Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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0014 The volume of ICF is fixed by the prevailing os-

motic pressure and the amount of solute that the cells

keep within them by metabolic activity. The kidney,

controlling the excretion of water under the influence

of ADH, fixes the osmotic pressure in the body, thus

setting the stage for the cells to maintain their appro-

priate volume of ICF. Fixing the osmotic pressure

throughout the body also determines the amount of

water to go with each unit of sodium in the ECF.

Hence, with osmotic pressure and the volume of ICF

stabilized by controlling the excretion of water, the

kidneys can also regulate the volume of ECF by

excreting or retaining salt.

See also: Body Composition; Hormones: Adrenal

Hormones; Potassium: Physiology; Renal Function and

Disorders: Kidney: Structure and Function; Sodium:

Physiology

Further Reading

Bray JJ, Cragg PA, Macknight ADC, Mills RG and Taylor

DW (1989) Lecture Notes on Human Physiology, 2nd

edn. Oxford: Blackwell Scientific.

Henderson LJ (1913) The Fitness of the Environment. New

York: Macmillan.

Robinson JR (1970) Water and life. World Reviews of

Nutrition and Dietetics 12: 172–207.

Robinson JR (1978) Water in the animal body. In: McIntyre

AK (ed.) Water: Plants, Planets and People, pp. 60–

70.Canberra: Australian Academy of Science.

Robinson JR (1988) Reflections on Renal Function, 2nd

edn. Oxford: Blackwell Scientific.

WATER ACTIVITY

Contents

Principles and Measurement

Effect on Food Stability

Principles and Measurement

Y H Roos, University College Cork, Cork, Ireland

Introduction

0001 Water is the main component of most food materials.

The control of water content is often used to increase

food stability, e.g., by dehydration. It is also known

that high concentrations of sugars and salts may

increase food stability by affecting the availability

of water to microbial growth, although the water

content may remain high. It is obvious that the

water contents of different foods vary substantially,

and water content as such cannot be used as a meas-

ure of water availability or food stability. The water

availability for microbial growth is a function of the

vapor pressure of water in a food, which has led to the

development of the water activity concept. It has been

shown that food storage stability and microbial

safety, i.e., rates of chemical and enzymatic reactions

and microbial growth, can be controlled by water

activity in addition to other factors, such as pH and

temperature. Therefore, the development of methods

to control water activity of foods and water activity

measurement has received much attention in the food

industry. Furthermore, reference values for water ac-

tivity are used in legislation and regulations to insure

food safety. This article discusses the basic concepts

of water activity, its definition, temperature depend-

ence, measurement, relationship with water content,

and mathematical approaches to predict water

activity in food systems.

Water Activity

0002The water activity, a

, in a solution or a food is

derived from its chemical potential, m

. The chemical

potential, m, of a pure substance is given by its molar

Gibbs free energy, G

. The Gibbs free energy is pres-

sure-dependent, and it can be shown that the Gibbs

free energy of a perfect gas at a pressure, p, can be

related to its value, G*, at a standard pressure, p*.

Then, the Gibbs free energy will be given by eqn (1).

G ¼ G* þ nRT ln

: ð1Þ

Assuming perfect behavior and taking into account

that for water vapor m

¼ G

/n, eqn (2), where m

*is

WATER ACTIVITY/Principles and Measurement 6089

the chemical potential at the standard pressure, p

*, is

obtained.



¼

* þ RT ln

:ð2Þ

The vapor pressure of pure water can be taken as

¼ p

*. Thus, the chemical potential is ex-

pressed by eqn (3). If water exists in a solution, its

chemical potential will be m

and vapor pressure

w(liquid)

. At equilibrium, the chemical potential in

the liquid and vapor states is equal, and, therefore,

the chemical potential of water in a solution, m

,is

defined by eqn (4). Eqns (3) and (4) can be combined

to give eqn (5) and the chemical potential of liquid

water in the solution. The activity, in the case of

water, the water activity, a

, is defined by eqn (6).



¼* þ RT lnp

ð3Þ



¼* þ RT lnp

ð4Þ



¼

þ RT ln

ð5Þ

:ð6Þ

As defined by the preceding equations, the chemical

potential, vapor pressures, and water activity are all

temperature-dependent. Therefore, the water activity

is a measure of water availability at equilibrium at a

constant temperature, e.g., room temperature.

0003 The relative humidity, RH, of an atmosphere is

defined as the ratio of the quantity of water present

to the saturation quantity at the temperature of the

atmosphere, and it is also the ratio of the vapor

pressure of water present to that of saturated water

vapor at the same temperature, i.e., the relative vapor

pressure (RVP), as given by eqn (7).

