AN INTERPRETATION OF THE MOISTURE DESORPTION ISOTHERM OF WHEAT

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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[image] 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[long dash]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 m8 = 0.1157 K/(1+K) is therefore given for predicting apparent surface moisture contents in the vacuum drying of wheat, where K = 3.05 X 10-4 exp(6270/RT).

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