Food Chemistry

2.5 Kinetics of Enzyme-Catalyzed Reactions 135

Fig. 2.37.Thermal inactivation of enzymes of milk.
1 Lipase (inactivation extent, 90%), 2 alkaline phos-
phatase (90%), 3 catalase (80%), 4 xanthine oxidase
(90%), 5 peroxidase (90%), and 6 acid phosphatase
(99%)

Fig. 2.38. Thermal inactivation (90%) of enzymes
present in potato tuber

such enzymes responsible for quality deteriora-
tion during storage. For example semiripened pea
seeds in which lipoxygenase is responsible for
spoilage. However, lipoxygenase is more sensi-
tive than peroxidase, thus a sufficient but gentle
blanching requires the inactivation of lipoxyge-
nase only. Inactivation of peroxidase is not neces-
sary.
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 con-

formational 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 en-

zymes.

Table 2.21 contains a list of technically useful

proteinases and their thermal stability. However,

these data were determined using isolated en-

zymes. They may not be transferrable to the same

enzymes in food because in its natural environ-

ment 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 ther-

mal stabilty data of isolated enzymes have been

developed. An example for the agreement be-

tween calculated and experimental results is pre-

sented in Fig. 2.40.

Peroxidase activity can partially reappear dur-

ing 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 influ-

enced by increasing the concentration of enzyme

Fig. 2.39.Pea seed lipoxygenase. Inactivation extent

at 65◦C as affected by pH