BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come
24 Chemistry and Biochemistry of Milk Constituents 451accelerates as the globules rise and clump. Ovine, caprine and
buffalo milk do not contain cryoglobulins and therefore cream
much more slowly than bovine milk.
In the past, creaming was a very important physicochemical
property of milk:
The cream layer served as an index of fat content and hence
of quality to the consumer.
Creaming was the traditional method for preparing fat
(cream) from milk for use in the manufacture of butter. Its
significance in this respect declined after the development
of the mechanical separator by Gustav de Laval in 1878
but natural creaming is still used to adjust the fat content
of milk for some cheese varieties, for example Parmigiano-
Reggiano. A high proportion (∼90%) of the bacteria in
milk become occluded in the clusters of fat globules.Homogenisation of MilkToday, creaming is of little general significance. In most cases,
its effect is negative and for most dairy products, milk is ho-
mogenised, that is subjected to a high-shear pressure that re-
duces the size of the fat globules (average diameter< 1 μm),
increases the fat surface area (four to six fold), replaces the nat-
ural MFGM by a layer of caseins and denatures cryoglobulins,
hence preventing the agglutination of globules.
Homogenization of milk is usually performed using a valve
homogeniser (developed by August Gaulin in 1899), in which
milk is forced at a pressure of approximately 20 MPa against
a spring-loaded valve. The residence time in the valve is very
short and many newly formed globules share emulsifier (proba-
bly casein micelles), leading to the formation of clusters that will
cream rapidly and increase the viscosity of the system. The clus-
ters are dispersed by a second homogenisation at approximately
3.5 MPa. Recently, a high-pressure homogeniser, operating at
a pressure up to 300 MPa, has been developed. This type of
homogeniser gives a very narrow distribution of fat globules,
resulting in improved product quality.
Homogenisation has several very significant effects on the
properties of milk:
If properly executed, creaming is delayed indefinitely due
to the reduced size of the fat globules, the denaturation of
cryoglobulins and the increased density of the fat globules,
due to the layer of adsorbed casein on the surface.
Susceptibility to hydrolytic rancidity is markedly increased
because indigenous LPL has ready access to the triglyc-
erides; consequently, milk must be heated under conditions
sufficiently severe to inactivate LPL before (usually) or
immediately after homogenisation.
Susceptibility to oxidative rancidity is reduced because pro-
oxidants in the MFGM, for example metals and xanthine
oxidase, are distributed throughout the milk.
The whiteness of milk is increased, due to the greater num-
ber of light-scattering particles.
The strength and syneretic properties of rennet-coagulated
milk gels for cheese manufacture are reduced; hence,
cheese with a higher moisture content is obtained. Con-sequently, milk for cheese manufacture is not normally
homogenised; an exception is reduced-fat cheese, in which
a higher moisture content improves texture.
The heat stability of whole milk and cream is reduced, the
magnitude of the effect increasing directly with fat content
and homogenisation pressure; homogenisation has no effect
on the heat stability of skimmed milk.
The viscosity of whole milk and cream is increased by
single-stage homogenisation due to the clustering of
newly formed fat globules, caused by sharing of casein
micelles; the clumps of globules are dispersed by a second
homogenisation stage at a lower pressure, which may be
omitted if an increased viscosity is desired.Lipid OxidationThe chemical oxidation of lipids is a major cause of instabil-
ity in dairy products (and many other foods). Lipid oxidation
is a free radical, autocatalytic process principally involving the
methylene group between a pair of double bonds in PUFAs;
oxygen is a primary reactant (see O’Brien and O’Connor 2011).
The process is initiated and/or catalysed by polyvalent metals,
especially Cu and Fe, ultraviolet (UV) light, ionising radiation
or enzymes, such as lipoxygenase in the case of plant oils, and
xanthine oxidase, which is a major component of the MFGM,
in milk. The principal end-products are unsaturated carbonyls,
which cause major flavour defects; the reaction intermediates,
that is fatty acid free radicals, peroxy free radicals and hydroper-
oxides, have no flavour. Polymerisation of free radicals and other
species leads to the formation of pigmented products and to an
increase in viscosity but it is unlikely that polymerisation-related
problems occur to a significant extent in dairy products.
Lipid oxidation can be prevented or controlled by the
following: Avoiding metal contamination at all stages of processing
through the use of stainless steel equipment.
Avoiding exposure to UV light by using opaque packing
(foil or paper).
Packaging under an inert atmosphere, usually N 2.
Use of O 2 or free-radical scavengers, for example glucose
oxidase and superoxide dismutase (an indigenous enzyme
in milk), respectively.
Use of antioxidants which break the free radical chain re-
action; synthetic antioxidants are not permitted in dairy
products but the level of natural antioxidants, for exam-
ple tocopherols (vitamin E), in milk may be increased by
supplementing the animal’s feed. Polyphenols are very ef-
fective antioxidants; their direct addition to dairy products
is not permitted but it may also be possible to increase their
concentration in milk by supplementing the animal’s feed.
Antioxidants are compounds which readily supply a H·to
fatty acid and peroxy radicals, leaving a stable oxidised rad-
ical. Many antioxidants are polyphenols, which give up a H·
and are converted to a quinone; examples are tocopherols
(vitamin E) and catechins. At low concentrations, ascor-
bic acid is a good antioxidant but at high concentrations it