Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 127

of water in all phases and forms must be equal at a
given temperature, T. The potential, , of the gas
phase may be expressed as:

w
RTln(p/pw),

where R is the gas constant (8.3145 J mol^1 K^1 ), p
is the partial water vapor pressure, and pwis the
vapor pressure of pure water at T. The ratio p/pwis
called the water activity aw (p/pw), although this
term is also called relative vapor pressure
(Fennema and Tannenbaum 1996). The difference is
small, and for simplicity, awas defined is widely
used for correlating the stability of foods. For ideal
solutions and for most moist foods, awis less than
unity (aw1.0; Troller 1978).
The Clausius-Clapeyron equation, mentioned ear-
lier, correlates vapor pressure, P, heat of phase tran-
sition,
H, and temperature, T. This same relation-
ship can be applied to water activity. Thus, the plot
of ln(aw)versus 1/Tgives a straight line, at least
within a reasonable temperature range. Depending
on the initial value for awor moisture content, the
slope differs slightly, indicating the difference in the
heat of phase transition due to different water con-
tent.
Both water activity and relative humidity are frac-
tions of the pure-water vapor pressure. Water activi-
ty can be measured in the same way as humidity.
Water contents have a sigmoidal relationship (Fig.
5.16). As water content increases, awincreases: aw
1.0 for infinitely dilute solutions, aw0.7 for
dilute solutions and moist foods, and aw0.6 for
dry foods. Of course, the precise relationship
depends on the food. In general, if the water vapor in
the atmosphere surrounding the food is greater than
the water activity of the food, water is adsorbed;
otherwise, desorption takes place. Water activity re-
flects the combined effects of water-solute, water-
surface, capillary, hydrophilic, and hydrophobic in-
teractions.
Water activity is a vital parameter for food moni-
toring. A plot of awversus water content is called an
isotherm. However, desorption and adsorption
isotherms are different (Fig. 5.16) because this is a
nonequilibrium system. Note that isotherms in most
other literature plot water content against aw, the
reverse of the axes of Figure 5.16, which is intended
to show that awis a function of water content.
Water in food may be divided into tightly bound,
loosely bound, and nonbound waters. Dry foods

contain tightly bound (monolayer) water, and aw

rises slowly as water content increases; but as loose-

ly bound water increases, awincreases rapidly and

approaches 1.0 when nonbound water is present.

Crisp crackers get soggy after adsorbing water from

moist air, and soggy ones can be dried by heating or

exposure to dry air.

Water activity affects the growth and multiplica-

tion of microorganisms. When aw0.9, growth of

most molds is inhibited. Growth of yeasts and bacte-

ria also depends on aw. Microorganisms cease grow-

ing if aw0.6. In a system, all components must

be in equilibrium with one another, including all

the microorganisms. Every type of organism is a

component and a phase of the system, due to its

cells or membranes. If the water activity of an

organism is lower than that of the bulk food,

water will be absorbed, and thus the species will

multiply and grow. However, if the water activity

of the organism is higher, the organism will dehy-

drate and become dormant or die. Thus, it is not

surprising that water activity affects the growth of

various molds and bacteria. By this token, humidity

will have the same effect on microorganisms in resi-

dences and buildings. Little packages of drying

agent are placed in sealed dry food to reduce vapor

pressure and prevent growth of bacteria that cause

spoilage.

Figure 5.16.Nonequilibrium or hysteresis in desorption

and adsorption of water by foodstuff. Arbitrary scales

are used to illustrate the concept for a generic pattern.