152 Chapter 6
Gelling
Milks do not gel at their natural pH. However,
they gel on acidifi cation as the pH is reduced
to pH 4.6. Gelation is a consequence of the
reduction of the charges on the milk proteins
as the pH approaches the isoelectronic point.
Acid milk gels may be formed by addition of
an acidulant such as glucono - delta - lactone or
with the use of cultures as in the production
of yogurt. The strength of milk gels may be
measured using standard texture analyzers
(e.g., Instron, TA - XT2 texture analyzer).
The acid gelation properties of milk are
affected by the milk composition and the heat
treatment applied during powder manufac-
ture. Firmer gels are made with milks that
have been given a high heat treatment.
Improved yogurt properties are obtained with
increasing whey protein denaturation in milk
powder (Augustin et al., 1999 ).Foaming and Emulsifying Properties
Milk powders offer good emulsifying and
foaming capabilities that are required for
some applications. In skim milk powder, the
main surface - active components are the milk
proteins, whereas in full - cream milk powders,
there is also the phospholipid component of
the milk fat globule membrane. Caseins,
whey proteins, and phospholipids are able to
stabilize the air - water interface of air bubbles
in foams and the oil - water interfaces of fat
droplets in emulsions due to their amphiphi-
lic properties.
Foaming capacity can be measured by
simple methods using domestic mixers with
milk solutions at set times and temperatures
and measuring the resultant foam generated
(Phillips et al., 1987 ). The emulsion capacity
of milk powder solution can be determined
by the principle of pumping oil into a protein
solution while homogenizing and monitoring
the electrical resistance of the solution. A
decrease in electrical resistance is observed
when the solution changes from an oil - in -
water emulsion in which water is the continu-of concentrated milks to withstand subse-
quent heat treatment is not recommended
because other factors (e.g. pH, mineral
balance of milk) can have a more signifi cant
effect on heat stability.
Viscosity
Milk powder is used to infl uence the viscos-
ity of products in a range of applications.
Viscosity control is particularly important in
high - solids products such as recombined
sweetened condensed milk.
The viscosity of milks reconstituted from
milk powders is usually measured by a
method aligned with the application in which
the powder is intended. A single - strength
solution at a specifi ed temperature is a
good starting point for many applications.
However, in applications in which a higher -
than - single - strength solution is to be used,
then specifi c tests must be undertaken that
mimic the environment of the application.
For example, tests for assessing the suit-
ability of milk powders for recombined
sweetened condensed milk involve making
a mixture of skim milk powder, sugar, and
water in the same proportions as the fi nal
product; heating the mixture under stan-
dardized conditions that are representative
of the process used in industry; and mea-
suring the viscosity of the fi nal mixture
(Kieseker and Southby, 1965 ; Weerstra et al.,
1988 ). Alternatively, pilot scale trials can
also be carried out to test suitability of
powders for recombined sweetened con-
densed milk applications.
The major factor that infl uences the vis-
cosity of recombined sweetened condensed
milk is the preheat treatment of the skim milk
applied during powder manufacture.
Generally, medium - heat milk powders are
suitable for this application. Increasing the
extent of whey protein denaturation in the
powder to higher than 50% results in marked
increases in viscosity of recombined sweet-
ened condensed milk (Cheng et al., 2000 ).