150 Chapter 6
ibility is reported in terms of the mass of the
test portion and the values for water content
and total solids (Pisecky, 1997 ).
To facilitate dispersibility, the agglomera-
tion process must be controlled to produce
few if any agglomerates exceed 250 microns
in size.
White fl ecks: White fl ecks are particles
that remain undissolved in a milk solution
after reconstitution. They can be observed
when the solution is spread to form a thin
fi lm, for example, on the back of a spoon
after the solution has been allowed to stand
for several minutes. The white fl ecks can also
form a surface layer. They tend to be more
prevalent when high total solids solutions are
prepared. Although white fl ecks tend to be
rather soft, they can cause physical problems
when the reconstituted milk is used in pro-
cessing operations, because they can clog
fi lters and sieves and can be visibly undesir-
able in the fi nal product.Physical Functionality of Powders
When a milk powder is used in an applica-
tion, either as the primary or secondary ingre-
dient, it imparts physical attributes to the
fi nal products which are often essential for
the success of the application. These physical
attributes include solubility, heat stability, gel
forming, thickening or viscosity control,
foaming, and binding characteristics.Solubility
Solubility is a prerequisite for most other
functional attributes because if the powder
cannot be effi ciently solubilized then it
cannot impart the desired attribute effec-
tively. If the powder is not completely dis-
solved it can cause problems in processing
such as clogging of fi lters and losses of mate-
rial due to sedimentation. There is also the
need for subsequent removal of undissolved
material.The degree of wettability is strongly infl u-
enced by several factors; two of the most
signifi cant are the free fat content of the
powder and the state of the lactose. Under
some conditions of manufacture or storage,
the amorphous lactose may be changed to a
crystalline state and damage the fat globule
membrane (Kelly et al., 2003 ), causing an
increased level of free fat in full - cream milk
powders. One way to overcome the problem
associated with reduced wettability is to add
surfactants such as lecithin to the powder.
Wettability is also reduced when there is an
increase in interstitial air within the powder
particles.
Sinkability: A closely aligned attribute to
wettability is the sinkability of powders. Once
the powder particle has been initially wetted
it then must be able to sink into the water
for complete dispersion and solubility.
Sinkability may be measured by recording
the time required for the disappearance of
powder from the water surface after a portion
of a milk powder has been added to water and
stirred with an impeller under fi xed condi-
tions (Schober and Fitzpatrick, 2005 ).
The conditions used for reconstitution
infl uence the sinkability of a powder. The
creation of a vortex and maintaining it during
reconstitution is crucial for sinkability. The
particle density is also an infl uencing factor
in sinkability in that the heavier the particle/
unit volume the more likely it is to sink.
Thus, low interstitial air content is a prereq-
uisite to good sinkability.
Dispersibility: During the process of dis-
solving powders the agglomerates need to
instantly disintegrate into single particles to
facilitate wettability and dissolution.
The dispersibility of powders is measured
by systematically placing a weighed amount
of powder (typically 10 g) onto the surface of
a set amount of water (250 ml at 25 ° C; 77 ° F),
stirring the solution for a set time in a rota-
tional pattern, sieving the contents, and after
drying, weighing the residue. The dispers-