Processing Principles of Dairy Ingredients 119higher protein content in the concentrated
milk results in a fi rmer acid gel in yogurt.
Membrane processes are extensively used in
the manufacture of whey protein and milk
protein concentrates and isolates.Concentration
The two common delivery forms of dairy
ingredients are liquid concentrates and
powders. Starting with milk, fat separation
generates cream, which is a concentrated
milk fat source. The fat content of cream is
controlled by the centrifugal operations and
varies from 10% to 70%. In these concen-
trated milk fat sources, the serum content is
between 30% and 90%. To minimize the
serum content, cream is converted to butter
by phase inversion. Butter has approximately
15% moisture. The residual moisture can be
decreased in the manufacture of butter oil
that has less than 1% moisture. The other
product of milk separation is skim milk or
milk serum. This product has more than 90%
water and must be concentrated by partial
removal of water. Products such as con-
densed skim milk, sweetened condensed
skim milk, and skim milk concentrate are
concentrated sources of milk - solids - not - fat
(MSNF). These concentrated sources of
MSNF still contain more than 55% water.
Further reduction of moisture content is
achieved by drying skim concentrate to a
solid with less than 4% moisture, a product
sold as non - fat dried milk.
Milk is concentrated by vacuum evapora-
tion. Water removal by atmospheric boiling
leads to denaturation of milk proteins and the
development of an undesirable cooked fl avor.
Because the boiling point of liquids is lower
when under pressures below atmospheric
pressure, concentration can be performed at
temperatures that do not produce deleterious
effects. Designs for evaporators, which are
used to concentrate milk, can be falling fi lm,
rising fi lm, or plate; the former two use a
shell - in - tube heat exchanger. When multiplethe molecular weight of the smallest mole-
cule that cannot pass through the pores of the
membranes.
The fi lter modules themselves are avail-
able in various geometries. The most common
are spiral wound; others are plate and frame,
tubular, and hollow fi ber. Tubular fi lters can
be made out of ceramics or polymers.
Membrane separation capacity depends
on a number of factors. Foremost is mem-
brane resistance, which is determined by
membrane thickness, surface area, and pore
diameter. Next is transport resistance (also
known as fouling effect), which occurs on the
membrane surface as fi ltration proceeds. The
formation of a layer of deposit leading even-
tually to membrane fouling is due to the fl ow
of macromolecules at right angles to the
direction of fl ow. A concentration gradient
leads to diffusion in the opposite direction.
Parallel to the membrane, the macromole-
cules present in the layer close to the mem-
brane move at varying velocities depending
on the axial fl ow rate. The concentration
polarization is not uniformly distributed,
especially when the pressure drop gives dif-
ferent transmembrane pressures along the
membrane surface. The upstream end of the
membrane clogs fi rst and gradually spreads
across the whole surface of the membrane,
reducing capacity and making cleaning
necessary.
Membrane operations can be batch or
continuous. Continuous processes are more
desirable in dairy plants. Process tempera-
tures are maintained at around 50 ° C (122 ° F)
to minimize microbial growth and to improve
membrane fl ux.
The use of membrane processing in the
cultured dairy products area is restricted to
concentration of skim milk for fat - free yogurt
manufacture. Some of the lactose and miner-
als are removed from skim milk, thereby
increasing the protein content. This process
can concentrate skim milk with 9% solids to
12% solids. There is still enough lactose in
the retentate to facilitate fermentation. A