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24 Chemistry and Biochemistry of Milk Constituents 459development and functioning of the mammary gland, or both
(Fox and Flynn 1992, Pihlanto-Leppala 2011, Wynn 2011).Milk Protein-Derived Bioactive PeptidesThe six lacto-proteins contain sequences, which when released
on proteolysis display various biological activities(Pihlanto-
Leppala 2011, Fosset and Tome 2011). Some of these peptides
are formed in the GIT in vivo but it not known whether or not
they are active in vivo.Indigenous Milk EnzymesMilk contains about 60 indigenous enzymes, which represent a
minor but very important part of the milk protein system (for
review, see Fox and Kelly 2006, Fox 2003b). The enzymes orig-
inate from the secretory cells or the blood. Many of the indige-
nous enzymes are concentrated in the MFGM and originate in
the Golgi membranes of the cell or the cell cytoplasm, some
of which occasionally becomes entrapped as crescents inside
the encircling membrane during exocytosis. Plasmin and LPL
are associated with the casein micelles and several enzymes are
present in the milk serum, many of which are derived from the
MFGM, which is shed during the storage of milk.
There has been interest in indigenous milk enzymes since
lactoperoxidase was discovered in 1881. Although present at
relatively low levels, the indigenous enzymes are significant for
several reasons:Nutritional and Protective Some indigenous enzymes have
a protective effect on the neonate or perhaps on the mammary
gland of the mother, for example lysozyme, lactoperoxidase,
superoxide dismutase and xanthine oxidoreductase. Many of
the enzymes have the potential to participate in digestion but
probably the only enzyme significant in this regard is bile salts
stimulated lipase (carboxyl ester hydrolase, CEH) which is be-
lieved to make a significant contribution to lipolysis; human milk
is particularly rich in CEH but it is also present in the milk of
several other species.Technological Plasmin causes proteolysis in milk and some dairy prod-
ucts; 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,
in which the coagulant is extensively or completely dena-
tured, for example Emmental, Parmesan and Mozzarella.
LPL may cause hydrolytic rancidity in milk and butter but
contributes positively to cheese ripening, especially that
made from raw milk.
Acid phosphatase can dephosphorylate casein and mod-
ify its functional properties; it may contribute to cheese
ripening.
Xanthine oxidoreductase is a very potent pro-oxidant 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 ex-
ploited for the cold-sterilisation of milk.Indices of Milk Quality and History
The standard assay to assess the adequacy of HTST pas-
teurisation is the inactivation of alkaline phosphatase. Pro-
posed assays for super-pasteurisation of milk are based on
the inactivation ofγ-glutamyltranspeptidase or lactoperoxi-
dase.
The concentration/activity of several enzymes in milk
increases during mastitic infection and some have been
used as indices of this condition, for example catalase, acid
phosphatase and especiallyN-acetylglucosaminidase.Antibacterial
Milk contains several bactericidal agents, two of which are
the enzymes, lysozyme and lactoperoxidase.MILK SALTS
Milk contains six inorganic elements, Ca, Mg, Na, K, Cl and P, at
substantial concentrations (macroelements) and about 20 more
at trace levels (microelements). These elements are referred to as
milk salts or, incorrectly, as minerals. In addition, milk contains
citrate that interacts with and affects the state of inorganic salts.
Although they are relatively minor constituents, the milk salts
of milk have major effects on its technological and biological
properties and there have been several reviews, including Holt
(1997) and Lucey and Horne (2009).
When milk is heated in a muffle furnace at 500◦C for approx-
imately 5 hours, an ash derived mainly from the inorganic salts
of milk and representing approximately 0.7%, w/w, of the milk,
remains. However, the elements are changed from their origi-
nal forms to oxides or carbonates and the ash contains P and
S derived from caseins, lipids, sugar phosphates or high-energy
phosphates. The organic salts, the most important of which is
citrate, are oxidised and lost during ashing; some volatile met-
als, for example sodium, are partially lost. Thus, ash does not
accurately represent the salts of milk. However, the principal
inorganic and organic ions in milk can be determined directly
by potentiometric, spectrophotometric or other methods. The
typical concentrations of the principal elements, often referred
to as macro-elements, are shown in Table 24.3. Considerable
variability occurs, due, in part, to use of poor analytical methods
and/or to samples from cows in very early or late lactation or
suffering from mastitis. The micro-elements are very important
from a nutritional viewpoint, some, for example Fe and Cu, are
very potent lipid pro-oxidants and some, for example Fe, Mo,
Zn, are enzyme co-factors.
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