Dry Milk Ingredients 151heat is magnifi ed in concentrated milk solu-
tions such as evaporated milk.
Several alternative methods have been
developed in an attempt to measure heat sta-
bility of powders in general as well as for
specifi c end uses. Typical of the common
methods is measuring the coagulation time of
a milk solution at a specifi c total solids, at
temperatures in the range of 120 ° C to 140 ° C
(248 ° F to 284 ° F). Another method using time
coagulation criteria is the ethanol stability
test (Horne and Parker, 1980 ) in which mix-
tures of reconstituted milk with various
amounts of ethanol are used. The drawback
of these tests is that they measure the heat
stability of the milk solution under defi ned
conditions, which do not directly predict
the heat stability in the intended application
in which the environment can be quite
different.
The closer the conditions of the test are to
the conditions used in the intended applica-
tion, the better the correlation. An objective
laboratory - scale method for examining the
suitability of skim milk powders for recom-
bined evaporated milk manufacture has been
developed. The method involves heating
concentrated milk to 120 ° C (248 ° F) for 13
minutes and measuring the viscosity of the
sterilized concentrated milk. This method has
proven to be a good guide to the stability of
recombined evaporated milk during retorting
(Kieseker and Aitken, 1988 ). This laboratory -
based method is an alternative to costly and
time - consuming pilot plant trials.
Many factors affect the heat stability of
milk powders. Heat treatment of milk applied
during powder manufacture has been used to
manipulate the heat stability of milk powders.
A high - heat treatment of milk, which results
in a high level of whey protein denaturation
(i.e., a low whey protein nitrogen index,
WPNI less than 1.5 mg denatured whey
protein/g powder), is desired for adequate
heat stability of concentrated milks. However,
reliance solely on WPNI to assess the abilityPowders are tested for insolubility by
determining the amount of insoluble material
remaining after a prescribed method for dis-
persion of the powder at a nominated total
solids concentration at defi ned temperature
and mixing techniques. The most common of
these is the method used by the American
Dry Products Institute (ADPI) (ADPI, 2002 ).
The insoluble material is usually made up of
denatured protein (typically β - lactoglobulin)
complexed with casein and lactose in various
ratios.
A range of factors are known to contribute
to the formation of insoluble material in milk
powder. The most critical factor controlling
the insolubility of powders is the temperature
of the particle during the removal of water in
the dryer when the moisture content is
between 10% and 30%. Other factors that
contribute to insolubility include the preheat
treatment of the milk during manufacture,
when higher temperatures more often lead
to higher insolubility, type of dryer used
(roller dryers are worse than spray dryers),
confi guration of the spray dryer (such as the
type of atomization) and single - stage versus
multi - stage drying, and the physical proper-
ties of the concentrate prior to drying (e.g.,
viscosity).
Another very critical factor that infl uences
powder solubility is the temperature at which
the milk powder is reconstituted. Solubility
is usually highest between 40 ° C and 60 ° C
(104 ° F and 140 ° F), particularly when prepar-
ing a high - solids reconstituted concentrate
from powder.
Heat Stability
Heat is used in some form during the process-
ing of most products. Therefore, milks recon-
stituted from powders, when incorporated
into a product, are subjected to heat of various
degrees. During heating the milk is required
to not unduly thicken or coagulate, depend-
ing on the application. The susceptibility to