electrons

are directed to remaining corners.

Partial charges located in the tetrahedron corners

lead to interactions between neighbouring molecules. A

hydrogen bond is formed, which nature is not strictly

electrostatic. Molar energy of hydrogen bond is esti-

mated as 20–35 kJ, and the distance between oxygen

atoms is from 0.26 to 0.30 nm (Franks, 1988). Hydrogen

bond is linear (Fig. 2) but it can be stretched or bent. In

solutions interactions between water and solute mole-cules also occur. Hence, properties of solution are de-

termined by interactions water–water, water–solute and

solute–solute.

Interactions between water and solute molecules are

called hydration. In the presence of polar molecules

these interactions are dominated by hydrogen bonding.

In solutions of small molecules undergoing dissociation

interactions between ion and solvent molecules are

present. Around ion a hydration shell is formed which

density increases with the increase of concentration.

Hydration shells are susceptible to deformation (Franks,

Journal of Food Engineering 61 (2004) 483–495

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* Tel./fax: +48-22-8434602.

E-mail address: [email protected] (P.P. Lewicki).

0260-8774/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0260-8774(03)00219-X

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1988). It was shown that ions Ca2þ, Naþ, ClÀ are sur-

rounded by 6 water molecules while ion Liþ is hydrated

by 4 water molecules (Franks, 1975). Ordering of water

molecules in a hydration shell is dependent on the sur-

face charge density of the ion. Ions with large surface

charge density form well ordered hydration shells. These

ions are called structure formers. Ions Liþ, Mg2þ, FÀ,

SO2À4 , PO3À

4 belong to that group. Ions with small

surface charge density are not able to overcome inter-

actions between water molecules. In consequence struc-

ture of solvent in the vicinity of the ion is distorted. Ions

IÀ, Sþ, NOÀ3 , ClOÀ

4 are considered as structure breakers.

Strong interactions between water and ions orient

spatially water molecules in hydration shell. That in turn

results in spatial arrangement of neighbouring water

molecules. In effect the ions are surrounded by few

layers of oriented spatially water molecules. The spheri-

cal structure is not compatible with tetrahedral one, and

water molecules are strongly distorted as far as theirstructure and dynamics are concerned.

Hydration of polar molecules cannot be estimated on

the number of groups able to form hydrogen bonds.

This is because interactions between water and polar

molecule can alter spatial conformation of a solute and

solvent as well. For example hexoses contain 5 –OH

groups and should interact with 5 water molecules.

Measurements show that hydration shell contains 3.7

water molecules. This is due to different hydration of

optical isomers of hexose. Glucose as the a-isomer in-

teracts with 3 water molecules and as b-pyranose has 4

active groups (Franks, 1988). Moreover, some –OH

groups are favoured in comparison with others. In the

pyranose ring equatorial groups are preferentially hy-

drated in comparison with axial groups.

In macromolecules such as proteins or polysaccha-

rides intramolecular interactions deform electron cloud

and surplus or deficit of electrons occurs. Hence, hy-

drogen bonds can be formed and water molecules are

built in the structure of biopolymers. Dissociation of

some groups leads to ionic interactions. Moreover, polar

groups present in macromolecule can form hydrogen

bonds and the polymer is surrounded by hydration shell.Two states of water can be considered (Fig. 3). One

state, in which water molecules are immobilised in the

structure of a macromolecule, is often called the struc-

ture water. Another state is the one in which water

molecules movement is not completely restricted. Water

molecule can reorient in respect to ionic or hydrogen

bond. This water is called hydration water. It is, there-

fore, evident that interactions between water and mac-

romolecules create and stabilise spatial conformation of

biopolymers.

Number of water molecules in the hydration shell

depends on the nature of macromolecule and its energy

and ionisation state. For example phosphatidilcholine issurrounded by 10 (Crowe, Clegg, & Crowe, 1998), urea

by 7 (Nandel, Verma, Singh, & Jain, 1998) while full

hydration of lyzosyme needs 360–400 water molecules.

Water is an essential component of starch structure.

In a lately proposed model 4 water molecules are pre-

sent in the A structure unit and 36 water molecules in

the B structure (Imberty, Buleon, Tran, & Perez, 1991;

Imberty & Perez, 1988).

Apolar compounds and polar compounds containing

apolar groups interact with water and reduce its degrees

of freedom. It results in a certain stabilisation of water

Fig. 3. Hydration of macromolecule. Shaded area––structure water,

plain area––hydration water.

Fig. 1. Structure of water molecule.

Fig. 2. Hydrogen bond.

484 P.P. Lewicki / Journal of Food Engineering 61 (2004) 483–495



molecules in space (Fig. 4) and the liquid acquires a

structure similar to the structure of a solid. This pheno-

menon is called hydrophobic hydration, and can beconsidered as the trial to reject solute molecule by the

solvent.

Hydrophobic and hydrophilic interactions lead to

formation of structures in which properties of water

differ from those of bulk water.

In numerous experiments it was shown that part of

water is so strongly bound to food constituents that is

not able to form ice crystals during freezing. Unfreezable

water in blood plasma is equal to 0.47 g/g, in collagen

0.26 g/g (Simatos, Faure, Bonjour, & Conach, 1975) in

egg albumen 0.55 g/g, in maize starch 0.35 g/g and in

cellulose 0.16 g/g (Steinberg & Leung, 1975). Moreover,

it was shown that freezing of water in the presence of soy

proteins occurs at temperatures which span for 40 K

(Johari & Sartor, 1998). This is due to interactions be-

tween water and proteins, which affect ice crystallisation.

