METER GROUP AquaLab 4TE - Page 43

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AQUALAB
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) which 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.
3.4.2.1. 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 AQUALAB 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.
Another effect of temperature on water activity occurs when samples are near saturation.
A sample that is close to 1.0 a
w
and is warmer than the sensor block will cause water to
condense within the chamber. This causes errors in the measurement and in subsequent
measurements until the condensation disappears. For example, a sample at 0.75 a
w
only
needs to be 4 °C above the sample chamber temperature to cause condensation to form.
The AQUALAB warns the user if a sample temperature is more than 4 °C above the chamber
temperature, but for high water activity samples, the operator needs to be aware that
condensation can occur if a sample is warmer than the sample block.
There are several advantages in having a temperature-controlled water activity meter. A few
major reasons are detailed here.
1. Research purposes.
Researchers can use temperature control to study the effects of temperature on the
water activity of a sample, make a comparison of the water activity of different samples
independent of temperature, and conduct accelerated shelf-life studies or other
water activity studies where temperature control is critical. There are many shelf-life,
packaging, and isotherm studies in which temperature control would be very benecial.