where an attempt is made to keep foods fresh without intense heat treatment
and without adding large concentrations of bactericidal components.
8.5 RECAPITULATION
Water Activity. The reactivity of water in a food is precisely given
by its water activity, which is mostly expressed as a fraction, thus ranging
from 0 to 1. In a dilute and ideal solution,awequals the mole fraction of
water, but in most foods there are several nonidealities, and it may be very
difficult to predictaw from composition. This means that it has to be
determined, which can be done by measuring the relative vapor pressure of
air in equilibrium with the food.
Many food properties correlate better with water activity then with
water content, especially if the food contains substances that are more or
less indifferent to water, like crystals or oil droplets. This does not mean that
relations between a property andaware the same for all foods, not even in a
relative sense. Some properties, notably rheological ones, generally correlate
better with water content than withaw.
Sorption. The relation betweenawand mass fraction of waterw,at
constant temperature, is called asorption isotherm. It is a useful relation,
giving information about hygroscopicity and about drying conditions to be
applied. For almost all foods, it does not reflect true water adsorption, since
much water is in a (concentrated) solution rather than adsorbed onto a well-
defined surface. It mostly takes a very long time to determine a sorption
isotherm, especially at the lowawend, presumably because the diffusivity of
water then becomes very small. The only way to obtain (near) zero water
content generally is drying at high temperature, where diffusivity is much
greater. Moreover, it is commonly observed that going to progressively
lower aw values (‘‘desorption’’) gives a sorption curve that differs
significantly from one obtained by progressively increasing aw
(‘‘adsorption’’). This hysteresis indicates that no true equilibrium is
reached, implying that water activity is actually undefined. Nevertheless,
sorption isotherms can be of great practical use.
Hydration. Hydration or true binding of water to food components
generally involves only small quantities of water. Only polar groups, and to
a lesser extent dipoles, can ‘‘bind’’ water molecules, i.e., immobilize them for
a short time. It is often stated that bound water does not react, but water
reactivity is just proportional toaw. Bound water would not be available as
a solvent, but nonsolvent water is primarily due to negative adsorption of a