Water is a very good solvent, but its solvent proper-

ties depend on the solute and interactions between

molecules. In the mixture NaCl-casein absorbing water

mobility of salt molecules is observed at aw > 0:3 (Gal,

1975; Kinsella & Fox, 1986). Mixture of starch with

glucose or sucrose when moistened shows mobility of

sugar molecules at aw ¼ 0:8 (Duckworth, 1981). Enzy-

matic hydrolysis of starch with b-amylase is initiated at

aw 0.65–0.70. Energy of mixing of moistened wheat flourincreases at water contents larger than 28% (Daniels,

1975). This is due to interactions between water and

protein molecules. At water contents higher than 28%

protein chains become mobile, structure of dough is

formed and mixing energy increases. Starch and gluten,

two main components of wheat flour interact with water

to a different degree. In starch, at 2% water content,

nuclear magnetic resonans shows no presence of water

protons. In gluten this effect occurs until 7.5% water

content is reached (Chinachoti, 1998). Protein mobility

begins at water content 0.1–0.2 g/g but it does not mean

that at higher water contents properties of water are

those of bulk water. In collagen water shows latent heat

of fusion identical with that of bulk water at water

content 0.6 g/g. Full hydration of lysozyme is estimated

at 0.45–0.50 g/g but lowering its water content below

0.75 g/g increases temperature of denaturation (Gre-

gory, 1998). These data show that water properties are

different from those of bulk water not only in hydration

shell but also in the vicinity of that shell.

Numerous literature data and examples presented

above picture the state of water in food. At low water

activities properties of water differ substantially from

those of bulk water. Water molecules are strongly

bound by hydrophilic centres, polar groups and charge

bearing moieties. This water does not freeze, and its heat

of evaporation exceeds that of bulk water. It is not

available to chemical reactions and does not serve as a

solvent. Moreover, it does not work as a smear, hence

mobility of one structures against others is not possible.

Formation of subsequent hydration shells is depen-dent on surface and spatial conformability of substrates

with water molecule structure. Biopolymers induce

structuring of water by hydrogen and hydrophobic in-

teractions. Moreover, swelling, rotational and transla-

tional mobility of chains, cross-linking and interactions

polymer–polymer affects number of molecules hydrating

biopolymers. Co-operative binding of water and pres-

ence of crystalline and amorphous domains also influ-

ence amount of water in hydration shells.

Interactions between biopolymers and water are

modified by the presence of small molecules, especially

those, which undergo dissociation. Interactions between

biopolymers and salts expel water molecules and change

spatial conformation of a newly formed structure.

Competition for water molecules can lead to redistribu-

tion of water in the material. Parts with higher and lower

water content can be formed. Effect of small molecules

on hydration of polymers is evident. Some salts desta-

bilise and others stabilise structure of biopolymers and

affect the extent of hydration shells. The effect of small

molecules on hydration of biopolymers is pronounced at

water content at which water has solvent properties.

Above presented description of the state of water in

food shows that interactions between water molecules

and remaining components of the material must affect itsproperties biological, chemical and physical as well.

Water can act as a solvent, it can structure macromole-

cules by internal bonding and hydrophilic and hydro-

phobic interactions, and it can act as a plasticizer making

possible movement of one structure in respect to others.

2. Effect of water on rheological properties of food

Water influences rheological properties of food

in liquid and solid state as well. In liquid state it is

Fig. 4. Hydrophobic interactions. Shaded area––apolar constituent of

the system.

P.P. Lewicki / Journal of Food Engineering 61 (2004) 483–495 485



pronounced by changes in viscosity and consistency,

while in solid foods water affects their response to force.

Water is a Newtonian liquid and its viscosity is

temperature dependent. Small molecules dissolved in

water interact with water molecules, hydration shells are

formed and some structuring of water surrounding hy-

drated solutes can occur. This loss of entropy leads to

the increased viscosity. Small ions and multivalent ions

increase water viscosity due to polarisation, structuring

and electrostriction of surrounding water molecules.

Ions of Liþ, Naþ, H3Oþ, Caþ2, Baþ2, Mgþ2, Alþ3, OHÀ

and FÀ show this effect on water viscosity. On the other

hand large monovalent ions such as Kþ, NHþ4 , ClÀ, BrÀ,

JÀ, NOÀ3 , ClOÀ

4 distort water structure and practically

increase water flowability.

Increased temperature reduces number of hydrogen

bonds and increases vibration of molecules. It results in

decreased effect of small molecules and ions on water

viscosity. On the other hand increased concentration of

solute increases viscosity of solution. Over limited ran-ges of concentration the effect of solids concentration on

viscosity of liquid food can be described by either ex-

ponential (Vitali & Rao, 1984), or a power type of re-

lationship (Rao, 1986). The equation of the form:

l ¼ l0 Á ca

can be written in another way showing the effect of

water concentration on viscosity of fluid food.

l ¼ l0 Á ð100 À wÞa

where l0 is the viscosity of water, Pa s; w is water con-

tent, %; c is the solubles concentration, %; a is constant.

Macromolecules soluble in water affect strongly

rheological properties of solution. Solution becomes

non-Newtonian and either time independent or time

dependent. Macromolecules acquire structure, which in

large part is due to interactions with water molecules.