RH ¼

 100% :ð7Þ

If a solution or a food is placed in an atmosphere, it

will lose or gain water until equilibrium is reached

and the chemical potential and, therefore, the water

activity in the atmosphere and the solution or food

have become equal. This has a great importance in a

measurements, because a

is often derived from the

equilibrium relative humidity (ERH) of the headspace

of a food system.

Raoult’s Law

000 4 The vapor pressure of ideal solutions is related to the

mole fraction of water, as defined by Raoult’s law.

Raoult’s law is based on the observation that

the partial vapor pressure of each component in a

solution is a function of its mole fraction and the

vapor pressure of a pure liquid at the same tempera-

ture. In the case of water, assuming that the solution

behaves as an ideal solution, Raoult’s law is given by

eqn (8). However, Raoult’s law assumes linearity, it

applies for ideal solutions, and often the vapor pres-

sure deviates from ideal behavior with increasing

solute concentration. For real solutions, an activity

coefficient is defined, and the water activity is related

to the mole fraction of water, x

, by eqn (9), where g

is the activity coefficient of water. Therefore, water

activity may also be considered as the ‘effective’ mole

fraction of water.

p ¼ x

ð8Þ

¼

:ð9Þ

Raoult’s law is important in understanding the

boiling temperature elevation and freezing tempera-

ture depression of solutions. The freezing temperature

depression may also be measured to derive water

activity.

Water Activity Measurement

0005The water activity of simple, ideal solutions can be

derived from Raoult’s law by applying eqn (8) and

real solutions from eqn (9). Unfortunately, food

systems are very complex, the water activity is diffi-

cult to estimate, and it must be measured for most

foods. A simple, but rough, water activity estimation

is based on placing samples in several environments

(chambers) with various relative humidities and

estimating the water activity from a plot showing

weight gain or loss against RH.

0006The water activity of food materials can be meas-

ured using various systems to observe the vapor pres-

sure of water in a food sample or by measuring the

relative humidity of air in equilibrium with a food

sample. The measurement of the vapor pressure of

water can be carried out using a manometer and other

relatively simple laboratory equipment. Simple, com-

mercial instruments may apply hair hygrometry. Such

devices are not very accurate and require that appro-

priate time is allowed for the sample material in a

container to equilibrate with the measuring unit.

Various other commercial hygrometers have been ap-

plied in water activity measurements. For these meas-

urements, an appropriate amount of sample material

often needs to be placed in a closed container or jar at

a constant temperature. The sensor of the instrument

is placed in the headspace, and the relative humidity

at equilibrium is recorded and expressed as water

activity.

6090 WATER ACTIVITY/Principles and Measurement

0007 Several commercial water activity measurement

devices have been developed for rapid and accurate

water activity determinations. These devices use

either a chilled-mirror dew-point technology or meas-

ure relative humidity with sensors that detect electric

resistance or capacitance. The dew-point instruments

provide relatively rapid and accurate water activity

measurements. The main principle of such instru-

ments is based on cooling air at equilibrium water

vapor pressure with a food sample without changing

the water content and determination of the tempera-

ture at which water in the air saturates at a chilled-

mirror surface. The water activity of the sample is

given by the ratio of the saturation vapor pressure at

the dew-point temperature and the saturation vapor

pressure at the sample temperature. Instruments

using relative humidity sensors require equilibration

with the sample at the same temperature in a closed

container or jar, as described earlier for hygrometer

measurements.

Instrument Calibration

0008 Water activity measurements require adequate and

accurate instrument calibration. This is usually

carried out with the help of saturated salt solutions

or other substances with known water vapor pres-

sures at a given temperature. Different instruments

may require different specific calibration procedures,

but most instruments should be checked for correct

water activity readings. Saturated salt solutions are

probably the most common means of producing

atmospheres with known relative humidities or

water vapor pressures. Saturated salt solutions can

be prepared to give a wide range of water activities

in a number of temperatures. Although the solutions

are relatively easy to prepare, accurate, and safe,

appropriate procedures and safety precautions should

be followed. The water activities of common satur-

ated salt solutions are listed in Table 1. Other mater-

ials, which in various concentrations in solutions with

water can be used to adjust water activity, include

glycerol and sulfuric acid (H

Water Sorption

0009It is often beneficial to establish water sorption

isotherms, which show the relationship between

water content and water activity for particular food

systems. Sorption isotherms can be obtained experi-

mentally by using dehydrated, for example freeze-

dried, samples of food solids. The samples should be

initially stored over a ‘zero’ relative humidity in a

hermetic container with CaSO

or P

. These

samples can be used to obtain the ‘anhydrous’ weight,

and the samples are then placed in hermetic contain-

ers, preferably evacuated desiccators at a constant

temperature over saturated salt solutions with

known water activities. The samples in each con-

tainer are weighed at intervals until subsequent

weighings level off, and the weight gain can be con-

sidered as equilibrium water content corresponding

tbl0001 Table 1 Water activities of selected saturated salt solutions at various temperatures

