146 Chapter 6
Homogenization
Homogenization of the full - cream milk con-
centrate prior to spray drying is a routine step
in the traditional process for manufacture of
full - cream milk powder (Figure 6.1 ). During
homogenization, the milk concentrate is fed
under high pressure through a small orifi ce
which disrupts the native milk fat globule.
The size of the globule is reduced, and this
is accompanied by an increase in the surface
area of the fat. The natural milk fat globule
membrane is insuffi cient to cover the
increased area and a new membrane is
formed, comprising a mixture of the original
milk fat globule membrane and adsorbed
milk proteins.
The purpose of homogenization is to
decrease the surface free fat in the milk
powder (Vilder de et al., 1979 ). Generally,
full - cream milk powders are manufactured to
obtain a low level of free fat in powder. This
is because a high surface free fat (above 2%
of powder) in full - cream milk powders is
undesirable because fl owability, wettability,
and storage stability of the powder are
adversely affected.Drying
In the drying stage, dryers remove water
from a milk concentrate to produce a shelf -
stable product. Early commercial milk
powder drying plants used roller dryers. The
concentrate was fed over rotating steam - or
oil - heated drums to evaporate the water from
the concentrate. The resultant sheet of powder
was then powdered in a hammer mill to a
predetermined size. The powders produced
have sharp edges and are made up of compact
particles (Caric and Kalab, 1987 ). Except for
the manufacture of full - cream milk powder
with high free fat content for the chocolate
industry, today most industrial milk powder
plants use spray dryers (Figure 6.2 ).
In spray drying, milk concentrates are fed
into dryers with positive displacement pumps.is primarily because of the change in the
mineral salt equilibria of milk because the
milk is concentrated by water removal.
Concentration causes transfer of calcium and
phosphate to the colloidal phase of milk
(Graet le and Brule, 1982 ), causing a re -
establishment of the mineral salt equilibria
with the release of hydrogen ions. The high
solids and the lower pH of evaporated milk
concentrates make them more susceptible to
aggregation.
Milks also may be concentrated by mem-
brane processing. This method of concentra-
tion is used for the production of milk protein
concentrates (Figure 6.1 ). In contrast to the
removal of water only by thermal evapora-
tion, concentration using membranes results
in the fractionation of the milk ’ s components.
The partitioning of the milk components
depends on many factors including the size
of the membranes, the extent of ultrafi ltration
and diafi ltration, and the conditions (e.g., pH
of feed, temperature) used for these separa-
tion processes. Membrane processing leads
to concentration of the milk protein in the
retentate streams which are subsequently
dried to produce milk powders with higher
protein content than that of traditional skim
milk powder.
Milk protein concentrate (MPC) powders
with varying protein contents (40% to 85%
protein) have been made by ultrafi ltration
and/or diafi ltration or evaporation of the
ultrafi ltered retentate prior to drying. Milk
protein concentrate powders may be made as
low - or high - heat products (Getler et al.,
1997 ; Huffman and Harper, 1999 ). The total
solids of the concentrate that can be fed into
the dryer depends on the protein content of
the MPC produced, but is much lower than
that used in the production of skim milk
powders. The high - protein and low - lactose
content of the concentrates prepared by
ultrafi ltration/diafi ltration leads to the higher
viscosity, which limits the solids content of
the concentrates that can be fed into the
dryer.