Chemistry Reference
In-Depth Information
Figure 11.2.
Water content and a
w
of cheeses (Banon and Hardy, 2002).
water between two compartments having different initial relative vapour
pressures in a multi-domain food system or between a food product and its
environment during drying or osmotic dehydration or during storage (ISO-
POW, 2000). The rate of these exchanges is controlled by the diffusivity of
water, or possibly by the presence of a barrier against moisture transfer.
Sorption isotherms can also be used to estimate the local water content, and
then the local glass transition temperature in multi-component products
(Mousia et al., 2000).
11.2.3.
Models
For systems where the a
w
concept is valid, numerous models, of a
theoretical or empirical nature, are available to describe (sometimes predict)
a
w
in non-electrolyte or electrolyte solutions (Sereno et al., 2001). The Norrish
equation (Norrish, 1966)
log
ΒΌ
k
2
X
2
2
(9)
allows calculation of the water activity coefficient,
, for solutions of low
molecular weight non-electrolytes; values for the k
2
coefficient can be found
