Processing Principles of Dairy Ingredients 109of factors. The operational time cannot be
predicted and varies from factory to factory.
Refrigeration is another aspect of cooling;
it involves the removal of heat from a product,
a process in which the product cools down
and the medium removing the heat warms up.
In the dairy industry, refrigeration is com-
monly achieved by chilled water or polyeth-
ylene glycol in some cases. The water is
chilled by contacting it with a refrigerant
such as ammonia or other fl uorocarbon gases.Mixing Operations
The manufacture of many dairy products
involves mixing ingredients into milk. For
example, in the manufacture of fl avored
milks, sweeteners, stabilizers, and fl avorings
are added to milk prior to processing. To
fortify solids in certain types of yogurts, milk
solids are added to milk prior to pasteuriza-
tion. In other instances, storage of raw milk
in silos necessitates periodic agitation of the
contents of the silo. In batch pasteurization,
the milk is heated in a tank with an agitation
system to ensure uniform heat transfer. In all
these instances, mixing is required and is
achieved by a number of means.
Batch and continuous processes are avail-
able to incorporate solid ingredients into
milk. The simplest batch blending system is
a funnel or hopper to feed the dry material to
a closed - circuit circulation of the process
fl uid. A centrifugal pump is used to circulate
the process fl uid after the tank is fi lled (Figure
4.8 ). The centrifugal pump can be placed at
either the suction or discharge sides of the
hopper. If the hopper is on the suction side of
the pump, powders are rapidly and effi ciently
dispersed as the mixture of powder and fl uid
come in contact with the impeller of the
pump. The disadvantage is that frequent
blockages may occur in the hopper. Placing
the hopper on the discharge end of the pump
avoids the blockage problem. This confi gura-
tion requires a venturi to facilitate the mixing
(Figure 4.9 ).Figure 4.6. Temperature profi le for a product in a
countercurrent heat exchanger. Red line/fi ll, heating
medium; blue line/fi ll, product fl ow; t i , inlet tempera-
ture; t o , outlet temperature; subscripts 1 and 2, prod-
uct and heating medium, respectively. Reproduced
with permission from Tetra Pak.
°CTimeti2ti2ti1ti1to1to1to2to2Figure 4.7. Heat transfer in a concurrent heat
exchanger. Red line/fi ll, heating medium; blue line/fi ll,
product; T i , inlet temperature; t o , outlet temperature;
subscripts 1 and 2, product and heating medium,
respectively. Reproduced with permission from Tetra
Pak.
°CΔtmTimeti2ti2ti1ti1to1to1to2to2