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PAWKIT
3.3.3 WATER POTENTIAL
Water activity is closely related to a thermodynamic 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. This means the measured water potential of the vapor phase and can be
used to determine the water potential of the liquid phase. Gradients in µ are driving forces
for moisture movement. 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
). Moisture content is not a
driving force for water movement, except in homogeneous materials.
3.3.4 SORPTION ISOTHERMS
Changes in water content affect both the osmotic and matrix binding of water in a product.
Therefore a relationship exists between the water activity and moisture content. This
relationship is called the sorption isotherm and is unique for each product. Besides being
unique to each product, the isotherm changes depending on whether it was obtained by
drying or wetting the sample. Typically, large safety margins are built into water content
specifications to allow for these uncertainties.
While the sorption isotherm is often used to infer water activity from moisture content, one
could easily go the other direction and infer the moisture content from water activity. This
is particularly attractive because water activity can be measured much more quickly than
water content. This method gives particularly good precision in the center of the isotherm. In
order to infer moisture content from water activity, an isotherm for the particular product is
needed. METER can run the isotherm for a fee. Visit metergroup.com/food/services/water-
activity-and-isotherm-testing for more information.
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