Decagon Devices AquaLab 4TE Operator's Manual page 20

AquaLab
3. Water Activity Th eory
called the sorption isotherm, and is unique for each product. Besides
being unique to each product, the isotherm changes depending on
whether it was obtained by drying or wetting the sample. Th ese fac-
tors need to be kept in mind if one tries to use water content to infer
the stability or safety of a product. Typically, large safety margins are
built into 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. Th is is particularly attrac-
tive because water activity is much more quickly measured than wa-
ter content. Th is method gives particularly 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. Decagon
sells an Isotherm Generator called the AquaSorp IG or you can also
have Decagon run the isotherm for a fee.
For example, if one were using the AquaLab to monitor the water
content of dried potato fl akes, one would measure the water activity
and water content of potato fl akes dried to varying degrees using the
standard drying process for those fl akes. 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. We
have an upgrade available to Series 4TE users that would allow you
to determine moisture content and water activity simultaneously.
Th is instrument is called the Series 4TE DUO.
Th e importance of the concept of water activity of foods, pharma-
ceuticals, and cosmetics cannot be over emphasized. Water activity is
a measure of the energy status of the water in a system. More impor-
tantly, the usefulness of water activity in relation to microbial growth,
chemical reactivity, and stability over water content has been shown.
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