35
AQUALAB TDL/TDL 2
reduced. Work would need to be done to extract the water from this matrix. This reduction in
energy status of the water is not osmotic, because the cellulose or protein concentrations
are far too low to produce any significant dilution of water. The reduction in energy is the
result of direct physical binding of water to the cellulose or protein matrix by hydrogen
bonding and van der Waals forces. At higher water activity levels, capillary forces and surface
tension can also play a role.
3.3.2 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 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 and, therefore, cannot be used to predict the direction of
water movement, except in homogeneous materials.
3.3.3 MOISTURE CONTENT
Moisture content implies a quantitative analysis to determine the total amount of water
present in a sample. There are two primary methods for determining moisture content: loss
on drying and Karl Fisher titration. Secondary methods, such as near infrared (NIR) and
nuclear magnetic resonance (NMR), may also be used. Moisture content determination
is essential in meeting product nutritional labeling regulations, specifying recipes, and
monitoring processes.
However, moisture content alone is not a reliable indicator for predicting microbial
responses and chemical reactions in materials. The limitations of moisture content
measurement are attributed to differences in the intensity with which water associates with
other components.
3.3.4 SORPTION ISOTHERMS
Changes in moisture content affect both the osmotic and matric binding of water in a
product. Thus, 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 moisture content
specifications to allow for these uncertainties.
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