106 Chapter 4
most common operations involve pumping or
transferring fl uids, heat transfer (cooling and
heating), mixing ingredients, separation (fat
standardization), and microbial transforma-
tion of milk (acid gel formation). These
aspects are discussed below.Overview of Processing
Equipment in a Dairy Plant
Fluid Transfer Operations
Fluid transfer processes involve using pumps
to transfer milk from the receiving tankers to
storage silos and then to appropriate unit
operations. The two main categories of
pumps used in the dairy industry are centrifu-
gal and positive displacement pumps. There
are different types of pumps within each
category.
The selection of the right type of pump for
use in an operation depends upon a number
of factors including fl ow rate, product to be
handled by the pump, viscosity, density, tem-
perature, and pressure in the system. Pumps
should be installed as close to the tanks and
with as few valves and bends in the line as
feasible. Any devices to restrict fl ow should
be placed at the exit or discharge side of the
pump. Cavitation is a pumping problem that
is caused by too low of pressure at the inlet
end of a pump relative to the vapor pressure
of the fl uid being transferred. As cavitation
progresses, pumping effi ciencies decrease
and eventually the pump ceases to transfer
the fl uid. The appropriate size of the pump
required for the transfer depends upon fl ow
rate and head, required motor power, and the
net positive suction head. Engineers using
charts and formulas easily calculate these
parameters.Centrifugal Pumps
In a centrifugal pump, a motor drives an
impeller with vanes (Figure 4.4 ). The motion
is circular and the liquid being pumped enters
to the center of the impeller, which imparts aof milk in the silos, and protection devices.
Redundant visual (non - electronic) systems
may also be employed in some dairies.
Milk storage silos are cleaned in place and
periodic visual inspections of the interior sur-
faces are conducted. Silos are considered
confi ned spaces; therefore, entry into a silo is
strictly according to the standards recom-
mended by Occupational Safety and Health
Administration of the U.S. government.
The temperature of the milk in the silo
must be maintained at 4 ° C or below (below
40 ° F). Even at these temperatures, psychro-
trophs can cause proteolysis and lipolysis if
milk is stored for long periods. Therefore, it
is recommended that the silos be regularly
emptied, cleaned, and sanitized. The raw
milk in the silo is further processed; the main
elements in the processing are centrifugal
operations, thermal treatment, homogeniza-
tion, cooling, and packaging.
The processing steps may involve one or
more operations in combination, and the
Figure 4.3. Schematic of a milk silo with a propeller
agitator. Reproduced with permission from Tetra Pak.