The size and shape of hydrated macromolecule influ-

ences its behaviour in sheared liquid.

Spherical particles are rotating under the influence of

shearing forces. The angular velocity (x) in laminar flow

is

x ¼ 12

_cc

where _cc is shear rate, sÀ1.For particles with shape different than sphere the

equation is written as

x ¼ a Á _cc

where a is dependent on the shape of the hydrated

molecule.

Viscosity of a solution of spherical particles can be

calculated from the Einstein equation

l ¼ l0ð1 þ 2:5uÞwhere u is a fraction of volume occupied by spheres.

Translatory and rotational motion of stiff macro-

molecules results in energy dissipation. Energy lost due

to friction is called intrinsic viscosity and is related to

molecular weight of the polymer. According to Mark-

Houwink equation the intrinsic viscosity is equal to

lint

¼ K

Á M b

where K and b are constants characteristic for the given

polymer–solvent system, and M is molecular mass of

polymer.

Chain macromolecules influence solution viscosity

even more than the stiff symmetric or asymmetric mol-

ecules. Translatory and rotational motion of these

macromolecules is accompanied by changes in spatial

conformation due to shearing forces. Experimental data

shows that intrinsic viscosity of solution of chain macro-

molecules is proportional to effective hydrodynamic

volume and inversely proportional to molecular mass of

the polymer. The power b in the Mark-Houwink equa-

tion is in the range 0:56 b6 1.Biopolymers such as proteins, nucleic acids and

polysaccharides have spatial conformation much differ-

ent from the sphere or a chain. Moreover, biopolymers

can change the spatial conformation under the influence

of temperature or other substances. Nevertheless the

Mark-Houwink equation can be used to calculate in-

trinsic viscosity.

In the Mark-Houwink equation the power value is

relevant to solute–solvent interactions in diluted solu-

tions.

Intrinsic viscosity can be calculated from the equation

lint ¼ bðv2 þ h1v1Þwhere v1; v2 are the specific volumes of the solvent and

solute, h1 is the weight of solvent associated with a unit

weight of solute, b is the shape factor of the particle.

The volume fraction of the particles is obtained from

the equation

/ ¼ cðv2 þ h1v1Þwhere c is concentration.

The weight of solvent associated with a unit weight of

solute is sometimes called hydrodynamic hydration. It

was estimated to be between 0.2 and 0.5 g water/g of different proteins. These values are about the same range

as the range of unfreezable water.

The Mark-Houwink equation and intrinsic viscosity

apply to diluted solutions in which interactions between

polymer molecules can be disregarded. In solution

polymer molecules can interact directly or can act one

on the other through the solvent molecules. Relation-

ship between viscosity and polymer concentration (c)

was developed by Huggins (Rao, 1986) in the form:

lred ¼lÀ l0

l0 Á c¼ lint þ k 1l

2int Á cþ Á Á Á




where lred is the reduced viscosity, k 1 is the interaction

coefficient. Generally it is assumed that k 1 is not de-

pendent on molecular weight of the polymer. It is a

measure of affinity of the polymer to the solvent.

However, the interaction coefficient is sensitive to shear

rate. The higher the shear rates the shorter the time of

contact between interacting molecules. It results in

smaller values of k 1.

Flow behaviour of the solution is affected by inter-

actions between macromolecules. The larger the mac-

romolecule the lower are the concentrations at which

intermolecular interactions affect friction between mole-

cules and solvent. Increase of shear rate affects extent of

interactions and the influence of shear rate on shear

stress is observed. This influence is the stronger the

larger are the molecules. Moreover non-spherical

molecules subjected to shearing forces become oriented

according to the flow direction. This leads to the de-

pendence of shear force on shear rate. Shear thinning of

the liquid is observed and its consistency decreases withincreasing shear rate.

Special types of macromolecules are polyelectrolytes.

Their size and shape depend on the ionic strength of the

solution. When a solution is diluted with water the in-

crease of reduced viscosity is noticed. This is due to

swelling of a polymer. At high ionic strength of the so-

lution Debye–H€uuckel ionic layer reduces electrostatic

repulsion of polymer chains and the increase of reduced

viscosity is smaller than that observed in solutions with

weak ionic strength. For polyelectrolytes, the variation

of reduced viscosity with concentration follows the

empirical equation (Elfak, Pass, & Phillips, 1978):

lÀ l0

l0c¼ A

1 þ B ffiffiffic

p þ D

where A, B and D are constants.

The state of water in solids can be graphically or

analytically represented by the so-called sorption iso-

therms. The sorption isotherm shows relationship be-

tween water content and water activity in the material

being in the state of equilibrium. At low water activities

interactions between water and macromolecules are so

strong that water is devoid of solvent properties

(Duckworth, 1981). Under these conditions mobility of

small molecules as well as macromolecules is very muchreduced. Material is regarded as crisp or crunchy.

Increasing water content can lead to plasticizing or

antiplasticizing effect. Water acquires properties of sol-

vent and promotes mobility of polymer chains. This

plasticization of polymer chains facilitates deformation.

Material becomes soft, extensible and flowable and

losses crispness, hardness or toughness (Karel, 1985).

