3
WATER ACTIVITY THEORY
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Most critical is the measurement of the difference between sample
and capacitance sensor temperature. Best accuracy is therefore ob-
tained when the sample is near chamber temperature.
3.3
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 products. 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 liq-
uid 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 therefore can not be used to predict the direction of water move-
ment, except in homogeneous materials.
Factors In Determining Water Activity
The water activity of the water in a system is influenced by factors
that effect the binding of water. They include osmotic, matric, and
pressure effects. Typically water activity is measured at atmospheric
pressure, so only the osmotic and matric effects are important.
Osmotic Effects
: Osmotic effects are well known from biology and
physical chemistry.
Water is diluted when a solute is added.
If
this diluted water is separated from pure water by a semi-permeable
membrane, water tends to move from the pure water side through
the membrane to the side with the added solute. If sufficient pressure
is applied to the solute-water mixture to just stop the flow, this pres-
sure is a measure of the osmotic potential of the solution. Addition
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