Another important property of instant powders is the ability to disperse
in water by gentle stirring. This means that the powder should disintegrate
into agglomerates which again should disintegrate into the single primary
particles.
To obtain a good dispersibility of a powder it is necessary that the powder
is wettable and that the agglomeration is optimal, i.e. no fine particles
should be present.
The analytical method is very difficult to define and
perform and the reproducibility is very poor. There are numerous methods, and
the results cannot be compared.
Being aware of that, IDF has developed a new
dispersibility test. This test is based on determining the capability of a
powder (25 g of skim or 34 g of whole milk powder) being poured on a surface of
water (250 g, 25ºC) to disintegrate into particles capable of passing through a
150-micron sieve when applying the prescribed manual stirring for 20 sec. The
amount of powder passing the sieve and being dissolved or dispersed is found by
the determination of total solids of the filtrate and expressed in percentage
as dispersibility. See Fig. 111.
Fig. 111 Determination of
dispersibility
The powder is considered instant by IDF, if the dispersibility is at least
85% (whole milk) or 90% (skim milk). However, plants with new drying technology
easily produce powders with a dispersibility of 95%.
There is no doubt that this test presents a more reliable basis for
assessment of instant milk powders than the wettability test. On the other
hand, it is a test requiring relatively high expenditure of work, so it can
hardly be used as a routine test. Besides, even when using skilled workers the
reproducibility is rather poor.
A more simple method is to pour 10 g of skim milk powder or 13 g of whole
milk powder into 100 ml of water at room temperature and then manually stir
with a teaspoon until the powder is dispersed leaving no lumps on the bottom of
the glass. The time used is measured by means of a stop watch.
After some training the reproducibility is fairly good, and the method is
quick. Furthermore, it has the supreme advantage that it is just what the
housewife does when she wants to prepare a glass of milk.