Numerous authors (Harris & Peleg, 1996; Katz & La-

buza, 1981; Lewicki & Wolf, 1995; Sauvageot & Blond,

1991; Wollny & Peleg, 1994) reported such an effect of

water on texture of brittle food. In all these publications

it is shown that there is a narrow range of water acti-

vities within which there is an abrupt change in me-

chanical properties of the material. Further increase in

water activity leads to little changes in mechanical

properties of the material. However, depending on the

kind of food, properties of moist food can be appreci-

ated or not accepted by consumers. Moistening of dried

fruits or vegetables can lead to properties resembling

fresh produce, while sogginess or chewiness of biscuits

or snacks means end of acceptability.

In some cases moisture toughening is observed. In-

creased mobility of macromolecules can expose new

centres for interaction with water molecules. This can

add some strength to the material and reduce its brit-

tleness. Georget, Parker, and Smith (1995), Nichols,

Appelquist, and Davies (1995), Harris and Peleg (1996)

and Seow, Chean, and Chang (1999) reported such an-

tiplasticizing effect of water on food mechanical pro-

perties. Our recent work showed that failure stress of flat

wheat bread increased as moisture was absorbed (Fig. 5)and reached a peak at an aw range 0.5–0.6 (Marzec,

2002).

Solvent properties of water can also be manifested by

structure collapse. Fast dehydration or freezing of so-

lutions and dispersions can prevent crystallisation and a

meta-stable system is formed. Amorphous state is very

sensitive to temperature and water (Karel, 1985). In the

amorphous state, due to a very high viscosity, material

acquires properties of a solid. Increased temperature

lowers viscosity and mobility of molecules becomes

possible. The same effect is observed when material

absorbs water. Phase transition goes through the rub-

bery state and shrinkage of the material is recorded. At

specific water content viscosity of the matrix is too low

Fig. 5. Influence of water activity on breaking force of flat bread.

Breaking means that flat bread is disintegrated into pieces; flow means

plastic deformation without disintegration. Points represent replicate

experiments.




to resist the flow caused by gravity and surface tension

forces. For a given material the water content at which

phase transition is initiated is strongly dependent on

temperature. Further sorption of water leads to crys-

tallisation and material attains stable state.

Collapse of structure and formation of rubbery and

crystalline states affects strongly physical properties of

the material. The most pronounced effects are stickiness

and caking phenomena. Moreover, mechanical proper-

ties are much different in comparison to those of the

material in the amorphous state.

As it was stated above the amorphous state is meta-

stable. It means that the rate at which the phase transition

proceeds is dependent on the increment of temperature or

water content. At small increments the rate is low and

structure collapse or crystallisation can last for days or

even months. Hence, properties of the material become

storage time dependent.

3. Influence of water on thermal properties of food

This part of the paper presents the influence of water

on those thermal properties of food, which can be di-

rectly measured. These include specific heat and thermal

conductivity.

Water has a high specific heat in comparison to other

food components. Hence, even small amount of water in

food affects its specific heat substantially. It is generally

accepted that specific heat, as a physical property, obeys

the rule of additivity. It means that specific heat of a

product is equal to the sum of the fractional specific

heats of the main constituents. The simplest models for

low-fat foods have a form:

cp ¼ aþ b Á wwhere a and b are constants specific for a product, w is

water content in decimal. Such models were proposed by

Lamb (1976), Siebel (1982) and reviewed by Sweat

(1986) and Miles, van Beek, and Veerkamp (1983).

Constants a and b are dependent also on the tempera-

ture range at which specific heat is calculated. In all

these formula when w ¼ 1 the specific heat is equal to

the specific heat of pure water.

Another approach to specific heat calculation as-

sumes that food is composed of two constituents that is

water and dry matter. For fruits and vegetables it was

proposed to calculate specific heat from the formula

(Pijanowski, Mrozewski, Horubala, & Jarczyk, 1973)

cp ¼ 4:187ð1 À 0:0066 sÞwhere s is dry matter content in %.

Using additivity principle specific heat can be calcu-

lated from the following equation:

cp ¼ cpww þ cpp p þ cpf f þ cpcc

where cp is specific heat at constant pressure, and

w; p ; f ; c as well as subscripts w, p, f, c denote water,

protein, fat and carbohydrate content, respectively.

Different values are assigned to specific heats of pro-

teins, fat and carbohydrates. Nevertheless all the specific

heat models published until now show that water sub-

stantially influences specific heat of foods.

Analysis of this type of equations shows that most of

them converge on the specific heat of water at w ¼ 1, but

the lower is the water content the more the difference

between predictions (Miles et al., 1983).

Calculation of specific heat of frozen food must ac-

count for specific heat of ice and specific heat of un-

frozen water. Hence, mass fraction of water in food is

divided into this portion which crystallises to ice and

that which does not undergoes phase change. The frac-

tion of water, which does not freeze out, depends on

temperature and interactions between water molecules

and food components.

Foods containing large amount of fat have a specificheat which accounts not only for a fraction of fat but

includes also term arising from the change of phase of

fat.

Thermal conductivity of food depends on structure

and chemical composition of the sample (Sweat, 1986).

Thermal conductivity increases with increasing water

content for all food products at temperatures above

freezing. Most of models used to calculate thermal

conductivity (k) of foods with high water content has a

form:

k

¼c1

þc2w

where c1 and c2 are constants and w is water fraction in

food.

Similarly as with specific heat most of the equations

converge on the thermal conductivity of water at w ¼ 1.

Predictions agree at high water contents, and at low

water contents differences are most marked.

Additivity principles are used to calculate thermal

conductivity of most of liquid foods and many solid

foods. Water, protein, carbohydrate and fat are taken

into account. In some calculations ash is also included.

