2009_Book_FoodChemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 135
Fig. 2.37. Thermal inactivation of enzymes of milk.
1 Lipase (inactivation extent, 90%), 2 alkaline phosphatase
(90%), 3 catalase (80%), 4 xanthine oxidase
(90%), 5 peroxidase (90%), and 6 acid phosphatase
(99%)
such enzymes responsible for quality deterioration
during storage. For example semiripened pea
seeds in which lipoxygenase is responsible for
spoilage. However, lipoxygenase is more sensitive
than peroxidase, thus a sufficient but gentle
blanching requires the inactivation of lipoxygenase
only. Inactivation of peroxidase is not necessary.
All the changes which occur in proteins outlined
in section 1.4.2.4 also occur during the heating of
enzymes. It the case of enzymes the consequences
are even more readily observed since a slight conformational
change at the active site can result in
total loss of activity.
The inactivation or killing rates for enzymes and
microorganisms depend on several factors. Most
significant is the pH. Lipoxygenase isolated from
pea seeds (Fig. 2.39) denatures most slowly at its
isoelectric point (pH 5.9) as do many other enzymes.
Table 2.21 contains a list of technically useful
proteinases and their thermal stability. However,
these data were determined using isolated enzymes.
They may not be transferrable to the same
enzymes in food because in its natural environment
an enzyme usually is much more stable. In
additional studies, mostly related to heat transfer
in food, some successful procedures to calculate
the degree of enzyme inactivation based on thermal
stabilty data of isolated enzymes have been
developed. An example for the agreement between
calculated and experimental results is presented
in Fig. 2.40.
Peroxidase activity can partially reappear during
storage of vegetables previously subjected to
a blanching process to inactivate enzymes. The
reason for this recurrence, which is also observed
for alkaline phosphatase of milk, is not known
yet.
Enzymes behave differently below the freezing
point. Changes in activity depend on the type of
enzyme and on a number of other factors which
are partly contrary. The activity is positively influenced
by increasing the concentration of enzyme
Fig. 2.38. Thermal inactivation (90%) of enzymes
present in potato tuber
Fig. 2.39. Pea seed lipoxygenase. Inactivation extent
at 65 ◦ C as affected by pH