Chemistry Reference
In-Depth Information
Figure 11.20.
Rate of thermal inactivation of
-galactosidase in dairy model systems, either
anhydrous (filled symbols) or exposed to 22% RH (open symbols)(
, *: lactose,
&
, h: sweet
whey powder,
~
,
M
: skim milk powder, : lactose+caseinate 1/1) (Burin et al., 2002).
in contrast, modelling enzymatic activity according to WLF kinetics may
be applicable in some situations.
11.5.6.
Stability of Frozen Food Products
As is well known, the rate of physical, chemical and biochemical
changes in frozen products is strongly affected by the storage temperature.
This is shown for instance by the high values of the empirical coefficient,
Q
10
, which can be determined from temperature-time tolerance diagrams
(Jul, 1984). These values, which range from 2 to 30 in the usual temperature
range of frozen storage, are often much higher than the Q
10
values observed
for common chemical reactions, particularly in food systems at tempera-
tures above 08C; i.e. the Q
10
is around 2 for most chemical or biochemical
reactions and 3-4 for Maillard reactions. The slowing down of changes with
decreasing temperature has been attributed to the greatly increased viscosity
of the freeze-concentrated phase, resulting from the combined effects of
concentration and temperature. Levine and Slade (1988, 1989, 1990) have
promoted the idea that the glass transition temperature of the maximally
freeze-concentrated phase (T
g
0
) is a threshold of instability in frozen food
systems and that kinetics above this temperature are controlled by the
difference between the temperature of storage and the specific T
g
0
of the
product, according to WLF kinetics.