Salt Water activityat temperature (



5 10 15 20253035404550

CsF 0.055 0.049 0.043 0.038 0.034 0.030 0.027 0.024 0.022 0.021

LiBr 0.074 0.071 0.069 0.066 0.064 0.062 0.060 0.058 0.057 0.055

ZnBr 0.087 0.085 0.082 0.079 0.078 0.076 0.076 0.075 0.076 0.077

LiCl 0.113 0.113 0.113 0.113 0.113 0.113 0.113 0.112 0.112 0.111

CaBr 0.216 0.202 0.185 0.165

LiI 0.217 0.206 0.196 0.186 0.176 0.166 0.156 0.146 0.135 0.124

COOK 0.234 0.234 0.231 0.225 0.216

MgCl

0.336 0.335 0.333 0.331 0.328 0.324 0.321 0.316 0.311 0.305

NaI 0.424 0.418 0.409 0.397 0.382 0.362 0.347 0.329 0.310 0.292

0.431 0.431 0.432 0.432 0.432 0.432

MgNO

0.589 0.574 0.559 0.544 0.529 0.514 0.499 0.482 0.469 0.454

NaBr 0.635 0.622 0.607 0.591 0.576 0.560 0.546 0.532 0.520 0.509

CoCl

0.649 0.618 0.586 0.555 0.526 0.500

KI 0.733 0.721 0.710 0.699 0.689 0.679 0.670 0.661 0.653 0.645

NaNO

0.786 0.775 0.765 0.754 0.743 0.731 0.721 0.710 0.700 0.690

NaCl 0.757 0.757 0.756 0.755 0.753 0.751 0.749 0.747 0.745 0.744

KBr 0.851 0.838 0.826 0.817 0.809 0.803 0.798 0.794 0.792 0.790

KCl 0.877 0.868 0.859 0.851 0.843 0.836 0.830 0.823 0.817 0.812

KNO

0.963 0.960 0.954 0.946 0.936 0.923 0.908 0.890 0.870 0.848

0.985 0.982 0.979 0.976 0.973 0.970 0.967 0.964 0.961 0.958

WATER ACTIVITY/Principles and Measurement 6091

to the water activity of the particular salt solution.

The equilibration times may vary depending on

the container design, water activity, and sample

material among other factors. However, equilibration

in evacuated desiccators is rapid and often occurs

in a few hours and should not take more than a

week. Equilibration in nonevacuated containers

may take substantially longer and is more subject

to errors caused by nonequilibrium conditions and

water transfer between the container and external

atmosphere.

0010 Food materials exhibit two basic types of sorption

isotherms reflecting their either amorphous or crys-

talline state. Most foods are noncrystalline or par-

tially crystalline systems and have a sigmoid shaped

sorption isotherm. Crystalline materials, e.g., crystal-

line sugars, show a low water sorption until solubil-

ization, and other changes occurring at high water

activities result in a substantial increase in sorbed

water (Figure 1). Food materials also have a hysteresis

between desorption and adsorption isotherms, i.e.,

the water content in desorption at the same water

activity is higher than in adsorption (Figure 2). The

hysteresis results most likely from retention of water

in capillaries and structural differences. Sorption iso-

therms are extremely useful in practical applications,

as only water content needs to be known to derive

water activity. Furthermore, changes in storage rela-

tive humidity of low moisture foods can be related to

corresponding changes in water content.

0011 Water activity and water sorption isotherms are

temperature-dependent. The temperature dependence

of water activity follows the well-known Clausius–

Clapeyron relationship (eqn (10)), where a

and

refer to water activity at absolute temperatures

and T

, respectively, Q

is the heat of sorption,

and R is the gas constant (8.3144 J K

1

mol

1

). A

schematic representation of the temperature depend-

ence of water activity is shown in Figure 3.





: ð10Þ

In general, the water activity of a food with a constant

water content increases with increasing temperature,

while the water sorption decreases with increasing

temperature at the same water activity. This is

observed from sorption isotherms determined for

the same food at different temperatures (Figure 4).

Understanding the temperature dependence of water

sorption is extremely important, for example, in esti-

mating the effects of temperature fluctuations on

water activity changes inside a food package. The

0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Water activity

Water content (g per 100 g)

Typical food system

Crystalline material

fig0001 Figure 1 Hypothetical sorption isotherms typical of food

systems and crystalline materials, such as sugar crystals.

Water activity

Adsorption

Desorption

Water content

fig0002Figure 2 Hysteresis between water desorption and adsorption

typical of foods.

−1

)

In a

Increasing water content

fig0003Figure 3 Temperature and water content dependence of water

activity. The temperature dependence of water activity increases

with decreasing water content.

