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SYSTEM
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 that water associates with other components.
Researchers measure the water activity of a system by equilibrating the liquid phase water in
the sample with the vapor phase water in the headspace and measuring the relative humidity
of the headspace. In the AQUALAB3, a sample in a sample cup is sealed inside the sample
chamber, which contains a fan, a dew point sensor, and an infrared thermometer. The fan
speeds equilibrium and controls the boundary layer conductance of the dew point sensor.
The dew point sensor measures the dew point temperature of the air in the chamber, and the
infrared thermometer measures the sample temperature. From these measurements, the
relative humidity of the headspace is computed as the ratio of the dew point temperature
saturation vapor pressure to the saturation vapor pressure at the sample temperature. When
the water activity of the sample and the relative humidity of the air are in equilibrium, the
measurement of the headspace humidity gives the water activity of the sample. Water activity
is the relative humidity of air in equilibrium with a sample in a sealed chamber.
In addition to equilibrium between the liquid phase water and the vapor phase, the internal
equilibrium of the sample is important. If a system is not at internal equilibrium, one might
measure a steady vapor pressure (over the period of measurement) that is not the true water
activity of the system. An example of this might be a baked good or a multicomponent food.
Initially out of the oven, a baked good is not at internal equilibrium; the outer surface is at
a lower water activity than the center of the baked good. One must wait a period of time in
order for the water to migrate and the system to come to internal equilibrium. It is important
to keep in mind that by this definition, water activity is an equilibrium measurement.
The water activity of a sample is influenced by factors that affect the binding of water.
These factors include temperature, osmotic, matric, and pressure effects. Typically, water
activity is measured at atmospheric pressure, so only the osmotic, temperature, and
matric effects are important.
TEMPERATURE EFFECTS
Temperature plays a critical role in water activity determination. Most critical is the
measurement of the difference between sample and dew point temperature. For water
activity measurements to be accurate to 0.001, temperature difference measurements need
to be accurate to 0.017 °C. The AQUALAB3 infrared thermometer measures the difference
in temperature between the sample surface and the sample block. The thermometer is
carefully calibrated to minimize temperature errors; however, achieving 0.017 °C accuracy is
difficult when temperature differences are large. Most accurate measurements are obtained
when the sample is near sample chamber temperature.
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