Water activity (aw) is a useful measure of water availability for the growth of various
microorganisms (Roos 2002) and physicochemical stability of biological materials
in general. Relationships between aw and growth of moulds, yeasts and bacteria were
described by Beuchat (1981); the microbial proliferation starts at about aw = 0.6.
Moisture adsorption (desorption) isotherms provide a graphical representation of the water
adsorbed (desorbed) by a hygroscopic material at various near-ambient air humidity at
a given temperature and pressure. Each point of the moisture sorption isotherm (MSI)
corresponds to the equilibrium moisture content (EMC) of the wet material under nearambient
air conditions. Under these circumstances, the heat and mass exchange between
the material and surrounding atmosphere does not occur.