Dry Milk Ingredients 145higher degree of concentration and a longer
holding time of the concentrate. The viscos-
ity of the concentrate is affected by the
natural variations in milk composition and
by the heat treatment applied prior to
concentration.
A high protein concentration in milk can
signifi cantly increase the viscosity of the
concentrate (Bloore and Boag, 1981 ).
Increasing heat treatment, resulting in a
higher extent of whey protein denaturation,
also increases the viscosity of the concen-
trate. Recent work on reconstituted whole
milk concentrates confi rmed that solids
content, heating, and storage temperature
affected their rheological behavior, with
Newtonian behavior observed at lower total
solids and non - Newtonian behavior occur-
ring at higher total solids and higher heat-
ing temperatures promoting non - Newtonian
behavior at lower solids (Binh et al., 2007 ).
It has been suggested that the total solids
of the concentrate can be increased to greater
than 50% when milks are exposed to low - or
medium - heat treatments prior to concentra-
tion, but should not exceed 50% when high -
heat treatment of milk is applied (Vilder de
and Moermans, 1983 ). However, others have
demonstrated that increasing the solids
content can pose potential problems. Jensen
and Hansen (1974) found that although
increasing the solids of full - cream milk con-
centrates from 43% to 49% did not affect the
initial solubility of the powder produced, the
loss in solubility was more pronounced
during 12 months of storage at 30 ° C (86 ° F)
when concentrates had a higher content of
solids.
The initial pH of single - strength milk is
pH 6.7 (i.e., the natural pH of milk) and it
decreases on concentration. Concentrated
milk at 20% total solids has a pH of approxi-
mately 6.45. Further concentration to 45%
total solids reduces pH even further, and at
this concentration the pH is approximately
0.5 to 0.6 units lower than the pH of single -
strength milk (Bienvenue et al., 2003 ). ThisFull - cream, buttermilk, and cream pow-
ders are not generally given a heat classifi ca-
tion. Nevertheless, the trends in WPNI relate
to the intensity of the heat treatment. High -
heat treatments have been shown to extend
the shelf life of full - cream milk powder
(Baldwin and Ackland, 1991 ). The treatment
inactivates lipase present in milk and devel-
ops the natural antioxidant activity of the
milk components. Heat treatment causes
exposure of the sulphydryl groups and the
Maillard reaction; both of these events con-
tribute to an increased oxidative stability of
the milk powder (Taylor and Richardson,
1980 ; McGookin and Augustin, 1997 ).
Concentration
In the production of skim and full - cream
milk powders, the milk stream is thermally
concentrated, typically to 45% to 50% total
solids, prior to drying in a spray dryer. A
multi - stage falling fi lm evaporator is nor-
mally used, and the evaporation is performed
under vacuum. There is little additional
heat damage during concentration after the
preheat treatment of the milk (Oldfi eld et al.,
2005 ) because the residence times in each
stage of the evaporator are short (approxi-
mately 60 seconds). The maximum tempera-
ture the milk is exposed to is 72 ° C (161.6 ° F)
in the fi rst stage, with lower temperatures in
subsequent stages (Singh and Newstead,
1992 ). Because the thermal evaporation of
water is cheaper than removing water by
spray drying, it is desirable to feed concen-
trates with high total solids content into the
dryer.
The viscosity of concentrates fed into the
dryer infl uences the properties of milk
powder; in particular, increasing viscosity
leads to a signifi cant reduction in the solubil-
ity of the powder (Baldwin et al., 1980 ). High
viscosity affects the effi ciency of the drying
process (Bloore and Boag, 1982 ). Hence, vis-
cosity must be controlled and monitored. The
viscosity of the concentrate increases with a