Although
water does not contribute any nutritional or caloric value to food
products, it is nevertheless one of the most abundant and most important
ingredients in many food products. In food it is measured as water
acitivity (aw), which is the available water, which is not bound up. The
aw-value is measured between 0 and 1 (water: aw = 1).
Water plays a role:
as a solvent for other nutrients (sugar, salt, etc.)
a medium for chemical reactions (e.g. hydrolysis, enzyme-catalyzed reactions, etc.)
is needed by microorganisms
tin exture (e.g. bread, sauces, etc.)
One of the oldest preservation methods to stop or at least slow-down
harmful enzymeactivity and microbial growths, is the reduction of the
aw-value, through e.g. drying or addition of salt and sugar, which are
both hydrophilic and bind up water.
Dry foods like pasta, biscuits, etc. have an aw-value between 0.2 and
0.5. Dried fruits, depending on the desired chewiness, have an aw-value
of around 0.6 which translates to a water content between 13-15%.
Fat-containing snacks and nuts should have an aw-value of 0.4, as the
undesireable oxidationreactions of fats are higher below and above this value.
Above an aw-value of 0.7 yeasts, molds and bacteria start to grow.
See in the image some more examples of aw-values of foods:
