19 Chemistry and Biochemistry of Milk Constituents 447Golgi membranes of the cell or the cell cytoplasm,
some of which occasionally becomes entrapped as
crescents inside the encircling membrane during
exocytosis. Plasmin and lipoprotein lipase are asso-
ciated with the casein micelles, and several enzymes
are present in the milk serum; many of the enzymes
in the milk serum are derived from the MFGM,
which is shed as the milk ages.
There has been interest in indigenous milk en-
zymes since lactoperoxidase was discovered in 1881.
Although present at relatively low levels, the indige-
nous enzymes are significant for several reasons:
Technological- Plasmin causes proteolysis in milk and some
dairy products; it may be responsible for age
gelation in UHT milk and contributes to
proteolysis in cheese during ripening, especially
in varieties that are cooked at a high temperature
and in which the coagulant is completely or
extensively denatured, for example, Emmental,
Parmesan, and Mozzarella. - Lipoprotein lipase may cause hydrolytic
rancidity in milk and butter but contributes
positively to cheese ripening, especially in
cheeses made from raw milk. - Acid phosphatase can dephosphorylate casein
and modify its functional properties; it may
contribute to cheese ripening. - Xanthine oxidase is a very potent prooxidant and
may cause oxidative rancidity in milk; it reduces
nitrate, used to control the growth of clostridia in
several cheese varieties, to nitrite. - Lactoperoxidase is a very effective bacteriocidal
agent in the presence of a low level of H 2 O 2 and
SCN-and is exploited for the cold sterilization of
milk.
Indices of Milk Quality and History- The standard assay to assess the adequacy of
HTST pasteurization is the inactivation of
alkaline phosphatase. Proposed assays for
superpasteurization of milk are based on the
inactivation of -glutamyltranspeptidase or
lactoperoxidase. - The concentration/activity of several enzymes in
milk increases during mastitic infection, and
some have been used as indices of this condition,
e.g., catalase, acid phosphatase, and especially,
N-acetylglucosaminidase.
Antibacterial- Milk contains several bactericidal agents, two of
which are lysozyme and lactoperoxidase.
MILK SALTS
When milk is heated in a muffle furnace at 500°C
for approximately 5 hours, an ash derived mainly
from the inorganic salts of milk and representing
approximately 0.7% by weight of the milk, remains.
However, the elements are changed from their origi-
nal forms to oxides or carbonates, and the ash con-
tains P and S derived from caseins, lipids, sugar
phosphates, or high-energy phosphates. The organic
salts, the most important of which is citrate, are oxi-
dized and lost during ashing; some volatile metals,
for example, sodium, are partially lost. Thus, ash
does not accurately represent the salts of milk. How-
ever, the principal inorganic and organic ions in milk
can be determined directly by potentiometric, spec-
trophotometric, or other methods. The typical con-
centrations of the principal elements, often referred
to as macroelements, are shown in Table 19.3. Con-
siderable variability occurs, due in part, to poor ana-
lytical methods and/or to samples from cows in very
early or late lactation or suffering from mastitis.
Milk also contains 20–25 elements at very low or
trace levels. These microelements are very impor-
tant from a nutritional viewpoint, and some, for ex-
ample, Fe and Cu, are very potent lipid prooxidants.
Some of the salts in milk are fully soluble, but
others, especially calcium phosphate, exceed their
solubility under the conditions in milk and occur
partly in the colloidal state, associated with the ca-
sein micelles; these salts are referred to as colloidal
calcium phosphate (CCP), although some magne-
sium, citrate, and traces of other elements are also
present in the micelles. The typical distribution of
the principal organic and inorganic ions between the
soluble and colloidal phases is summarized in Table
19.3. The actual form of the principal species can be
determined or calculated after making certain
assumptions; typical values are shown in Table 19.3.
The solubility and ionization status of many of
the principal ionic species are interrelated, especially
H, Ca^2 , PO 4 3-, and citrate3-. These relationships
have major effects on the stability of the caseinate
system and, consequently, on the processing proper-
ties of milk. The status of various ionic species in
milk can be modified by adding certain salts to milk,