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. Seve ral models of isotherms were evaluated for their ability to predict wa ter adsorption. Crystallization of amorphous lactose resulted in loss of adsorbed water. Crystallization of pure lactose occurred at lower r elative humidity (> 40%) than did crystallization of lactose in skim m ilk powder (>50%) after 1 d at 24-degrees-C. Milk fat decreased the ra te of crystallization, but the amount of sorbed water after complete c rystallization became constant and was independent of fat content. Lac tose crystallization in milk powders was accounted for by water plasti cization and depression of the glass transition temperature to below a mbient temperature. Crystallization did not occur in skim milk powder with hydrolyzed lactose. The Guggenheim-Anderson-de Boer model was con sidered 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.