All milk powder has to meet a requirement for residual moisture. For skim milk it is usually 4% and for whole milk usually 2.5%. There may naturally be deviations from country to country.

The moisture content will have an influence on the keeping quality of the powder. High moisture content (high water activity Aw) will thus decrease the keeping quality, as the proteins will denaturate and the lactose, which is found in an amorphous stage, will crystallize causing the free fat to increase in whole milk powders, and oxidation of the fat will be the result. The Maillard reaction, which is a reaction between the NH2 group in the aminoacid lysine, and lactose, becomes more pronounced, and the powder may even become brown and lumpy. The Maillard reaction is directly proportional to the storage time, temperature and residual moisture content. The moisture can be controlled by the outlet temperature of the dryer or by applying more heat to the Vibro-Fluidizer. Moisture absorption should be avoided, and dehumidification of the cooling air is recommended in humid areas.

The packing material should be of such a quality that very little vapour will penetrate the bag or container. As there will always be some vapour diffusion (and the diffusion direction is determined by the water vapour pressure) it is recommended to store the powder in a dry, cool place, where the water vapour pressure will be low.

Residual moisture is determined by a simple drying oven method. The powder is dried at 102-105ºC for three hours. The difference in weight (i.e. weight loss) is determined and the moisture calculated in per cent of the powder weight, see Fig. 103.

Fig. 103  Determination of moisture

Various quick methods for determination of moisture have also been developed. They usually work with a powerful heating lamp, the voltage of which can be adjusted. This type of equipment will never be so accurate as the drying oven method, but is a great help during operation of a plant, as the operator can have quick response from the laboratory enabling him to find suitable drying parameters.

Automatic control of the moisture content is measured with infrared light. The reflection from the sample is direct proportional to the moisture content, and the output is used to control the outlet temperature by regulation of either the feed pump or the heat applied to the heating section of the Vibro-Fluidizer.