6092 WATER ACTIVITY/Principles and Measurement

temperature change under normal storage conditions

may result in a water activity shift exceeding 0.1 a

Water Sorption Models

0012 A number of mathematical models have been used to

fit experimental water sorption data of food mater-

ials. The most common models are the Brunauer–

Emmett–Teller (BET) model (eqn (11)) and the

extended Guggenheim–Anderson–De Boer (GAB)

model (eqn (12)). These models originate from stud-

ies of sorption of gases on solid surfaces. The BET

model relates the water content (m) to water activity

using two parameters, m

and C, defined as the

monolayer value and a constant related to the excess

enthalpy of sorption, respectively. The monolayer

value gives the amount of water sorbed initially to

first available sorption sites, and it is often found to

correspond to the water content of low moisture food

systems with improved stability. The BET sorption

model can be written to the form of eqn (12), which

gives a linear relationship between a

/(1 a

m) and

. Thus, experimental water activity and water con-

tent values obtained from sorption studies can be

used to derive the constants m

and C. The

BET equation, however, fits experimental data only

between 0 and 0.5a

1  a

ðÞ1 þ C  1ðÞa

½

ð11 Þ

1  a

ðÞm

C  1

ð12 Þ

1  Ka

ðÞ1 þ C

 1ðÞKa

½

ð13Þ

 1



1 



ð14Þ

The GAB model (eqn (13)) has been shown to fit

experimental sorption data for most food materials

over the entire a

range. Experimental data may also

be used to derive the three parameters, the constants

and K, and the monolayer value, m

, of the GAB

model. This can be carried out, for example, by using

nonlinear regression or rearranging the equation to a

form of a second-order polynomial and plotting

experimental values of a

/m against a

, as suggested

by eqn (14). However, the BET and GAB monolayer

values are not equal, and neither of these values can

be considered as a water content giving maximum

stability in a low-moisture state.

Water Activity in Food System

0013Water is the main component of most fresh foods, and

their water activities are high and close to 1. The

water activity has increasing importance in concen-

trated foods, dehydrated foods or systems that con-

tain salts, sugars, and other compounds reducing

water activity to provide stability, for example highly

salted meats and fish or confectionery and candies.

Therefore, it is often of great interest in food product

development to estimate effects of food composition

and recipe components on end-product water activ-

ity. It is also important in packaging of mixtures of

different foods, such as dry ingredients, to understand

how water activity in a sealed package with several

components may change owing to differences in com-

ponent water activities. Furthermore, water activity

in different parts of foods may differ and result in

time-dependent water migration until equilibrium is

attained. This is often the case in bakery products,

which may have a high water activity filling in a lower

water activity matrix. However, it is important to

note that lipids and fat are not water-soluble, and

water activity is mainly affected by water-soluble

and water-miscible food components only. Therefore,

high-fat products with relatively small amounts of

water, such as butter and margarine, may have a

relatively high water activity.

0014The water activity of dilute sugar and salt solutions

can be calculated using Raoult’s law (eqn (8)) and for

more concentrated solutions when the activity coeffi-

cient is known using eqn (9), as discussed above.

Many food systems, however, deviate from ideal

Water activity

Water content (g per 100 g of solids)

Temperature increases

Constant water content

Temperature decreases

Constant water activity

fig0004 Figure 4 Schematic representation of water sorption iso-

therms determined at different temperatures.

WATER ACTIVITY/Principles and Measurement 6093

behavior, and Raoult’s law cannot be used. Then,

either sorption isotherms or other water activity cal-

culation methods must be followed to estimate water

activity in food product development. Several equa-

tions are available for predicting water activity of a

food product based on the product composition.

These include the Norrish equation, which extends

the use of mole fractions of food components with

additional coefficients to calculate water activity,

and the Grover equation, which uses different em-

pirical constants for the various components in

a polynomial. The Ross equation (eqn (15)) gives

probably the most accurate water activity predictions

for complex solutions. The equation is relatively

simple and does not require the use of empirical

constants.

¼ a

 a

...a

:ð15Þ

Mixtures of two or more ingredients or food com-

ponents with different water activities in a sealed

package will exchange water until the water activity

of the components becomes the same. The water

content of the components initially and after equili-

bration may differ significantly, owing to differences

in sorption properties. There are several approaches,

which can be applied to estimate the final water

activity of a food mixture in a sealed package. These

methods are based on knowledge of the sorption

isotherms and their use to predict the equilibrium

water activity. A simple approach is to establish a

sorption isotherm for the mixture and predict water

activity using the total water content of the ingredi-

ents. Another approach assumes that the sorption

isotherms of the components over the applied water

activity range follow a straight line. This leads to

eqn (16), which can be used to predict the equilibrium

water activity, a

. The method is also useful in esti-

mating an appropriate ratio of food components to

keep the final, equilibrium water activity at an accept-

able level.

