When producing whey powder, especially the non-caking type, the following analyses, and some of those mentioned under Analyses for Milk Powder, are appropriate:

• Total Moisture and Free Moisture
• Hygroscopicity and Caking Properties

Total Moisture and Free Moisture

During the manufacture of non-caking whey powder the lactose is crystallized during which process the lactose picks up one molecule of water. In order to determine the degree of crystallization the water bound to the crystals (crystal water) is measured as the difference between the total moisture and the free moisture content.

The total moisture is determined by the Karl Fisher titration, the principle of which is a reaction between iodine and sulphur dioxide in the presence of water. The free moisture content is determined in a drying oven at 86ºC in 6 hours.

% crystal water = % total moisture - % free moisture (23)

The degree of crystallization is calculated as follows:

%crystallization = %crystal water x 19 x 100 / % Lactose (24)

For rapid routine tests of sweet whey the figure of 74% lactose may be used.

 

Hygroscopicity and Caking Properties

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%.