AquaLab TDL 3 WATER ACTIVITY THEORY
Temperature Effects
Temperature plays a critical role in water activity determination.
The AquaLab infrared thermometer measures the difference in tem-
perature between the sample and the block. It is carefully calibrated
to minimize temperature errors, but when temperature differences
are large water activity can change during testing. Best accuracy is
therefore obtained when the sample is near chamber temperature.
Another effect of temperature on water activity occurs when sam-
ples are near saturation. A sample that is close to 1.0 a
w
and is
only slightly warmer than the sensor block condenses water within
the block. Condensation causes errors in the measurement, and in
subsequent measurements until it evaporates. A sample at 0.75 a
w
needs to be approximately 4
◦
C above the chamber temperature to
cause condensation. The AquaLab warns the user if a sample is more
than 4
◦
C above the chamber temperature, but for high water ac-
tivity samples the operator needs to be aware that condensation can
occur if a sample that is warmer than the block is put in the TDL.
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 va-
por 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
therefore can not be used to predict the direction of water movement,
except in homogeneous materials.
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