Pawkit
Theory
45
safety margins are built in to water content specifi-
cations to allow for these uncertainties.
While the sorption isotherm is often used to infer
water activity from water content, one could easily
go the other direction and use the water activity to
infer the water content. This is particularly attractive
because water activity is much more quickly mea-
sured than water content. This method gives partic-
ularly good precision in the center of the isotherm.
In order to infer water content from water activity,
one needs an isotherm for the particular product;
produced, ideally, using the process that brings the
product to its final water content.
For example, if one were to monitor the water con-
tent of dried potato flakes, one would measure the
water activity and water content of potato flakes
dried to varying degrees using the standard drying
process for those flakes. An isotherm would be con-
structed using those data, and the water content
would be inferred using the measured water activity
of samples and that isotherm.
The importance of the concept of water activity of
foods, pharmaceuticals, and cosmetics cannot be
overly emphasized.
Water activity
is a measure of
the energy status of the water in a system. More
importantly, the usefulness of water activity in rela-
