112 Chapter 4
separator. This is counterbalanced by con-
trolling the pressure of skim outlet and
depends on the make and throughput of the
separator.
In paring disc separators the volume of
cream discharged is controlled by a cream
valve with a built - in fl ow meter (Figure
4.10 ). The size of the valve aperture is con-
trolled by a screw and the throttled fl ow
passes through a graduated glass tube with an
indicating device. Balancing the cream fl ow
and the skim milk pressure produces the
desired fat content in the cream.
In the more common hermetically sealed
separators, milk is supplied to the bowl
through the bowl spindle. It is accelerated to
the same speed as the rotation of the bowl
and continues through the distribution holes
in the disc stack. The bowl of a hermetic
separator is completely fi lled with milkIn addition to lactic acid being produced
during fermentation, caseins are also being
modifi ed by proteolytic enzymes. Other
changes occurring in milk may produce poly-
saccharides which can alter the viscosity of
the milk. Some lactic acid bacteria metabo-
lize citric acid to produce aroma volatiles
such as diacetyl.
Milk fermentation is necessary for the
manufacture of yogurt, buttermilk, kefi r, and
cheeses, whereas the fermentation of cream
is essential for the manufacture of sour
cream, cream cheese and other types of
cheeses, and cultured cream butter. Some of
these aspects are discussed in greater detail
in Chapters 6 , 11 , 16 , 17, and 18). Common
milk processing steps are discussed below.
Centrifugal Operations
Centrifugal operations remove some or most
of the fat, a step called standardization. One
method is to completely remove all of the fat
as cream, leaving skim milk. The cream and
skim milk can then be recombined in desired
ratios to obtain low - fat, light, and whole milk
with 1%, 2% and 3.25% fat, respectively.
This standardization usually is performed in
a continuous manner.
Cream is separated from milk in a cream
separator. Often the separator has the ability
to remove sediment from milk as well as
separate the cream from milk. Depending on
the design of the separator - clarifi er, the sedi-
ment collected can be manually or automati-
cally removed. Typically, milk can have 1 kg
of sediment/10,000 liters (1 lb/1,100 U.S.
gallons). Automatic discharging separator -
clarifi ers are hermetically sealed and clean-
able in place. This is less cumbersome than
opening up the bowl assembly and manually
cleaning both the sediment and disc stacks of
a separator.
Fat content in cream is controlled by a
paring disc in conjunction with a cream fl ow
meter. A throttle valve at the cream discharge
controls the volume of cream leaving the
Figure 4.10. Paring disc separator with manual con-
trols. 1, skim milk outlet with regulator; 2, cream
throttling valve; 3, cream fl ow meter. Reproduced
with permission from Tetra Pak.231