Chemistry Reference
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was generally reported to cause only little change in the sorption behaviour.
From a study of dried yogurt, for instance, it was concluded that the sorption
capacity of powder might be influenced more by differences in raw material
than by the drying methods (Kim and Bhowmik, 1994). The amounts of water
sorbed by caseins and caseinate are strongly dependent on pH, particularly at
high a
w
(above
0.5). SI show typical hysteresis loops, water contents retained
during the initial desorption being higher than the amount of water sorbed
during subsequent sorption.
The isotherm for a multi-component dairy product can be predicted using
a simple additive model, in which the amount of water sorbed at any given a
w
is
the weighted addition of the water that the components would sorb alone,
assuming that no interactions between components occur (Berlin et al., 1968;
Foster et al., 2005). Isotherms for commercial dairy powders could be predicted
based on the isotherms for amorphous lactose, micellar casein and whey pro-
teins, with the difference between predicted and measured water contents being
mostly less than
10%, i.e. within the measure error limit. The predicted water
contents, however, were in general slightly greater than those measured, which
could suggest some interactions between components (Foster et al., 2005).
The amounts of water sorbed by various dairy powders (lactose, whey
protein isolate, milk protein concentrate and micellar casein) showed no
obvious temperature dependence between 4 and 388C, but were lower at
508C (Foster et al., 2005). Desorption isotherms for skim and whole milk
powders also showed slightly lower residual water contents as temperature
increased between 53 and 908C, at low RH, but increasing with temperature
at RH
>
30-40%. This phenomenon, which has been reported previously for
various sugar-containing foods, is explained by the increase in solubility of
lactose (Lin et al., 2005). The a
w
measured for six different cheeses between 5
and 308C showed no significant temperature dependence. SI, however,
revealed that a slightly greater amount of water sorbed at 58C than at 258C
in the intermediate moisture range (Ruegg, 1985).
Variability in published sorption data results not only from variations
in composition and methods of sample preparation but also from variations
in water content determination, equilibration time, etc. Literature SI, there-
fore, even taken from careful compilations (Iglesias and Chirife, 1982; Wolf
et al., 1985) should be used only as a first approximation.
11.3.
State Diagrams
State diagrams are now commonly used to represent the interactions of water
with food materials, as they can help to design many processing operations or
storage conditions. Figure 11.4 represents the state diagram for milk, with
