Chemistry Reference
In-Depth Information
agglomeration now takes place in the primary drying chamber (straight-
through drying/agglomeration process), it is likely that most agglomerating
particles in this zone of the dryer are wet and have a surface composition more
typical of the matrix before migration occurs during the final stages of
particle dehydration.
Later, surface properties will also prevail during reconstitution along
with other factors, such as the interplay between liquid (water), gaseous (air
between particles) and solid phases (powder particles) during powder wetting.
Efficient wettability and dispersibility are essential in order to prevent a
viscous layer forming at the interface around grouped particles which delays
dissolution of the powder in water.
Powder agglomeration may be regarded as intentional caking as a result
of forced compaction under controlled conditions - particles being forced to
interact and form granules (Listiohadi et al., 2005). Some insights into the
effects of force during the recycling of fines when spray drying are gleaned
from the descriptions of P´seck ´ (1997). When the fines return is positioned
close to the spray atomizer, considerable penetration of wet primary particles
occurs, which in turn, become covered by concentrate from the incoming
spray. These newly forming agglomerates possess high moisture, plasticity
and stickiness. On the other hand, if the fines return is positioned at a distance
from the atomizer, less compact agglomerates, displaying ''raspberry- like''
and ''grape-like'' microstructures, are formed. The objective is to strive as far
as possible to achieve a ''compact grape'' structure as an optimal process
condition where the powder has simultaneously good instant properties and
sufficient mechanical strength to withstand the rigours of subsequent hand-
ling and packaging (Table IV.2). ''Onion-structured'' agglomerates have also
been described. These have high mechanical strength, bulk density and
appear as slowly dispersible particles on reconstitution.
Two distinct processes for agglomerating milk powders are used in
practice (Wulff, 1980), the straight-through method (accomplished during
spray drying) and the rewet method (using powder that has already been
prepared). Both processes exploit similar principles of instantization - (i)
wetting of particle surfaces (by steam, water or a mixture in the case of the
rewet method), (ii) agglomerating, (iii) drying or redrying (rewet), (iv) cooling
and (v) classifying according to particle size in order to remove particles that
are too large or too small.
IV.4.5.
Maillard Reactions
Non-enzymatic browning of milk powders during prolonged storage
has been associated with reactions taking place between proteins and lactose
functioning as a reducing sugar (Thomas et al., 2004; see Chapter 7). Early
