WATER SORPTION AND TIME-DEPENDENT PHENOMENA OF MILK POWDERS

Water sorption properties of dehydrated milk products with various fat contents, the effect of lactose hydrolysis on water sorption, and the effect of milk fat on lactose crystallization were investigated. Several models of isotherms were evaluated for their ability to predict water adsorption. Crystallization of amorphous lactose resulted in loss of adsorbed water. Crystallization of pure lactose occurred at lower relative humidity (> 40%) than did crystallization of lactose in skim milk powder (>50%) after 1 d at 24-degrees-C. Milk fat decreased the rate of crystallization, but the amount of sorbed water after complete crystallization became constant and was independent of fat content. Lactose crystallization in milk powders was accounted for by water plasticization and depression of the glass transition temperature to below ambient temperature. Crystallization did not occur in skim milk powder with hydrolyzed lactose. The Guggenheim-Anderson-de Boer model was considered to be the most applicable for prediction of water adsorption. Time-dependent changes determine storage stability of dehydrated milk products and should be considered in modeling water sorption.