Chemistry Reference
In-Depth Information
which will be in equilibrium with each other, or the formulation allowing the
desired a
w
to be obtained; it is an indispensable tool for designing drying
operations, or packaging. On the other hand, the use of the BET and GAB
models to obtain information on the interactions of water with the other
constituents is very questionable. First, the hysteresis observed between
sorption and desorption points to the non-equilibrium character of the iso-
therms. Although correlations could be found between the chemical struc-
tures of solutes and BET or GAB parameters, for instance between the
''monolayer'' GAB values and the number of polar groups in proteins
(Timmermann et al., 2001), it is increasingly admitted that the basic assump-
tions of the BET model are not fulfilled in the case of water sorbed on polar
materials (energetic equivalence of all sites on the sorbing solid surface).
Moreover, the plastifying action of water on the solid certainly plays a role
in the form of the sorption curves. It would most likely be more justified to
consider low-water-content food materials as solid solutions (Kuntz and
Kauzmann, 1974). Models taking into account the plastifying effect of
water on the sorption process have been proposed recently. While these
models do not allow more accurate mathematical description of isotherms
than the previously reported ones, they advance our understanding of the
processes that occur as water vapour is taken up into amorphous solids.
Zografi and co-workers (Hancock and Zografi, 1993; Shamblin et al., 1998;
Zhang and Zografi, 2000) combined the Flory-Huggins solution model with
the Vrentas model based on the free volume changes resulting from the water
sorption to describe sorption isotherms of water on poly(vinyl pyrrolidone)
(PVP), sugars and their mixtures. Benczedi et al. (1998a,b) analysed the
sorption isotherms of water on starch as a combination of the Freundlich
adsorption model (a monolayer adsorption model with independent, differ-
ent sorption sites) and the Flory-Huggins model of polymer solution. The
approach has been further refined by Ubbink et al. (2007), considering that
the composition of carbohydrate systems influences the sorption of water
differently in the glassy and rubbery states and that the two models of
Freundlich adsorption and of Flory-Huggins solution therefore apply suc-
cessively during the sorption process. The authors satisfactorily described the
isotherms of mixtures of a maltopolymer with various concentrations of
maltose.
Sorption energies are often calculated from sorption isotherms
obtained at various temperatures using a relationship derived from the Clau-
sius-Clapeyron law. As discussed for the BET/GAB monolayer values, the
''net isosteric heat of sorption'', Q
nst
, thus obtained cannot be used to char-
acterize the interaction of water with the sorbing solid. A maximum in the
Q
nst
values for a given water content has often been reported. This suggests a