þ a

...þ a

þ b

...þ b

, ð16Þ

where a

, a

...a

, b

...b

,andw

, w

...w

refer to the initial component 1, 2 ...i water activity,

slope of the sorption isotherm, and weight of dry

solids, respectively.

0015 The role of water activity as a food stability param-

eter has been well recognized. Water activities can be

measured with commercially available equipment,

and sorption isotherms are available for most food

components. Furthermore, computer software for

water activity calculations and establishment of sorp-

tion isotherms has been developed and used widely in

the food industry.

See also: Water: Structures, Properties, and

Determination; Physiology; Water Activity: Effect on Food

Stability

Further Reading

Barbosa-Ca

novas GV and Welti-Chanes J (eds) (1995) Food

Preservation by Moisture Control: Fundamentals and

Applications. Lancaster, PA: Technomic.

Bell LN and Labuza TP (2000) Moisture Sorption. Practical

Aspects of Isotherm Measurement and Use. St. Paul,

MN: American Association of Cereal Chemists.

Fontana AJ (2000) Understanding the importance of water

activity in food. Cereal Foods World 45: 7–10.

Iglesias HA and Chirife J (1982) Handbook of Food Iso-

therms. New York: Academic Press.

Jouppila K and Roos YH (1997) Water sorption isotherms

of dehydrated milk products: Applicability of linear and

nonlinear regression analysis in modeling. International

Journal of Food Science and Technology 32: 459–471.

Labuza TP (1968) Sorption phenomena in foods. Food

Technology 22: 263–265, 268, 270, 272.

Labuza TP (1980) The effect of water activity on reaction

kinetics of food deterioration. Food Technology 34(4):

36–41, 59.

Rockland LB and Stewart GF (eds) (1981) Water Activity:

Influences on Food Quality. New York: Academic Press.

Roos YH (1995) Phase Transitions in Foods. San Diego,

CA: Academic Press.

Roos YH (2000) Water activity and plasticization. In: Eskin

NAM and Robinson DS (eds) Food Shelf Life Stability.

Boca Raton, FL: CRC Press.

Roos YH, Leslie RB and Lillford PJ (eds) (1999) Water

Management in the Design and Distribution of Quality

Foods. Lancaster, PA: Technomic.

Seow CC, Teng TT and Quah CH (eds) (1988) Food Preser-

vation by Moisture Control. London: Elsevier.

Simatos D and Multon JL (eds) (1985) Properties of Water

in Foods. Dordrecht: Martinus Nijhoff.

Slade L and Levine H (1991) Beyond water activity: Recent

advances based on an alternative approach to the assess-

ment of food quality and safety. Critical Reviews in Food

Science and Nutrition 30: 115.

Effect on Food Stability

Y H Roos, University College Cork, Cork, Ireland

Introduction

0001Water activity, together with pH and temperature, is

often an important factor in controlling food stability.

These three parameters all, but not alone, influence

microbial activity, rates of chemical and enzymatic

reactions, and changes in food texture. Water activity,

6094 WATER ACTIVITY/Effect on Food Stability

however, has a significant role in controlling texture

formation and textural changes in intermediate-

moisture foods (IMF) and low-moisture foods. In

such foods, removal or sometimes sorption of water

results in concentration changes of food solids and

thereby has an effect on reaction rates. Enhanced

reaction rates resulting from water removal may be

desired, as in baking to provide color and flavor, or

they may be detrimental as in production of many

food powders, such as dairy powders. That makes the

time–temperature–water or water activity control in

food production and storage extremely important.

0002 In some cases, the maximum food stability at low

water contents is achieved at a water activity cor-

responding to the Brunauer–Emmett–Teller (BET)

monolayer water content. Furthermore, water affects

the physical state and water plasticization, i.e.,

softening of water miscible food solids. As a plasti-

cizer, water controls the glass transition of food com-

ponents, such as carbohydrates and proteins. The

glass transition is a well-known property of all inor-

ganic and organic amorphous materials. The glass

transition occurs over a temperature range, which is

often referred to with the glass transition tempera-

ture, T

. The physico-chemical properties of amorph-

ous materials change considerably over the glass

transition, as the transformation includes a change

between the liquid-like and solid-like material states.

There are also established and possible relationships

between the physical state, water plasticization,

and rates of diffusion-controlled reactions, such as

nonenzymatic browning, enzymatic reactions, and

oxidative changes.

0003 In general, water activity provides valuable infor-

mation of the effects of water content on water

availability and the physical state of food solids.

Such information can be described using sorption

isotherms and state diagrams. They provide critical

values for water content, water activity, and tempera-

ture, which are important in the characterization of

food behavior in processing and in establishing cri-

teria for packaging requirements and appropriate

storage conditions. These areas and the role of

water activity in controlling food stability are

discussed in the present article.

Water Activity and Food Stability

0004 Most fresh foods are high-moisture materials, and

their shelf-life is reduced by enzymatic changes, the

growth of microorganisms, and mechanical damage.

