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.

Spray dryer with vibro fluidizer as agglomerator
Fig. 115  Spray dryer with Vibro-Fluidizer as agglomerator/instantizer, (SDI)

The agglomeration is in detail discussed on page 153.

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.

Agglomeration by fines return - Forced secondary 
agglomeration
Fig. 116  Agglomeration by fines return (Forced secondary agglomeration)

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:

  • good instant properties

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.