Dairy Chemistry And Biochemistry

348 DAIRY CHEMISTRY AND BIOCHEMISTRY

h

min.

T("C)
Figure 9.1 The time needed (1') at various temperatures (T) to inactivate some enzymes and
cryoglobulins; to kill some bacteria and spores; to cause a certain degree of browning; to
convert 1% of lactose to lactulose; to cause heat coagulation; to reduce available lysine by 1%;
and to make 10% and 75% of the whey proteins insoluble at pH 4.6 (from Walstra and Jenness,
1984).

Table 9.1 Approximate values for the temperature dependence of some reactions in heated milk
(modified from Walstra and Jennes, 1984)

Reaction Activation energy (kJ mol- ') Qlo at 100°C

Many chemical reactions
Many enzyme-catalysed reactions
Autoxidation of lipids
Maillard reactions (browning)
Dephosphorylation of caseinate
Heat coagulation of milk
Degradation of ascorbic acid
Heat denaturation of protein
Typical enzyme inactivation
Inactivation of milk proteinase

Killing vegetative bacteria
Killing of spores

(plasmin)

80-130

40 - 60

40-100

100-180

110-120

150

60-120

200-600

450

75

200-600

250-330

2.0-3.0

1.4-1.7

1.4-2.5

2.4-5.0

2.6-2.8

3.7

1.7-2.8

6.0-175.0

50.0

1.9

6.0-175.0

9.0- 17.0

varies widely, as depicted in general terms in Figure 9.1 and Table 9.1. The
most significant of these changes, with the exception of the killing of
bacteria, will be discussed below. In general, the effect(s) of heat on the
principal constituents of milk will be considered individually, although there
are interactions between constituents in many cases.