High-moisture foods have an a

of 0.90–0.999, and

they often contain more than 50% (w/w) water

(Table 1). These foods include fresh meats and sea-

food, various dairy products, fruits, vegetables, and

beverages. Most bacteria, molds, and yeasts are likely

to grow in high-moisture foods. However, the types of

spoilage microorganisms and the growth of various

species are highly dependent on pH, temperature, and

water activity.

0005IMF have a water activity within the range of 0.60–

0.90a

, and their water contents normally vary

between 10 and 50% (w/w) (Table 1). These foods

include many traditional low-moisture foods, such as

grains, nuts, and dehydrated fruits, but also a number

of processed foods or foods designed and manufac-

tured to have a known composition to provide stabil-

ity. Such foods may have particular applications and

requirements for stability, e.g., when used as fillings

in bakery products or confectionery. Although micro-

bial spoilage is prevented below 0.60a

, and many

microbes do not grow in IMF, their stability and shelf-

life are reduced by deleterious changes, such as struc-

tural transformations, enzymatic changes, browning

reactions, and oxidation, depending on a

, pH, and

temperature. The rates of such changes are often at

least to some extent affected by the physical state of

the materials and the extent of water plasticization

of water-miscible solids.

0006Low-moisture foods obviously have the lowest

water contents, often below 10% (w/w), and their

water activity is lower than 0.6a

(Table 1). Such

foods are not subject to microbial spoilage. Their

shelf-life, however, is often limited by chemical and

textural changes, particularly browning and other

changes in color and flavor as well as oxidation.

These foods may be exceptionally hygroscopic and

exhibit water sorption from their surroundings.

Water sorption is reduced by the use of protective

packaging, but sorption occurs during storage as a

result of permeation or damage of protective pack-

aging. Many low-moisture foods have a solid appear-

ance, or they have a crispy, solid texture. Water

sorption in such foods may lead to stickiness, struc-

tural changes, such as loss of crispness and sogginess,

tbl0001Table 1 Examples of water activity ranges of various foods

Food systems Wateractivity

range

Fresh meat, fish, vegetables, <40% (w/w)

sucrose, <7% (w/w) salt

>0.95

Bread, cooked sausages, medium aged cheese 0.90–0.95

Salami, old cheese, >65% sucrose, >15% salt 0.87–0.90

Dried beef, sweet condensed milk, cereals with

15% (w/w) water

<0.86

Jam, marmalade, old salami, >26% (w/w) salt 0.80–0.87

Flour, cereals, nuts 0.75–0.80

Caramels, honey, toffee 0.60–0.75

Breakfast cereals, snack foods, food powders 0.20–0.60

WATER ACTIVITY/Effect on Food Stability 6095

and coincident increases in the rates of browning and

enzymatic changes. A number of products contain

amorphous components, for example, lactose in

dairy powders and sucrose in many bakery products

and confectionery. Such amorphous components may

crystallize during water sorption, as water activity

increases. It is important to notice that low-moisture

foods are often glassy materials with T

values above

their normal storage temperature. Water sorption and

associated plasticization may decrease the T

, and the

glass transition may occur over the range of the con-

current increase in water activity.

Stability Control

Microbial Stability

0007 Microbial growth requires a minimum a

, in addition

to pH, temperature, and other appropriate conditions

that are important for the growth of bacteria, molds,

and yeasts. The water activity of high-moisture foods,

especially processed foods, can be manipulated to

some extent by the addition of salts and sugars or

other ingredients, which are known to reduce water

activity. Such compositional alterations are highly

advantageous in product development and food

safety control. For many products, the effects of com-

positional changes on water activity can be predicted

on the basis of composition and confirmed by meas-

uring water activity of the final product. This allows

an understanding of storage requirements and estima-

tion of the product shelf-life in various storage condi-

tions. In high-moisture foods, the main role of water

activity control is to govern and reduce the risk of

the growth of pathogenic and spoilage bacteria.

Examples of water activity limits for the growth of

selected microorganisms are given in Table 2.

0008 The lowest water activity limit for microbial

growth of 0.60a

allows the growth of xerophilic

yeasts. Above this limiting water activity, IMFs have

an increasing possibility for the growth of various

microorganisms with increasing water activity. How-

ever, the water activities of IMFs are such that patho-

genic bacteria are unable to grow, but there is a

possibility for the growth of molds and yeast. The

growth of these microorganisms must be controlled

by careful adjustment of product water activity, use of

protective packaging to avoid contamination, and

selection of appropriate humectants, pH control, and

use of antimicrobial agents.

0009 Microbial stability is an obvious, and often the

most important, criterion in food preservation. The

limits for growth of various microorganisms,

as shown in Figure 1, are well established and suc-

cessfully used in food product development and

manufacturing as well as control of product safety.

