33
AQUALAB TDL/TDL 2
humidity of the headspace is computed as the ratio of the partial pressure measured
by the AQUALAB TDL to 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.
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 remember the restriction of the definition of water activity to equilibrium.
The water activity of the water in a system is influenced by factors that affect the binding
of water. They include osmotic, matric, and pressure effects. Typically, water activity is
measured at atmospheric pressure, so only the osmotic and matric effects are important.
3.3.1.1 TEMPERATURE EFFECTS
Temperature plays a critical role in water activity determination. The AQUALAB TDL infrared
thermometer measures the difference in temperature 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 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 approximately 4 °C above the chamber temperature to cause condensation to
form. The AQUALAB TDL warns the user if a sample 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 that is warmer than the block is put in the AQUALAB.
Samples not read at room temperature during the read cycle equilibrate with the AQUALAB
TDL temperature before the water activity is displayed. Large temperature differences cause
longer reading times, since the AQUALAB TDL cannot make a complete and accurate reading
until the sample and the instrument equilibrate to within 4 °C.
There are several advantages in having a temperature-controlled water activity meter. A few
major reasons are:
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
