The moisture sorption isotherm of a food
is obtained from the equilibrium moisture
contents determined at several water activity
levels at constant temperature. There are three
types of isotherm curves; adsorption (starting
from the dry state), desorption (starting from the
wet state), or working (native state). An
isotherm prepared by adsorption will not
necessarily be the same as an isotherm prepared
by desorption. This phenomenon of different
moisture contents for the same aw is called
moisture sorption hysteresis, and is exhibited by
many foods. Some reasons for hysteresis are