Dairy Chemistry And Biochemistry

HEAT-INDUCED CHANGES IN MILK^361

h c

.-

v E

E

F

Xanthine oxidase

0.1 :

0.01! I 1 I 1

60 70 80 90 100

Temperature ("C)

Figure 9.11 Time-temperature combinations required for which milk must be heated to a

certain temperature to inactivate some indigenous milk enzymes (from Walstra and Jenness,

1984).

enzymes can have undesirable effects in milk and dairy products. Although

not the primary objective of thermal processing, some of the indigenous

enzymes in milk are inactivated by the commercially used heat processes,

although many are relatively heat stable (Figure 9.1 1).

The thermal denaturation of indigenous milk enzymes is important from

two major viewpoints:

1. To increase the stability of milk products. Lipoprotein lipase is probably
   the most important in this regard as its activity leads to hydrolytic
   rancidity. It is extensively inactivated by HTST pasteurization but
   heating at 78°C x 10 s is required to prevent lipolysis. Plasmin activity is
   actually increased by HTST pasteurization due to inactivation of inhibi-
   tors of plasmin and/or of plasminogen activators.


2. The activity of selected enzymes is used as indices of thermal treatments,
   e.g. alkaline phosphatase (HTST pasteurization), y-glutamyl transpep-
   tidase (index of heating in the range 72-80°C) or lactoperoxidase
   (80-90°C).

Microbial enzymes. The widespread use of refrigerated storage of milk at
farm and factory for extended periods has led to psychrotrophs, especially