Glass Transitions and Crystallization in Milk Powders

Abstract

Glass transition temperatures were determined for dehydrated milk products with various water contents and water activities, and state diagrams were established. Crystallization behavior was studied with milk powders stored at various relative humidities. Glass transition temperatures of milk powders containing lactose were close to those of pure lactose. Skim milk powder with hydrolyzed lactose had considerably lower glass transition temperatures. Glass transition temperatures decreased as water content increased. The critical water content and water activity for stability at 24°C were predicted using data on glass transition temperature and water sorption. Lactose and milk powders containing lactose had the same critical water activity or storage relative humidity, but slightly different critical water contents. The critical values for skim milk with hydrolyzed lactose were much lower than those for regular milk powders. Loss of adsorbed water, indicating lactose crystallization, was observed in milk powders stored above the critical relative humidity. Glass transition temperatures and state diagrams can be used to evaluate changes during processing and storage of dairy foods.