Several different isotherm models have been proposed and
compared in the literature. These models are necessary to
predict the moisture content at a given water activity and are
used to evaluate thermodynamic functions of water in foods.
They are also necessary when using isotherm methods with low
data resolution to interpolate between isotherm data points.
While there are 270 proposed isotherm models, the most
commonly used models are the GAB and BET. Since the BET
model is only applicable up to 0.50 aw, the GAB model is widely
accepted as the most useful for characterizing isotherms across
the entire water activity range. Its coefficients also have
theoretical physical meaning such as providing monolayer
moisture content. A new empirical model called the Double Log
Polynomial (DLP) or Chi plot (Condon, 2006) has proven to be
even better than the GAB at characterizing complex isotherms.
The model equations are shown below.