The moisture relations of spray dried skimmed milk

Abstract

Water sorption was studied for both spray dried skimmed milk and lactose. The high lactose content of dried milk was found to influence the moisture relations and similar breaks were found in the adsorption curves for both, at about 50% relative humidity. Below this level, the lactose was in the form of an amorphous glass, but between 42 and 52% relative humidity, equivalent to between 7 and 9.5% moisture content in dried milk, the glass crystallised and released water, which produced a break in the curve. Above this level, the non-hygroscopic lactose crystals did not absorb water, but the other constituents of dried milk did, and its moisture content increased with relative humidity. When desorption of dried milk was begun from 7% moisture content, and the lactose was in the form of a glass, the hysteresis was conventional, but when desorption was begun from a high moisture content with the powder containing only crystals, it closely followed the adsorption down to the break, but then continued as a smooth line, and caused an apparent reverse hysteresis. The usual convention for setting the safe storage moisture content as that in equilibrium with 70% relative humidity, does not apply to dried milk. It was recommended that dried milk be kept at or below 6% moisture content, 40% relative humidity, to prevent lactose crystallisation occurring and lowering the quality of the product. Changes in lactose from pure solution, and lactose in dried milk were examined microscopically and by a scanning electron microscope. Small lactose crystals were found in freshly produced spray dried skimmed milk when the lactose was predominantly present in the form of a glass. A different form of crystal was observed in samples that had a high moisture content, and in old samples.