Water in Dairy Products - Advanced Dairy Chemistry - page 497

Figure 11.19. Browning rate in a caseinate-based model system (Na-caseinate/glucose 74/
25% (dry basis) and Na-caseinate/glucose/humectant 50/17/33% dry basis) at 0.11 < a w < 0.75
(Sherwin and Labuza, 2003).
Colour formation resulting from the thermal degradation of malto-
dextrins was also reported to show an Arrhenius-type temperature depen-
dence across the glass transition range (with an apparent activation energy
83 kJ/mol for a w ¼ 0.15) (Claude and Ubbink, 2006). The thermal inacti-
vation of -galactosidase in dairy model systems was found to occur in the
glassy state. The reaction rate showed no sharp increase above T g (Burin
et al., 2002). As shown in Figure 11.20, the rate of thermal inactivation in
various systems was more dependent on water content than on the (T-T g )
value.
To be able to carry out their specific activity, proteins need a mini-
mum amount of water. The water content threshold for activity was shown
to coincide with the water content for incipient mobility. In more complex
food systems at low water content, however, glass transition did not
appear to be the main factor controlling the enzymatic activity. For
instance, the rate of sucrose hydrolysis by invertase, in maltodextrins-su-
crose-lactose systems, significantly increased only for water contents cor-
responding to (T-T g )valuesashighas408C (Kouassi and Roos, 2001).
For -galactosidase in whey powder or dextrans, the small change in
reaction rate for increasing (T-T g ) suggests that T g is not a significant
factor; reduction of pH upon dehydration seems to be the main cause of
the low activity (Burin and Buera, 2002). As will be seen in the next section,