Dairy Chemistry And Biochemistry

8 Enzymology of milk and milk products

8.1 Introduction

Like all other foods of plant or animal origin, milk contains several

indigenous enzymes which are constituents of the milk as secreted. The

principal constituents of milk (lactose, lipids and proteins) can be modified

by exogenous enzymes, added to induce specific changes. Exogenous en-

zymes may also be used to analyse for certain constituents in milk. In

addition, milk and most dairy products contain viable micro-organisms

which secrete extracellular enzymes or release intracellular enzymes after the

cells have died and lysed. Some of these enzymes may cause undesirable

changes, e.g. hydrolytic rancidity of milk and dairy products, bitterness

and/or age gelation of UHT milks, bittiness in cream, malty flavours or

bitterness in fluid milk, or they may cause desirable flavours, e.g. in ripened

cheese.

This chapter is devoted mainly to the significance of indigenous enzymes

in milk. The principal applications of exogenous enzymes have been dealt

with in other chapters, e.g. rennets and lipases in cheese production

(Chapter lo), P-galactosidase to modify lactose (Chapter 2). Some minor or

potential applications of exogenous enzymes are presented here. Enzymes

derived from contaminating bacteria, which may be significant in milk and

some dairy products, will not be discussed. The interested reader is referred

to McKellar (1989) for a comprehensive review of enzymes produced by

psychrotrophs which are the principal spoilage microorganisms in refrig-

erated milk and milk products. The significance of enzymes from microbial

cultures in cheese ripening is discussed in Chapter 10.

8.2 Indigenous enzymes of bovine milk

8.2.1 Introduction

As many as 60 indigenous enzymes have been reported in normal bovine
milk. With the exception of cr-lactalbumin, which is an enzyme modifier in
lactose synthesis (Chapter 2) most, if not all, of the indigenous enzymes in
milk have no obvious physiological role. They arise from three principal
sources:

0 the blood via defective mammary cell membranes;