solute onto particles or macromolecules and greatly depends on the nature,
especially on its molecular size, of the solute. Bound water would not freeze,
but some water remains unfrozen when its diffusivity has become virtually
zero, so that its crystallization would take infinite time. And held or imbibed
water is certainly not bound, but just mechanically entrapped. In other
words, water fills space. Water binding as a general term therefore is
confusing.
Reaction Rates. Dry foods are generally more stable than those
with a higherw. This can be due to various mechanisms, and the relations
are generally not well understood. Most physical changes depend on
diffusivity, which becomes progressively smaller with smaller w. Most
chemical reactions proceed slower for a smallerw. If water is a reactant, a
lowerawmeans a slower reaction. At fairly loww, reactions may become
diffusion–limited, which generally is the ultimate cause of stability in very
dry foods. Activity coefficients may be greater at lowerw, and removal of
water may increase the concentration of reaction inhibitors or catalysts. It is
possible that a reaction proceeds faster at a smallerw, at least over a certain
range. For a bimolecular reaction this may be due to a higher concentration
of reactants; moreover, water itself may be an inhibitor.
Microorganisms generally do not grow at low aw, although great
variation is observed among various organisms. The prime cause for
inhibition is a high osmotic pressure. However, it is generally observed that
different components at the sameaw, which also means at the same osmotic
pressure, markedly differ in inhibitory capacity. This means that one cannot
state that a certainawis the minimum one at which a given organism can
grow, since it would depend on the presence and concentration of many
components.
BIBLIOGRAPHY
A somewhat different discussion, also involving glass transitions, and giving more
practical information, is
O. R. Fennema. Water and ice. In: O. R. Fennema, ed. Food Chemistry, 3rded.
Marcel Dekker, New York, 1996, Chapter 2.
Basic aspects of water activity are discussed by
T. M. Herrington, F. C. Vernier. In: S. T. Becket, ed. Physico-Chemical Aspects of
Food Processing. Blackie, London, 1995, Chapter 1.
A series of symposium reports, containing many interesting articles, comprises e.g.
L. B. Rockland, G. F. Stewart, eds. Water Activity: Influences on Food Quality.
Academic Press, 1981.