Water Activity :-
In 1952, Scott came to the conclusion that the storage quality of food
does not depend on the water content but on water activity (aw)
ERH = Equilibrium relative humidity
P = Partial vapour pressure
P0 = Saturation vapour pressure
Relation between water content and water activity is indicated by
sorption isotherm of food. At low water content (<50%) minor
changes in this parameter lead to major changes in water activity. It
is shown in figure.
In figure (b) the desorption isotherm, indicating the course of a
drying process lies slightly above the adsorption isotherm pertaining
to the storage of moisture sensitive food.
Decreased water activity retards the growth of micro organisms,
slow enzyme catalyzed reactions and lastly retards non enzymatic
browning. Foods with aw values between 0.6 and 0.9 are known as
“Intermediate Moisture Foods” Those foods are largely protected
against microbial spoilage.