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Effect of Temperature on Water Vapor Sorption by Dries Milk Powders

Dairy Products Laboratory, Eastern Utilization Research and Development Division¹, Washington, D.C. 20250

ABSTRACT

The influence of temperature on the equilibrium water vapor sorption values for dried milk powders varied with the relative pressure P/P_o, in the temperature range between 14 and 34 C, with both positive and negative temperature effects on adsorption observed. Thermodynamic treatment of the data indicate an initial high isosteric heat of adsorption, q_ST, for protein-water binding, followed by lower energy values, less than H_L, the heat of liquefaction of water, which are attributable to mobility of sorbed water molecules and solubilization and crystallization of lactose. The lactose crystallization-induced breakpoint in the isotherm occurred at lower P/P_o on elevating the temperature. In materials containing no amorphous lactose, only negative effects of temperature were observed with q_ST > H_L.

¹ Agricultural Research Service, U.S. Department of Agriculture.