Furthermore, in high-moisture foods and several IMF

products, water activity is relatively constant and

dependent on composition, especially solids content,

and the type of water-soluble components.

Chemical and Enzymatic Stability

0010The chemical stability of high-moisture foods is not

significantly affected by water activity, as the micro-

bial quality is the main determinant of product shelf-

life. The role of water activity in determining the

chemical and enzymatic stability of IMF and low

moisture foods is more important. As the water activ-

ity and water content decrease, the concentration of

reactants in the water phase of foods becomes obvi-

ously increased. Therefore, rates of several reactions

may increase with decreasing water activity. The rela-

tive rate of deteriorative changes in intermediate and

low moisture foods is traditionally related to water

content and a

, as shown by the ‘Food stability map’

in Figure 2. Early studies applying nuclear magnetic

resonance spectroscopy found ‘mobilization points’

(water activity allowing reactant mobility) for solutes

in low-moisture food matrices. The mobilization

tbl0002Table 2 Water activity (a

) limits for the growth of selected

pathogenic and spoilage microorganisms

Microorganism Minimum a

Bacteria

Bacillus cereus 0.930

Bacillus subtilis 0.900

Campylobacter jejuni 0.990

Clostridium botulinum 0.940

Clostridium perfringens 0.945

Escherichia coli 0.935

Halobacterium halobium 0.750

Lactobacillus plantarum 0.940

Listeria monocytogenes 0.920

Salmonella spp. 0.940

Shigella spp. 0.960

Staphylococcus aureus 0.860

Vibrio parahaemolyticus 0.936

Yersinia enterocolitica 0.960

Molds

Aspergillus candidus 0.750

Aspergillus flavus 0.780

Aspergillus niger 0.770

Erotum echinulatum 0.620

Penicillium citrinum 0.800

Penicillium expansum 0.830

Penicillium patulum 0.810

Rhizopus nigricans 0.930

Xeromyces bisporus 0.610

Yeasts

Saccharomyces bailii 0.800

Saccharomyces cerevisiae 0.900

Saccharomyces rouxii 0.620

6096 WATER ACTIVITY/Effect on Food Stability

point was peculiar to the system, and the level of

hydration needed to achieve mobility was solute-

dependent. No solute mobilization occurred below

the BET monolayer value, and, for example, experi-

mental data of nonenzymatic browning reaction rates

suggested that browning initiated at the mobilization

point. An increase in the reaction rate was apparent

with increasing a

, and the rate maximum occurred at

the water activity corresponding to a hydration level

allowing complete mobilization. However, several

other factors, including the glass transition, are

important in controlling changes occurring in inter-

mediate and low moisture foods.

0011Enzyme activity in low-moisture foods is often re-

lated to hydration. At low water activities enzymatic

activity is generally not observed, as water cannot

enhance diffusion of substrates to enzyme molecules.

Such water activity dependence applies to hydrolases

and oxidases, unless the substrates are nonaqueous

liquids, allowing changes to occur fairly independ-

ently of water activity. It seems that not only water

activity but also the ability of water to give a known

mobility to enzymes and substrates is important in

controlling enzyme activity. The amount of water

needed increases with increasing molecular size

owing to impaired diffusion, in particular for

enzymes, which are active in the water phase of

foods. Therefore, for example, lipase activity is not

necessarily related to the mobility provided by

water.

0012Lipids exist often in a separate phase apart from the

water phase. At low water activities and water con-

tents, lipids become more accessible to the atmos-

pheric oxygen, and oxidation rates may increase

(Figure 2). The rate of oxidation, however, decreases

rapidly with increasing water activity, as the lipids

may become protected from atmospheric oxygen

owing to the formation of protective aqueous layers

in the food microstructure. In several low-moisture

foods, lipids, flavors, and other oxygen sensitive com-

pounds are entrapped within the amorphous struc-

ture. Such encapsulated compounds also exhibit

reduced rates of oxidation as compared with ‘free’

lipids.

0013Traditional shelf-life predictions of low-moisture

foods were based on the information of rates of de-

teriorative changes and loss of nutrients at various

temperatures and water contents. An increase in

water content above the BET monolayer value at a

constant storage temperature often results in rapid

deterioration as reaction rates increase at intermedi-

ate water contents. The main factors affecting reac-

tion rates in low-moisture food materials, however,

can be controlled by a number of factors, which

include food composition and the type of the reac-

tion, temperature, pressure, water content, and pH.

In concentrated food systems, changes in viscosity

and relaxation times in the vicinity of the glass transi-

tion may also affect diffusion and thereby contribute

to reaction rates and product shelf-life.

