42 Chapter 2
albumin (BSA), sodium chloride, magne-
sium chloride, and calcium.
The fat globule membrane protects most
lipids from enzyme activity. However,
damage to the membrane results in lipase
activity and the production of short - chain
fatty acids that increase rancidity and produce
off fl avors, which render milk unacceptable
for consumption. Lipolysis can occur from
vigorous agitation and homogenization.Phosphatases
Two main phosphatases occur in bovine
milk: alkaline phosphatase and acid phospha-
tase. Both hydrolyze phosphoric esters and
have roles in the processing of milk and milk
products.
Alkaline phosphatase is an enzyme situ-
ated in the fat globule membrane that hydro-
lyzes phosphoric esters. Its optimum activity
is at pH 8.5 at 37 ° C. It is activated by diva-
lent metal ions and inhibited by metal chela-
tors, orthophosphates, and pasteurization. Its
concentration in bovine milk depends on the
individuality of the cow and the lactation
stage.
Acid phosphatase is an enzyme that
hydrolyzes tyrosine phosphorylated proteins
and aryl and acyl phosphates; it is approxi-
mately 42 kDa. It is predominantly located
in the milk serum and its optimum activity is
at 37 ° C and pH 7.9. Its concentration in
bovine milk depends on the lactation state
and disease status of the cow. Its activity is
lower than alkaline phosphatise; however,
its heat stability is not affected by pasteuri-
zation. Its activity is inactivated by UV light,
and inhibited by heavy metals, oxidizing
agents, orthophosphosphates, and polyphos-
phates. It may infl uence heat stability in dairy
products.Factors Affecting Composition,
Quality, and Safety of Milk
Several factors affect the composition, qual-
ity, and safety of milk. Environmental, physi-dimeric metallo - fl avoprotein that associates
with the fat globule membrane and is approx-
imately 30 kDa. It is a nonspecifi c oxidore-
ductase that catalyzes the oxidation of a
variety of substances, including nitrate to
nitrite in cheese, which inhibits the growth of
butyric acid bacteria. Higher concentrations
are present in bovine milk than human milk.
Superoxidase dismutase is located in the
milk serum and catalyzes the dismutation of
the superoxide anion to hydrogen peroxide
and triplet oxygen. It may inhibit the oxida-
tion of milk constituents including the autox-
idation of lipids. Sulfhydryl oxidase is
associated with lipoproteins in the fat globule
membrane. It catalyzes the oxidation of sulf-
hydryl groups to disulfi des and is inactivated
partially by pasteurization.
Plasminogen and Plasmin
Plasminogen is the principal proteolytic
enzyme in milk. It remains inactive until con-
verted to plasmin via plasminogen activators.
There are three plasmin types.
Plasmin is responsible for the production
of γ - casein and proteose peptone by hydroly-
sis of β - casein. It also hydrolyzes many other
proteins. Its optimal activity is at 37 ° C and
pH 8.0; however, approximately 82% of the
enzyme ’ s activity is lost after pasteurization
and is reduced by UV light. The concentra-
tion of plasminogen and plasmin is affected
by lactation stage, disease status, diet, age,
breed, and hormone use.
Lipoprotein Lipase
Lipoprotein lipase is the principal lipase in
milk bound mostly to casein micelles. It is a
glycoprotein that is approximately 10 kDa,
which hydrolyzes triglycerides and diglycer-
ides into fatty acids. Its optimum activity is
at 37 ° C in the pH range of 8.4 to 9.2. It is
inhibited by its products (i.e., fatty acids),
UV light, heat, acid, calcium chloride, man-
ganese chloride, and oxidizing agents. It is
stimulated by the presence of bovine serum