As an examples an equation developed by Choi and

Okos (Sweat, 1986) can be given:

k ¼ 0:61 xw þ 0:20 xp þ 0:205 xc þ 0:175 xf þ 0:135 xa

Sweat (1986) presents a modified best-fit equation

based on 430 points in the form:

k ¼ 0:58 xw þ 0:155 xp þ 0:25 xc þ 0:16 xf þ 0:135 xa

where x is a fraction of a food component and subscripts

w, p, c, f and a denote water, protein, carbohydrate, fat

and ash, respectively.

The above equations are good for non-porous foods.

For porous foods they are not accurate and adding air

as another component is not satisfactory.




In foods containing air the flow of heat is impeded.

The porosity or volume fraction of air is included into

calculation. The parallel model gives

k ¼ ewkw þ eoko þ ef kf epkp þ eckc

and for perpendicular model

I k¼ ew

kwþ eo

koþ ef

kf þ ep

kpþ ec

kc

where e is a volume fraction of food component and

subscripts w, p, c, f and o denote water, protein, car-

bohydrate, fat and air, respectively.

Since some foods have an internal structure or can be

described as a dispersed system some structural models

for thermal conductivity were developed. The models

assume the food consists in two different components

physically oriented so that heat travels either in parallel

or in perpendicular through each of them.

Assuming that food is composed of dry material and

wet material with infinite moisture and that both com-ponents are uniformly mixed the parallel model takes

the form (Maroulis, Saravacos, Krokida, & Panagiotou,

2002):

k ¼ 1

1 þ xkd þ x

1 þ x xkx

and perpendicular model is

k ¼ 1

1 þ xÁ 1

kd

þ x

1 þ xÁ 1

kx

À1

where kd is the thermal conductivity of the dry phase, kx

is the thermal conductivity of the wet material, x is thewater content, kg/kg d.m.

In dispersed system the volume fraction of dispersed

component as well as thermal conductivity of dispersed

and continuous component are taken into account

(Miles et al., 1983).

In modelling thermal conductivity of frozen foods the

presence of unfrozen water complicates calculations.

The volume fractions of unfrozen water, ice, fat, protein

carbohydrate and air are taken into account and either

parallel or perpendicular models are applied (Miles et al.,

1983).

Thermal conductivity of binary liquid mixtures is

curvilinearly related to mass fraction of water. Usuallythe relationship between thermal conductivity and mass

fraction of water is concave-upwards.

Specific heat and thermal conductivity are intrinsic

properties of the material. They represent the ability of

the material to accumulate and to transport heat, re-

spectively. It is evident that both properties are strongly

related to water content. Moreover, analysis of pub-

lished data shows that the less water in the material the

more discrepancies between predicted and measured

values. It seems reasonable to assume that the discrep-

ancies arise from the treatment of whole water in food as

bulk water. Interactions between water and food com-

ponents affect water properties. Hence, it is rather dif-

ficult to accept that the interactions do not affect

thermal properties.

4. Influence of water on mass transfer properties

Many unit operations in food processing are based on

mass transfer. These are drying, extraction, distillation,

absorption, crystallisation, salting, candying and many

others. Mass transfer can proceed in one phase or can be

done through interface between two phases. Two types

of mass transfer mechanisms are considered, i.e. mole-

cular and convective.

Molecular mass transfer called diffusion occurs in

solids and fluids. Random movement of molecules

causes it.

FickÕs first law gives the flux of component A diffus-

ing in a stationary component B:

nA ¼ D

À dcA

d x

where cA is the concentration of component A, x is the

diffusion path and D is the diffusivity or diffusion coef-

ficient. Diffusion coefficient is intrinsic property of the

system, hence it is affected by interactions between dif-

fusing substance and the medium.

In liquids the diffusivity can be predicted by empirical

relations, like the Wilke-Chang equation (Saravacos,

1986) in which there is an association parameter which

accounts for association of solvent molecules. And the

association arises from the interactions between mole-

cules. For water it is 2.6 and shows that interactions

between water molecules strongly affect diffusion of

small molecules.

Solving FickÕs first law is usually done with the as-

sumption that diffusion coefficient is constant and in-

dependent of concentration. Experimentally it was

shown that this assumption is false, especially at high

concentrations of solutes.

Diffusion of macromolecules, even at low concen-

trations, is more dependent on interactions with solvent

than that of small molecules. Einstein related diffusionof macromolecules with their microscopic properties

and introduced frictional coefficient. The Einstein

equation is

D ¼ kT

f ¼ RT

Nf

where N is the Avogadro number, R is gas constant, T is

temperature in K, f is frictional coefficient and k is the

Boltzmann constant.

The frictional coefficient f is directly related to

size and shape of the macromolecule. Assuming the




macromolecule is a solid sphere the Stokes equation can

be applied and diffusion coefficient is

D ¼ kT

6plsr

where ls is viscosity of the solvent and r is radius of a

sphere. In most cases the shape of macromolecules dif-

fers from the sphere, hence diffusion coefficient is mea-sured experimentally.

The frictional coefficient is affected by intermolecular

interactions. At relatively low concentrations it is lin-

early related to volumetric fraction occupied by mac-

romolecules in the solution:

f ¼ f 0ð1 þ anvÞwhere n is the number of macromolecules in a unit

volume, v is the volume of one macromolecule, a is

constant, f 0 is the frictional coefficient of solvent.

