Instant Milk Powders
During the last few decades there has been a
growing market for powders which are instantly soluble in cold water.
Ordinary non-agglomerated powders tend to lump when mixed with water,
and if strong mechanical stirring is not applied, it may result in an
inhomogeneous mixture which is not attractive to the consumer.
It has
for a long time been known that an agglomerated powder, i.e. a powder where the
single particles have formed bigger granulates or agglomerates, see Fig. 115,
possesses completely different properties when mixed with water than it is the
case with ordinary powder.
The plant is operated, so that the powder leaves the primary
drying stage with 2-10% higher moisture than wanted in the final product. The
cyclone fraction is returned to the atomizing device, where the dry fine
particles will collide with the primary particles thus forming agglomerates.
See Fig. 116.
The powder leaving the chamber is therefore
warm, moist and consists of stable agglomerates. Consequently by gentle
after-drying performed in integrated fluid beds and/or a Vibro-Fluidizer the
agglomerated product structures are maintained. The cooling should always be
done in a fluid bed.
The powder obtained by this process can be
characterized by:
- agglomerated product structure
- non-dusty
- lower bulk density than for powder from the pneumatic plant
- good flowability
The decreased drying air outlet temperature and consequently lower product
temperature will result in:
- improved solubility because of less thermal damage
- low content of occluded air, because in the critical stage of the drying,
with a water content of 30-10%, the blowing-up of the particles is avoided.
If we limit the above process to skim milk powder we will, by the mere
agglomeration, obtain a product with:
Besides the manufacturing equipment it is necessary to have control of
the quality of the final powder, especially regarding the instant properties.
It is not astonishing that the first method which was introduced to
distinguish instant products was based on determining the property called
wettability, as this is the most conspicuous feature of an instant product
compared with an ordinary product. It was measured as the time necessary for
wetting of a given amount of powder, i.e. the time from the first contact with
water until the powder had completely passed the water surface.
It has
later been realized that wetting is only a first step of a rather complicated
reconstitution process, and that this process consists of a number of phenomena
which can be described as follows:
| Reconstitution phenomenon: |
Milk Powder Property: |
| Wetting |
Wettability |
| Dispersing |
Dispersibility |
| Dissolving |
Solubility |
Splitting the reconstitution
process into the above steps helps to understand the process, for instance to
be able to find the reasons for the lack of instant property. On the other
hand, one must be aware that there is no sharp borderline between these
individual reconstitution steps, and it is therefore impossible to determine
the individual properties independently of the others. For a good evaluation of
product properties all these qualifications have to be considered.