Pawkit
Theory
41
ment of the headspace humidity gives the water
activity of the sample.
In addition to equilibrium between the liquid phase
water in the sample and the vapor phase, the inter-
nal equilibrium of the sample is important. If a sys-
tem 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 multi-component 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 impor-
tant to remember the restriction of the definition of
water activity to equilibrium.
Effect of Temperature on water
activity
Temperature plays a critical role in water activity
determinations. Most critical is the measurement of
the difference between sample and dew point tem-
perature. Best accuracy is therefore obtained when
the sample is near chamber temperature.
