Vapor Equilibrium Relationships of Dry Milk

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Abstract
Vapor equilibrium data was 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 the water sorption mechanisms for proteins, a similar mechanism was proposed. Absorption 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. Moisture sorption occurs completely with proteins since the lactose crystallization is completed at higher relative vapor pressures. 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. Nonfat dry milk with high preheat treatment consistently demonstrated the largest number of polar sites for water sorption. Increased percentages of fat in the product reduced the number of water sorption sites since the fat did not provide additional sites.

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