Dairy Chemistry And Biochemistry

HEAT-INDUCED CHANGES IN MILK 363

cooling. Since this mechanism does not apply to purified lipase, the

mechanism of LTI of lipase is not clear (for reviews on enzymes from

psychrotrophs see Driessen (1989) and McKellar (1989)).

9.6.2 Denaturation of other biologically active proteins

Milk contains a range of biologically active proteins, e.g. vitamin-binding

proteins, immunoglobulins, metal-binding proteins, antibacterial proteins

(lactotransferrin, lysozyme, lactoperoxidase), various growth factors and

hormones (Chapters 4 and 8). These proteins play important nutritional and

physiological functions in the neonate. All these proteins are relatively heat

labile - some are inactivated by HTST pasteurization and probably all are

inactivated by UHT and more severe heat treatments. Inactivation of these

biologically active proteins may not be particularly important when milk is

used in the diet of adults but may be highly significant in infant formulae;

consequently, supplementation of infant formulae with some of these pro-

teins is advocated.

9.6.3 Denaturation of whey proteins

The whey proteins, which represent about 20% of the proteins of bovine

milk, are typical globular proteins with high levels of secondary and tertiary

structures, and are, therefore, susceptible to denaturation by various agents,

including heat. The denaturation kinetics of whey proteins, as measured by

loss of solubility in saturated NaCl at pH4.6, are summarized in Figure

50 i

0 20 40 60 80 100

Heating time (rnin)

Figure 9.13 Heat denaturation of whey proteins on heating skim milk at various temperatures
("C) as measured by precipitability with saturated NaCl (from Jenness and Patton, 1959).