Dairy Chemistry And Biochemistry

(Steven Felgate) #1
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

Free download pdf