Water Plasticization

0014Intermediate and low moisture foods often exist in an

amorphous (noncrystalline, no defined structure

of molecular arrangement), elastic, ‘rubbery,’ or

‘leathery’ state or in a solid-like, glassy state. The

amorphous state is typical of liquids and remains in

molds

Water activity01

Relative rate

Nonenzymatic browning

Growth of:

bacteria

yeasts

Oxidation

fig0002 Figure 2 Food stability map describing the relationship

between water activity and food stability.

Xerophilic molds

Osmophilic yeasts

growth

Number of growing microbes

Most halophilic bacteria

Most molds, no growth of pathogenic

bacteria

Staphylococcus aureus may grow

Most yeasts, mycotoxin-producing molds,

spoilage often by molds and yeasts

Most cocci, lactobacilli, some molds, Salmonella,

lactic acid bacteria are major spoilage flora

Most bacteria, some yeasts,

pathogenic and spoilage organisms

0.91−0.95

0.80−0.87

0.75−0.80

0.65−0.75

0.60−0.65

<0.65

0.87−0.90

>0.95

>0.86

Growth

0.4 0.5 0.6 0.7 0.8 0.9 1.0

Water activity

fig0001 Figure 1 Growth of various microorganisms at different water

activity conditions.

WATER ACTIVITY/Effect on Food Stability 6097

many biological materials owing to complicated

chemical composition, rapid removal of solvent

water, or quench cooling of a liquid melt in processes

not allowing component crystallization. For example,

many sugars typical of foods do not crystallize in

dehydration, but remain in a supercooled, liquid

state after removal of the solvent water. Glassy,

solid-like foods may suffer the glass transition as a

result of water sorption, which may be observed from

rapid changes in structure and appearance, and

enhanced rates of chemical and enzymatic reactions.

0015 The glass transitions of food components have a

wide range of temperatures depending on the com-

ponent itself and also on its molecular weight. In

general, high-molecular-weight food components,

such as carbohydrate polymers and proteins, have

glass transition temperatures at very high

temperatures well above 100



C. Low-molecular-

weight components may have very low glass transi-

tion temperatures, for example, the glass transition of

amorphous water occurs at around 135



C, whereas

many anhydrous sugars have a T

between 0 and

100



C. The T

is often measured using differential

scanning calorimetry, and it can be observed with a

number of other thermal and spectroscopic tech-

niques. These include dielectric analysis, dynamic

mechanical analysis, electron spin resonance spec-

troscopy, infra-red and Raman spectroscopy, and

nuclear magnetic resonance spectroscopy. Examples

of glass transition temperatures of food components

are given in Table 3.

0016Carbohydrates and proteins in foods are water-

miscible components, and they are plasticized by

water. Water plasticization of the amorphous struc-

ture results in a decrease in T

. The decrease in T

substantial even at low water contents, and at high

water contents, the T

approaches that of amorphous

water. Therefore, at a constant temperature, e.g.,

room temperature or a typical storage temperature

of intermediate- and low-moisture foods, a water

activity or water content range can be established,

which corresponds to the glass transition of the ma-

terial. The relationships between glass transition,

water activity, and water content are described in

Figure 3. The water activity and water content,

which correspond to the T

depression to the storage

temperature can be defined as ‘critical water activity’

and ‘critical water content,’ respectively.

þ kw

, ð1Þ

where w

and w

and T

are the weight

fraction and glass transition temperature of solids

(1) and water (2), respectively.

0017The relationship between the glass transition tem-

perature depression and water content is often pre-

dicted with the Gordon–Taylor relationship (eqn (1)).

The model can be fitted to experimental T

and water

content data and used together with water sorption

data to establish relationships between the T

and

water sorption properties, as shown for amorphous

lactose in Figure 3. Such predictions are useful in food

product development, because they allow estimation

of food stability in terms of the physical state and

water activity.

tbl0003 Table 3 Examplesofmeltingandglasstransitiontemperatures

offoodcomponents

FoodcomponentMeltingtemperatureof

crystallinesolids

(DSC onset,



Glasstransition

temperature

(DSC onset,



Carbohydrates

Fructose1085

Galactose16330

Glucose14331

Lactose 223 101

Maltose 160 87

Sorbitol 85 9

Starch

na 250

Sucrose 173 62

Trehalose 215 100

Xylitol 89 29

Proteins

Gelatin na 207

Gliadin na 179

Glutenin na 189

Myoglobin na 149

Ovalbumin na 157

Estimated values.

Glass transition

Water activity

Critical water content

Critical

water activity

Water activity (at 25 ⬚C)

1.0

0.8

0.6

0.4

0.2

0.0

0 5 10 15 20 25 30 35 40

Water content (g per 100 g of solids)

100

−20

−40

−60

−80

Glass transition (⬚C)

fig0003Figure 3 Relationships between glass transition temperature,

water activity, and water content for amorphous lactose with

critical values for water content and water activity at room

temperature.

6098 WATER ACTIVITY/Effect on Food Stability