The concept of aw has been very useful in
food preservation and on that basis many processes
could be successfully adapted and new products
designed. Water has been called the universal
solvent as it is a requirement for growth,
metabolism, and support of many chemical
reactions occurring in food products. Free water in
fruit or vegetables is the water available for
chemical reactions, to support microbial growth and
to act as a transporting medium for compounds. In
the bound state water is not a available to
participate in these reactions as it is bound by water
soluble compounds such as sugar, salt gums, etc.
(osmotic binding), and by the surface effect of the
substrate matrix binding) [7].
These water-blinding effects reduce the
vapor pressure of the food substrate according to
Raoult’s Law. Comparing this vapor pressure with
that of pure water (at the same temperature) results
in ratio called water activity (aw). Pure water has an
aw of 1, one molar solution of sugar – 0.98, and one
molar solution of sodium chloride -0.9669. A
saturated solution of sodium chloride has a water
activity of 0.755. This same NaCl solution in a
closed container will develop an equilibrium
relative humidity (ERH) in a head space of 75.5%.