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W. Bushuk and I. Hlynka. Copyright 1960 by the American Association of Cereal Chemists, Inc. The toluene-displacement pycnometric method was applied to measure the change in volume of whole wheat with gain or loss of moisture after equilibration at various relative pressures of water vapor. The corresponding weight changes were determined also. A plot of the volume changes against the equilibrium relative pressure gave a volume isotherm with hysteresis between the sorption and the desorption branches similar to that in the weight isotherm. A definite hysteresis also exists between the sorption and the desorption branches of the plots of change in volume against change in weight. The sorption branch seems to be curvilinear. At low moistures, the weight changes faster than the volume; at intermediate moisture the change in volume varies directly with the change in weight; and, at moistures above about 15% (dry basis) the volume changes faster than the weight. There seems to be a net decrease in total volume for low sorption and a net increase for high sorptions. For the desorption branch the change in volume varies directly with the change in weight. The dry grain shows a net increase in volume after a complete sorption-desorption cycle in the relative pressure range 0 to 0.94. Wheat density changes approximately linearly with moisture for both branches of the isotherm. The densities during desorption are lower than the values during sorption at equivalent relative pressures. Wheat which was put through a sorption-desorption cycle in the moisture range 0 to 26.4% showed a net decrease in density of 0.014 g. per ml. |
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