Diffusion coefficient is affected by intermolecular in-

teractions and its value is strongly dependent on solu-tion concentration. In real solutions the relationship is

D ¼ kT

f ð1 þ 2a2 Mcþ 3a3 Mc2 þÁ Á Á Þ

where M is the molecular weight of macromolecule, c is

concentration, a2; a3; . . . are viral coefficients. Diffusion

coefficient depends on molecular weight, concentration

and viral coefficients, which account for intermolecular

interactions.

Since both diffusion coefficient and frictional coeffi-

cient are influenced by concentration a complex effect of

increased concentration (increased intermolecular in-

teractions) on mobility of macromolecules can be ob-served (Cwietkow, Eskin, & Frenkel, 1968).

The structure, which usually is heterogeneous and

interactions with solid matrix affect diffusion of gases,

vapours and liquids in solids. Diffusion in solids is an

unsteady process. Concentration gradients impose

stresses on the material and diffusion can be accompa-

nied by shrinkage, deformation or swelling. In some

cases strong interactions between diffusing substance

and solid matrix occur and adsorption or chemisorption

accompanies diffusion.

Changes in volume and/or sorption processes ac-

companying diffusion lead to a complex relationship

between diffusion coefficient and concentration. In most

cases diffusion in solids is interpreted with the help of

effective diffusion coefficient. The coefficient accounts

for different ways the molecule is transported within the

solid and includes porosity (e) and tortuosity (s) of the

diffusion path. It is defined as follows:

Deff ¼ e D

s

The diffusivity of water and other small molecules

decreases significantly at low moisture contents. The

decrease depends on the kind of diffusing molecule. It

was found that diffusion coefficient of water decreases

much less with the increased solids content than the

diffusion coefficient of other substances. On this basis, in

the mid 1960s, Thijssen proposed a selective diffusion

mechanism for volatile retention during drying (King,

1988).

At low water contents diffusivity of reactants is

strongly solvent and temperature dependent. The way

both parameters control the mobility of small molecules

and the flexibility of macromolecules is complex and

little known.

The Stokes–Einstein relationship describes transla-

tional diffusion coefficient of a molecule while the De-

bye–Stokes–Einstein equation gives the rotational

diffusion coefficient:

Dtrans ¼ kT

6plsr

Drt ¼kT

8pl sr 3

Both equations show that the diffusion coefficient is in-

versely related to viscosity of solvent. Moreover, the

influence of the hydrodynamic radius on mobility of the

molecule is presented.

In diluted solution individual or local motions of

molecules are present and the motion is affected by in-

teractions between molecules and solvent. Activation

energy provides information on the number of mole-

cules or the size of the region involved in the motion. At

low solvent concentrations motion of one molecule or a

polymer segment involves a displacement of the sur-

rounding segments or molecules. A strong cooperativityoccurs. Le Meste et al. (1999) measured sucrose diffusion

in concentrated solutions at temperatures close to the

glass transition temperature. In 90% sucrose solution and

at temperatures 60 K above glass transition temperature

diffusion coefficient was of the order 10À15 –10À17 m2/s

while in 30% solution and at room temperature it is of the

order of 10À10 m2/s.

Mobility of sucrose in starch–sucrose extrudates hy-

drated with water starts at a moisture content close to

8% (Farhat, Mitchell, Blanshard, & Debyshire, 1996).

5. Effect of water on electrical properties of food

The most intensively investigated electrical properties

of foods have been the relative dielectric constant and

loss factor. These dielectric properties determine the

energy coupling and distribution in a material subjected

to dielectric heating.

Primary determinants of electrical properties of food

are frequency, temperature, chemical composition and

physical structure (Mudgett, 1987). Water as one of the

major constituents of foods affects dielectric behaviour




of the material. Dielectric constant and loss factor varies

with moisture content. de Loor and Meijboom (1966)

and de Loor (1968) showed that high-moisture foods are

generally linear dielectrics. On the other hand data

published by Stuchly (1970) showed that the more water

contains food the more dielectric properties depend on

temperature. Dried solids showed little or no variation

of dielectric properties with temperature. Studies done

by Kent and Kress-Rogers (1987) and Laursen (1987)

showed that dielectric permittivity of sucrose solutions

are dependent on concentration. The higher was the

sucrose concentration the lower was the dielectric per-

mittivity. It is important to note that interactions of

water with other food constituents affect food dielectric

properties. Polarity of the water molecules is strength-

ened by the dissolved ions at high temperatures. The

influence of the dissolved ions is even more important at

lower frequencies (Ohlsson & Bengtsson, 1975).

Dielectric measurements done on hydrated com-

pressed pellets of lactose and maltose showed that 0.8water molecules per saccharide molecule are tightly

bound (irrotationally). It is reasonable to assume that in

both crystals probably only one oxygen atom per mole-

cule is accessible to water. All the water molecules in

excess of 0.8 per saccharide molecule are loosely bound

(Daoukaki-Diamanti & Pissis, 1987).

Electromagnetic field at radio- and microwave fre-

quencies interacts with food constituents. These inter-

actions result from the dipole rotation of free water and

conductive migration of charged molecules. Dipole ro-

tation results in the disruption of tetrahedryl hydrogen

bonding patterns (Mudgett, 1986).

Salts dissolved in water influence dielectric properties

of the solution. Water molecules are bound by counte-

rions of dissolved salts and it results in depression of

dielectric constant and increase of loss factor. Sodium

chloride binds 5.5 water molecules on average (sodium

ion binds 4 and chloride ion binds 7 water molecules).

