The hygroscopicity, i.e. the ability to absorb moisture, of the whey
powders is determined by the degree of crystallization of the lactose. But also
the salts and even the proteins absorb water, however, only in a limited amount
compared with the lactose. The hygroscopicity of a given powder is determined
by passing air with a given humidity (80% RH at 20ºC) through a given amount of
powder (see Fig. 112). When equilibrium has been obtained the absorbed moisture
is calculated by weighing the powder sample before and after the absorption.
Fig. 112 Determination of
hygroscopicity
% Hygroscopicity =
(% weight increase + % free water) x 100 / 100 + weight
increase (25)
in non-hygroscopic powder this value should be <10%.
The caking properties of whey powder are even more important, as they are a
measure for the tendency to form hard lumps in bags during storage.
There is no direct relation between the hygroscopicity and cakeness, as a
rich crystallization in itself is not enough to prevent caking. Thus it is of
importance that the crystals are small, and that they are evenly distributed in
all the powder particles. Due to the moisture absorption in non-crystallized or
badly crystallized powder, the crystallization of any remaining amorphous
lactose will start mainly on the surface of already existing crystals. The rate
is thus depending on the distance between the individual crystals. After
termination of the crystallization the vapour pressure of the powder moisture
becomes higher than that of the surrounding air, so that the moisture
absorption is followed by a moisture desorption. The sticky powder formed
during the moisture increase will during the crystallization become cemented in
hard lumps.
The method for determination of cakeness is carried out as follows:
The powder sample from the hygroscopicity test is dried at a temperature of
102ºC for 1 hour, after which it is cooled in a desiccator. The powder is then
sifted through a 500 micron sieve in a shaking apparatus for 5 min. Any powder
which has not passed through the sieve is weighed.
Degree of caking in % = g powder left on the sieve x 100 / g powder
used (26)
Non-caking whey powders should have a degree of caking of <10%.