Processing Principles of Dairy Ingredients 113during operation. There is no air in the center,
hence the name hermetic separator. It is a part
of the closed piping system of the dairy. The
pressure generated by the external product
pump is suffi cient to overcome the resistance
to fl ow through the separator to the discharge
pump at the cream and skim outlets.
An automatic constant pressure unit in a
hermetic separator is controlled by a dia-
phragm valve. The pressure on the valve is
controlled by compressed air above the dia-
phragm (Figure 4.11 ).
Direct in - line standardization of fat
content of milk is based on the principle of
keeping the pressure of the skim milk con-
stant. This pressure must be maintained
regardless of fl ow fl uctuations or pressure
drop caused by the equipment after separa-
tion. Pressure is maintained by a constant
pressure valve at the skim discharge side of
the separator. Precision standardization also
depends upon fl uctuations in fat content of
the incoming milk, throughput, and pre -
heating temperatures.
Figure 4.11. Hermetic separator bowl with an auto-
matic pressure unit on the skim milk outlet.
Reproduced with permission from Tetra Pak.
Some countries may use centrifugal oper-
ations to manufacture cultured dairy prod-
ucts. In yogurt manufacture, skim, 1%, and
3.25% milk is often used and in the more
indulgent types of yogurt higher fat contents
up to 8% may be used. All of these different
fat contents are obtained through centrifugal
operations involving standardization on line.
A schematic of an in - line standardization unit
is shown in Figure 4.12.
Separation temperature is also an impor-
tant variable. Cold separation of milk (below
4 ° C or 40 ° F) decreases the effi ciency of fat
recovery. Therefore, warm separation is com-
monly used where the effi ciency of fat
removal is greater because the fat is in a fl uid
state at temperatures of around 50 ° C (122 ° F).
Warming the milk can take place during the
regeneration phase of heat transfer (see
below).Thermal Processing Systems
The standardized milk is thermally processed,
or pasteurized, as required by law to render
the milk free from pathogens. Pasteurization
can be a batch process or a continuous pro-
cess. Batch processes are used by small
processors; they are uncommon in modern
dairies. The batch process is called long - time -
low - temperature (LTLT) pasteurization.
In the batch process, standardized milk is
heated to 62.5 ° C (145 ° F) and held at that
temperature for 30 minutes. The processing
tanks used for such purposes should have the
characteristics defi ned in the pasteurized milk
ordinance (PMO). Homogenization takes
place post pasteurization and is followed by
cooling. Homogenization may also take place
after the regeneration section and prior to
entering the heating section. If the tempera-
ture of the milk is around 40 ° C (104 ° F), lipol-
ysis can be enhanced by homogenization.
Therefore, homogenization temperature must
be above 45 ° C (113 ° F). At this temperature
milk lipase and many microbial lipases are
rendered ineffective.