Chemistry Reference
In-Depth Information
IV.4.3.
Stickiness and Caking of Milk Powders
Adhesion of powder particles to dryer chamber walls during processing is
generally referred to as the stickiness behaviour of a product. Excess stickiness
gives rise to product loss, fouling of process surfaces, risk of compromising
product quality because of increased residence of deposits which may be
dislodged and re-entrained with product flow and a risk of spontaneous
combustion within the drying chamber (Ozmen and Langrish, 2002).
Stickiness of powders arises from plasticization of particle surfaces.
Adsorbed water in contact with the surface of powder particles has a plasti-
cizing effect by lowering T
g
and also reducing viscosity. Direct methods for
measuring stickiness are based on the use of physical indices, such as resis-
tance to shear, viscosity and optical properties (Schuck et al., 2006). T
g
can be
correlated indirectly with stickiness. In fact, T
g
measured by DSC is virtually
the same as the sticky point temperature measured using a thermo-mechan-
ical test (Ozmen and Langrish, 2002). Using a blow tester-based method,
Patterson et al. (2005) used the parameter (T-T
g
) to characterize the rate of
stickiness development for a range of conditions (37-678C and 0.15-0.35 a
w
),
and found that at a given T-T
g
value, the level of stickiness increased linearly
with time. Patterson et al. (2007) also showed that stickiness curves for
powders run above and parallel those for the T
g
of amorphous lactose, and
succeeded in establishing a critical point (T-T
g
)
crit
to characterize the initia-
tion of powder stickiness.
Caking follows when inter-particle liquid bridges provide an environ-
ment for dissolution of milk components and the resulting lactose crystal-
lization transforms these interfaces into solid bridges (Thomas et al., 2004).
Sticking is typically a problem encountered during drying, while caking is
more prevalent during the subsequent storage of powder (Schuck et al., 2005).
Stickiness and caking of whole and skim milk powders are different due to the
significant difference in their surface compositions ( Ozkan et al., 2002). The
high-fat surface coverage and its melting behaviour was associated with
stickiness occurring at a lower temperature in whole milk powders, while
the higher temperature at which cakiness occurred in skim milk powder was
influenced by T
>
T
g
so that the rubber phase transition to the crystalline
phase facilitated the formation of strong junctions between skim milk powder
particles ( Ozkan et al., 2002).
A close relationship appears to exist between the measured sticky point
temperature of skim milk powder and that of lactose (Boonyai et al., 2004).
However, the approaches used to correlate the sticky point and T
g
are not
very precise. First, the sticky point temperature may be taken at 208C above
T
g
for general and simple estimations, but this may not be sufficient when
spray drying some products at a very high temperature where precise
