Vapor Equilibrium Relationships of Dry Milk

Abstract

Vapor equilibrium data have been used to predict and explain water-binding properties of spray-dried milk. Equilibrium moisture contents for nonfat products with low, medium, and high preheat treatment and low-heat product with 1, 13, and 26% of milk fat were obtained at storage temperatures of 35, 60, 86, and 100 F. Based on proposed water sorption mechanisms for proteins, a similar mechanism has been proposed for dry milk. Adsorption isotherms indicate a combination of sorption by lactose in the glass state and sorption on polar sites in proteins at relative vapor pressures less than 0.4. At higher relative vapor pressures, moisture sorption is by proteins, since the lactose crystallization is completed. Application of the B-E-T multimolecular adsorption theory provided information on the relative number of moisture sorption sites. Results indicated that lactose in spray-dried milk provides approximately 25% of the moisture sorption sites. An influence of preheat treatment on the relative number of polar sites in the protein was revealed. Since dry milk with high preheat treatment consistently demonstrated the largest number of polar sites for water sorption, results tend to confirm and explain the water-binding properties of high-heat nonfat dry milk. The presence of increased percentages of fat in the product reduced the number of water sorption sites, since the fat did not provide additional sites. © 1965, American Dairy Science Association. All rights reserved.