From Milk to Dairy Products 17
1.2. Biological and physicochemical aspects of milk processing
1.2.1.
The stability of fat globules
1.2.1.1.
Native fat globules
Native fat globules have a natural tendency to rise to the surface
(creaming), the rate of
which depends on globule size, the temperature
affecting the viscosity
of the continuous phase, the difference in density
between the continuous and dispersed phases and the
gravitational acceleration
(Stokes’ law, equation [1.1], Volume 2). These characteristics are taken into
account in the preparation of milk and cream for consumption. Flocculation
and the coalescence of fat globules accelerate creaming by increasing particle
size. In milk, flocculation or agglutination of fat globules is mainly due to the
IgM class of immunoglobulins that result in the formation of aggregates of up
to 1mm (up to 10
6
fat globules). These aggregates form at cold temperatures
and can be separated by stirring or reheating milk above 37°C. Fat globules of
heat-treated milk are less prone to agglutination due to IgM denaturation.
Coalescence of fat globules does not occur in
milk emulsions due to the
electrostatic barrier generated by the charge of the native MFGM (surface
potential of –13mV at the natural pH of milk) and the steric barrier formed by
hydrophilic carbohydrate chains of glycoproteins in the membrane. Lowering
pH that reduces the MFGM surface potential or increasing the ionic strength
that reduces the thickness of the electrical double layer,
decreases emulsion
stability by promoting flocculation and the coalescence of fat globules. In
addition, coalescence of fat globules can be obtained by the vigorous stirring
of cream (churning) or by a series of freeze–thaw cycles, which have a
destabilizing effect on the membrane.
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