Water a At equilibrium, the aw is related
to the equilibrium relative humidity (ERH) of the near-ambient air (Rao and Rizvi 1995).
Moisture sorption isotherm of biological materials, especially of food, is usually described
as a plot of an amount of water adsorbed/desorbed as a function of aw. Most of these
materials follow a sigma-shaped curve corresponding to type II of the BET classification
(Rao and Rizvi 1995). The course of the curve is the result of the additive effect of the
Raoult’s law, the capillary effect and surface–water interactions. There are two inflections,
one around an aw value of 0.1–0.3, and the other at 0.7–0.9.