Binding of water by macromolecules leads to low values

of dielectric constant. Takashima (1962) investigated

hydrated protein crystals and found that dielectric

constant increases rapidly when water content increased

from 20% to 35%. The moisture content and the state of

water influence dielectric properties of foods and that is

strongly emphasised in literature (Mudgett, 1986).Investigation of dielectric properties of freeze-dried

potato equilibrated to different water activities (Mud-

gett, Goldblith, Wang, & Westphal, 1980) showed that

the dielectric properties, i.e. dielectric constant and loss

factor increase significantly over those of dry solids at

water activity close to 0.86. The critical water activity

corresponded to moisture content of about 20%.

Electrical conductivity is a food property important

in heating and separation processes. It occurs in media

that containing electrically charged molecules. Low

molecular weight molecules are electrolytes with pre-

cisely defined electrical charge. With macromolecules

the situation is not as clear and there is still considerable

interest in methods for estimating the charge carried by

biopolymers.

Law of Coulomb gives the force experienced by a

molecule in an electrical field:

F ¼ neE

where n is the number of charges, e is the magnitude of

electronic charge and E is the electrical field, V/cm. In

the steady motion situation the force is equal to the

resistance to motion. Hence

fv ¼ neE

where f is the friction factor and v is the velocity.

The molecule mobility is

v ¼ neE

f

and if the particle is spherical then StokesÕ

law can beapplied:

v ¼ neE

6pl R

where R is molecule radius and l is the solvent viscosity.

From the above equation it is evident that viscosity of

liquid food affects pronouncedly mobility of ions, hence

it influences electrical conductivity. Moreover, it was

found that increased concentration of insoluble particles

decreases electrical conductivity (Sastry & Palaniappan,

1992). In general it is accepted that electrical conduc-

tivity of liquid foods increases up to about 30% dry

matter content. At higher concentrations the viscosity

effect prevails and mobility of molecules decreases.

Electrical conductivity of solids, especially of bio-

logical origin is dependent on such variables as size and

shape and temperature history. Electrical conductivity

increases as particles get smaller. Heating of material

over 60 °C substantially increases electrical conductivity.

Probably it is due to disintegration of cell membranes,

which create additional resistance to ion movement.

Decrease of water content in the material decreases

electrical conductivity. However at water activity lower-

than 0.75–0.80 electrical conductivity is small and

proves that water–substrate interactions predominate atlow material moistness.

6. Effect of water on optical properties of food

Among important physical properties of foods are

those properties, which are stimulated thermally. Ther-

mal radiation is released from all bodies and depends on

their material properties and temperature.

Thermal radiation is emitted, transferred and ab-

sorbed and extends in the electromagnetic spectrum




from 0.1 to 1000 lm. Between 0.78 and 1000 lm lies

infrared radiation, but practical application finds radi-

ation with wavelength below 5.6 lm (Ginzburg, 1969).

In food processing near infrared radiation with wave-

length around 1 lm is the most important.

Emission of thermal radiation comes normally from

molecules in a layer close to the surface. This layer is

around 1 lm thick; hence the infrared radiation is a

surface process. The emissivity of the material depends

on its chemical composition, shape, temperature and the

surface texture. Examples of emissivities of materials in

food processing are given in Table 1.

Infrared radiation falling on a body is partly ab-

sorbed and partly reflected at its surface. According to

Kirchhoff Õs law a close relationship exists between the

emission and absorption capabilities. Since water has a

high emissivity foods containing water show significant

ability to absorb infrared energy. Absorption of infrared

energy by food materials is not a pure surface process.

In some products infrared radiation is capable to pene-trate the material as deep as 30 mm, and penetration to

a depth of 4–5 mm is very common (Ginzburg, 1969).

Water as a constituent of food affects very strongly its

capability to absorb infrared energy. Spectral absorp-

tion coefficient of water shows three maxima at the

following wavelengths: 1.4, 3.0 and 6.0 lm. It means

that at those wavelengths water absorbs infrared energy

maximally. The wavelength affects the ability of infrared

radiation to penetrate the material. Generally the longer

the wavelength the more energy is absorbed in surface

layers. Hence, in food processing it is recommended to

use infrared radiation at wavelength close to the first

maximum, i.e. 1.3–1.4 lm.

There is not much research done on relationship be-

tween water content of food and its ability to absorb

and transmit infrared energy. From data published by

Ginzburg (1969) it can be inferred that decrease of water

content makes material more transparent for infrared

radiation. Layer 0.5 mm thick of tomato paste was

transparent to infrared radiation in 55.8% and 95% at

dry matter contents 15% and 40%, respectively. Simi-

larly wheat dough is much less transparent than bread,

however differences in structure must be here taken into

account. Drying of potatoes to 42% water content in-

creases their transparency to infrared radiation. Ac-cording to Asselbergs, Mohr, and Kemp (1960) spectral

characteristics of dry matter of apple becomes importantin final stages of drying. Nowak (2002) who showed that

temperature of apple undergoing drying does not

change much until 90% of water is removed (Fig. 6)

indirectly proved the major role of water in absorption

of infrared energy during drying. Further drying leads to

substantial increase of material temperature. It is sug-

gesting that at low moisture contents efficiency of water

evaporation decreases and a part of infrared energy is

absorbed by the components of dry matter of apple.

Although the data in the literature is scarce it can be

concluded that water is responsible, in large degree, for

infrared absorptivity of food materials.

Optical properties of food materials are responsible

for their colour and appearance. The appearance in-

cludes such attributes as translucency, gloss, uniformity

and pattern.

There is not much research done on the effect of water

on the colour and appearance of food. In liquid foods

some information on the effect of water on colour of the

product can be extracted from the research considering

the colour and pigment concentration. Aqueous solu-

tion of red anthocyanin pigment shows maxims for a

(redness) and b (yellowness) values in the CIELAB

System at pigment concentration of 0.5–2 mg/ml while

the L (whiteness) value decreases with increasing anto-cyanin concentration (Francis, 1987). Kent (1987) and

Brimelow (1987) who showed that L value decreases

with increased dry matter content in tomato paste report

similar results. The a value shows a maximum at con-

centration close to 20% d.m. while the b value decreases

with increasing dry matter content. On the other hand

dilution of evaporated milk results in a maximum in the

case of L and continuous decrease of a and b (Kent,

1987). Foods such as milk or tomato paste shows in-

ternal diffusion of light, which can be observed as

translucency and opacity. Research done by MacDou-

Table 1

Approximate values of emissivities (Fellows, 1988)

Material Emissivity

Dough 0.85

Lean beef 0.74

Beef fat 0.78

Water 0.955

Ice 0.97

Fig. 6. Relationship between temperature of apple slice and water

content during infrared drying. Parameter is air velocity.




gall (1987) showed that increasing total solids in the

evaporated milk up to about 10% increases the light

scatter, which increased reflectance. Further increase in

total solids decreases reflectance and product becomes

more opaque. For tomato paste the L value showed

maximum at total solids concentration close to 10%.

Influence of water on colour and appearance of solid

foods is also evident. The appearance of free water on

vegetables and fish increase their apparent shininess and

increase visual impression of freshness. Blanching of

vegetables make them to look more colourful. During

this process air in the plant tissue is replaced by water,

the refractive index changes and reduces light scattering.

In consequence blanched vegetables are perceived more

intense in colour (Hutchings, 1994).

Evaporation of water from the material affects colour

of material undergoing drying. Results published by

Lewicki and Duszczyk (1998) showed that luminance (Y

value in the CIE system) of potato, carrot and pumpkin

increased linearly with the decrease of water contentuntil the maximum was reached. The maximum oc-

curred in blanched potato at water content 2.18 g/g

d.m., while for carrot and pumpkin it was 0.91 and 0.68

g/g d.m., respectively. Chromaticity co-ordinates also

changed and the colour moved toward the white point,

in the case of carrot and pumpkin. Colour saturation

increased at water content below 0.5 g/g d.m. but it was

lower than that of raw material.

Changes of colour co-ordinates of material under-

going drying are due mostly to water removal, shrinkage

and alterations of the structure of the material surface.

Water is replaced by air in porous materials and they

appear as pale. Decreased luminance and more satu-

rated colour of the material during final stages of drying

are caused probably by the effect of concentration.

7. Effect of water on acoustic emission

Acoustic emission is a significant quality attribute of

many food products and is considered as a sign of

freshness. Drake (1963) showed that acoustic emission

of many foods differs by amplitude, frequency and in-

tensity. Vickers and Bourne (1976) who suggested that

frequency of 160 Hz is typical for this type of productsinvestigated crunchiness of food.

Tesch, Normand, and Peleg (1996) investigated effect

of water activity on acoustic emission of crunchy cereal

foods. It was shown that at aw > 0:65 acoustic emission

disappears and the product becomes ‘‘silent’’. Wollny

and Peleg (1994), Decremont (1995), Roudaut, Decre-

mont, and Le Meste (1998), van Hecke, Allaf, and

Bouvier (1995, 1998) and Duizer, Campanella, and

Barnes (1998) performed similar investigations.

Research done by Marzec (2002) showed that

acoustic emission of flat bread is strongly related to

water activity (Fig. 7). The lower was the water activity

the amplitude was the higher and acoustic emission was

generated even at very low stress. Increase of water ac-

tivity to 0.53 reduced the energy of acoustic emission

about 4-fold. Most of acoustic emission occurred in two

frequency ranges. These are 1–3 kHz and 7–15 kHz. At

water activities over 0.53 the acoustic emission was veryweak and some sound is present only at frequencies

larger than 10 kHz.

8. Conclusions

Water as a component of food affects its physical

properties. Practically there is no physical property of

food, important from the engineering or consumer point

of view, which is not affected by water. The influence of

water on food properties arises from interactions be-

tween water molecules and other constituents of food.

The extent and strength of interactions depends onchemical composition and determines the state of water

in food. Hence, many physical properties of food are

related to water activity.

At high water contents a rule of additivity is observed

and physical properties can be predicted with the use of

appropriate equations. The rule of additivity is applica-

ble in those cases in which properties of water are those

of bulk water. At low water contents, when interactions

between water molecules and food components prevail

and water activity is below 1, the influence of water on

physical properties is more complicated. It seems that

research relating mechanical properties to the state of

water in foods gives pretty thorough picture of the effectof water on those properties. However, other properties

of food are less elucidated from this point of view.

Hence, the influence of the state of water in foods on

such properties as thermal, electrical, optical, acoustic

and other could be a good topic for a future research.

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water as the determinant of food engineering properties. a review

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