8
Water Relations
The water content of foods varies widely, and several properties of foods
greatly depend on water content. This concerns, among other things, rates
of changes—notably various kinds of deterioration—rheological properties,
and hygroscopicity. The relations are far from simple, partly because water
is not a simple liquid. Most of the intricacy relates to the many constituents
in food, each of which may interact with water in a different way.
The reader is advised to consult Sections 2.2 (especially 2.2.1) and 3.2
before starting with this chapter.
8.1 WATER ACTIVITY
Many of the relations mentioned become simpler if water activity (aw) rather
than water content is considered. Following Eq. (2.6), the chemical
potentialmwof water in a solution is given by
mw¼mwþRTlnaw ð 8 : 1 Þwheremwis the chemical potential of pure water.awis expressed as mole
fraction and varies between 0 and 1. The chemical reactivity of the water, as
in a reaction formula, is exactly proportional toaw, rather than to water
concentration. Water activity is often a much better indicator for water-