AN INTERPRETATION OF THE MOISTURE DESORPTION ISOTHERM OF WHEAT

Abstract

It is shown that the moisture desorption isotherm of wheat can be interpreted in harmony with Cassie's theoretical assumptions regarding adsorption on localized sites by the equation[Formula: see text]where N is the number of adsorbed molecules per gram of wheat and S is the number of localized sites. Kf/(1 + Kf) is a Langmuir-type factor for adsorption of the low energy fraction of molecules, and K′fn is a factor for adsorption of the normally condensed fraction of molecules. In agreement with theory K′,n,and S are independent of temperature. The dependence of K on temperature gives a value of −6.27 kcal/mole for the partial molal enthalpy of dilution of the low energy fraction of molecules, which is in the order of magnitude of a hydrogen bond. The analysis also illuminates the practice of the drying of wheat. An exact correspondence appears to exist between the calculated values of S and the apparent surface (or equilibrium) moistures observed in studying the diffusion of moisture out of wheat. The theoretical relation[Formula: see text]is therefore given for predicting apparent surface moisture contents in the vacuum drying of wheat, where[Formula: see text]