Processing Principles of Dairy Ingredients 111bacterial content of milk. Sporeformers are
effectively reduced by this process, which is
more commonly used in treating milk for
powder and cheese manufacture.Microbial Transformation
Drying, condensing, and fermentation are all
methods of preserving milk. Fermentation is
the controlled acidifi cation of milk and
cream, in which the type of microorganisms
growing and the conditions for their growth
are carefully monitored and stopped. The
characteristics of the microorganisms used in
fermenting milk and cream are discussed in
greater detail in Chapter 6. The main con-
cepts of this transformation are outlined
below.
Lactic acid bacteria are the prime agents
of fermentation. Morphologically these are
rods and cocci and they stain Gram - positive.
The optimal temperatures for their growth
are either in the mesophilic range (20 ° C to
30 ° C; 68 ° F to 86 ° F) or thermophilic range
(35 ° C to 45 ° C; 95 ° F to 113 ° F). Lactic acid
bacteria use lactose to produce lactic acid.
The transport of lactose into the cells is facili-
tated by two enzyme systems: the phospho-
enol pyruvate dependent phosphotransferase
system and an ATPase dependent system.
Lactic acid bacteria are also classifi ed as
homofermentative or hetrofermentative.
Production of lactic acid only from lactose,
as is the case with most mesophilic lactic acid
bacteria, leads to such bacteria being labeled
homofermentative. One molecule of lactose
results in four molecules of lactic acid.
Hetrofermentative lactic acid bacteria,
including leuconostocs, lack the enzymes
called aldolases and cannot ferment lactose
via the glycolytic pathway. This class of bac-
teria ferments one molecule of lactose to two
molecules each of lactic acid, ethanol, and
carbon dioxide. Homofermentative lactic
acid bacteria do not produce ethanol or
carbon dioxide, whereas hetrofermentative
lactic acid bacteria do.Likwifi er, a high - speed blender, the powder
and process liquid are contacted and sheared
in the mixer. Silverson, another in - line mixer,
operates at high speeds and its action is
somewhat similar to homogenization.
Separation
It is necessary to separate the fat from the
milk. The principles used to separate fat from
milk are also applied to remove fi ne extrane-
ous material from milk and to reduce the
bacterial content of milk. Separation of fat
from milk is called cream separation, the
removal fi ne extraneous particle is termed
clarifi cation, and the reduction in microbial
numbers is obtained through bactofugation.
All of these processes rely on centrifugal
force to achieve their objective. The factors
that affect the effi ciencies of these processes
are diameter of the particles (d μ ), density of
the particle ( ρ (^) p kg/m^3 ), density of the con-
tinuous phase ( ρ (^) l kg m^3 ),viscosity of the con-
tinuous phase ( η kg/ms) and the gravitational
force (g = 9.81 ms^2 ). For example a 3 μ diam-
eter fat globule will rise at a velocity of
0.6 mm/h. To speed up this process centrifu-
gal force is applied and the sedimentation
velocity is increased 6,500 - fold. Specially
designed equipment called a cream separator
is used to achieve this separation under a
centrifugal force fi eld.
Another centrifugal operation in the dairy
industry is a variant of cream separation that
is used to remove solid impurities from milk.
This piece of equipment is called a clarifi er.
The principal difference between clarifi ca-
tion and separation is in the design of the disc
stack in the centrifuge bowl and the number
of outlets. In a clarifi er, the disc stack has no
distribution holes and only one outlet. In a
separator disc there are distribution holes and
are two outlets, one each for cream and skim
milk.
Bactofugation is a third application of
centrifugal force in dairy processing. In this
process centrifugal force is used to reduce the