Milk destined for
the production of powder must be of high chemical, organoleptical, and
bacteriological quality. Acidity of milk must be below 0.15% (expressed as lactic
acid), otherwise the solubility of the milk powder is reduced. Stored raw milk is
not good even when it is kept under refrigerated conditions because it may lead
to an increase in free fatty acidy in the resultant dried milk causing many
fold increase in lipolysis in the powder produced from such milk. High
bacterial counts increase the susceptibility of fat to oxidation during storage
of the powder. Oxidation of milk fat reduces the shelf life of the resulting
milk powder. Fat oxidation is accelerated by the presence of metals such as
copper or iron. Care must be taken that all equipments,pipelines and accessories
coming into contact with milk and cleaning solutions are made of stainless
steel exclusively.
The basic
operations in milk powder production consists of receiving and selecting milk,
filtration/clarification, cream separation/standardization, preheat treatment, condensing,
homogenisation, drying, packaging and storing of powder. Production of skim
milk powder differs slightly from whole milk powder production.
i.
Standadization
The objective of
standardization is to adjust the ratio of milk fat and total solids to the
level required in the final product. Production of skim milk powder differs
slightly from whole milk powder production. For skim milk powder, it is
necessary to reduce fat in skim milk to less than 0.1%.
ii.
Preheat Treatment
The preheating of
milk before condensing in the manufacture of dried milks is done for the
production of safe and better stable milk powders and also for inducing in it other
desirable attributes. Pasteurization at 72oC
for 15 s is sufficient to address the safety requirements associated with
processing of raw milk, though, higher temperatures and longer holding times
may be necessary to meet requirements relating to ther moduric pathogens and
specific food spoilage organisms. For milk powder solubility, which mostly
depends on the state of the milk protein system, the high temperature short
time (HTST) regime is more convenient when compared to prolonged thermal
treatment at low temperatures, and has the same microbiological effect. The
temperature most frequently used is in the range of 88-95oC
for 15-30 sec. HTST regimes with direct or indirect heating are also used with
temperatures ranging up to 130oC.
Preheat treatment
contributes significantly to the shelf life of dried milks (e.g. whole milk
powder) primarily through development of antioxidant or reducing substances.The
formation of reactive or free sulphydryl groups, resulting from the heat
treatment of milk, are responsible for preventing oxidized flavour development
by acting as free radical scavengers, and thus as antioxidants. Products of
Maillard reactions also contribute to the overall antioxidant effect in milk
powders.
Milk may be
heat-treated at temperatures to achieve various levels and types of protein
denaturation, according to final product requirements. Skim milk powder is commonly
graded according to the amount of undenatured whey protein nitrogen present in
non-fat milk solids expressed by the whey protein nitrogen index (WPNI), which
is defined as the amount of undenatured whey protein nitrogen measured in mg/g
powder. Table 14.1 shows the heat classifications for skim milk powders as was
introduced by the American Dry Milk Institute (ADMI) in 1971.
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iii. Condensing
For spray drying,
the milk is concentrated in multiple effect evaporators to 40-50 % total
solids. For roller drying, the milk is concentrated to 18% total solids.
Further concentration for spray drying would increase viscosity and cause
difficulties during atomisation of the milk. During roller drying, a higher
concentration of milk would form a thicker layer on the rollers, followed by
inhibited drying and intensive irreversible changes to proteins, lactose and
fat. Evaporators use much lesser thermal energy than driers for removal of
equivalent amount of moisture from milk.
iv.
Homogenization
Homogenization is
not an obligatory operation in milk powder manufacture, but is usually applied
to decrease free fat content. Higher free fat content in powder is,however,
demanded in certain confections. Homogenization also helps to prevent clumping
of fat during reconstitution and improves the keeping quality of powder.Homogenization
is conducted after evaporation, or in partly concentrated milk, the concentration
rate being not more than 3:1. At higher concentration, homogenization destabilizes
milk proteins, thus decreasing powder solubility. The customary homogenization
is 2500 to 3000 psi at 62.8 to 76.7oC.
v.
Drying
Though a number of
drying systems are available, practically only spray drying and roller drying
are commercially used in dairy industry. Since the product quality and process
economy are much better and are being constantly improved, spray drying has the
highest potential today and in the foreseeable future
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