116 Chapter 4
cool the milk to below 4 ° C in the cooling
section. Cooling is achieved by chilled water
or cold glycol as the refrigerant. The water is
chilled by a refrigeration system which com-
monly uses ammonia as the refrigerant. Other
hydrocarbons also may act as refrigerants.
Because the pasteurized milk transmits con-
siderable heat to the cold raw milk, less
refrigeration capacity is required to cool the
milk to below 4 ° C.
In yogurt manufacture the cooling system
may not be used. Once the pasteurized milk
has been cooled to around 43 ° C to 45 ° C it
may be pumped to the fermentation tanks for
further processing. Obviously, reheating cold
pasteurized milk to the incubating tempera-
tures of 43 ° C to 45 ° C requires more energy
than avoiding this step in the fi rst place.Homogenization
Homogenization reduces the size of fat glob-
ules to prevent creaming (separation of a fat -
enriched layer from the aqueous phase). The
globule size is reduced through a combina-
tion of turbulence and cavitation in a
homogenizer.
Cold milk cannot be homogenized effi -
ciently because the milk fat is still solid.
Therefore, homogenization occurs best at
temperatures greater than 37 ° C (99 ° F).
Another necessity for effi cient homogeniza-
tion is the presence of protein. A minimum
value of 0.2 grams of casein/gram of fat is
recommended.
Homogenizers are manufactured as single -
stage and dual - stage machines. In single -
stage homogenization the whole pressure
drop is used over one device. It is used for
products with low fat content and in products
requiring a high viscosity (e.g., sour cream,
coffee cream, whipping cream). Two - stage
homogenizers are used in breaking down the
fat globule in two stages. This is effective for
high fat content, high solids content, or prod-
ucts in which low viscosity is desired (Figure
4.14 ).A sensor at the exit of the holding tube
transmits a signal to the temperature monitor.
As soon as the temperature falls below a
preset minimum value the monitor switches
the fl ow diversion valve to diverted fl ow. In
diverted fl ow, the hot milk returns to the
balance tank because it is not considered pas-
teurized. The reason for the fl uctuation is
determined and corrected and if the correct
temperature is maintained at the exit point of
milk from the holding tube, further fl ow is
continued past the fl ow diversion valve.
Often a booster pump may be added after the
milk exits the holding tube. The hot pasteur-
ized milk enters the regeneration section of
the pasteurizer to heat the incoming raw
milk.
In the regeneration section unpasteurized
milk fl ows on one side of the plate and hot
pasteurized milk fl ows on the other. If there
are pin holes in the plates of the heat
exchanger, unpasteurized milk can commin-
gle with pasteurized milk. This violates the
integrity of the pasteurized milk and the fl uid
is not considered pasteurized. To avoid such
a problem, the pasteurized milk is always at
a higher pressure than the raw milk. A pres-
sure differential meter is often installed on
the control panel to measure the pressures. If
the pressure differential between raw and
pasteurized milk drops below a preset value,
a signal is sent to the fl ow diversion valve to
open. Thus, the two different causes for fl ow
diversion are temperature falling below
preset values and the pressure differential
between raw and cold milk falling below a
certain preset limit. The milk is not consid-
ered pasteurized if either of these events
occurs. For milk to be designated as pasteur-
ized, every drop must be heated to and held
at the specifi ed minimum temperature for a
specifi ed amount of time.
Pasteurized milk in the regeneration
section is cooled, giving off its heat to the
cold incoming raw milk. This cools down the
milk but not to the desired 4 ° C (40 ° F) or
below. The fi nal step in pasteurization is to