Pawkit
Theory
42
Water Potential
Some additional information may be useful for
understanding what water activity is and why it is
such a useful measure of moisture status in prod-
ucts. Water activity is closely related to a thermody-
namic property called the water potential, or
chemical potential (
µ
) of water, which is the change
in Gibbs free energy (G) when water concentration
changes. Equilibrium occurs in a system when
µ
is
the same everywhere in the system. Equilibrium
between the liquid and the vapor phases implies
that
µ
is the same in both phases. It is this fact that
allows us to measure the water potential of the
vapor phase and use that to determine the water
potential of the liquid phase. Gradients in
µ
are
driving forces for moisture movement. Thus, in an
isothermal system, water tends to move from
regions of high water potential (high
a
w
) to regions
of low water potential (low
a
w
). Water content is
not a driving force for water movement, and there-
fore can not be used to predict the direction of
water movement, except in homogeneous materials.
Factors in determining Water
Potential
The water potential of the water in a system is influ-
enced by factors that effect the binding of water.
They include osmotic, matric, and pressure effects.
