The sorption isosteric heats of rice grains in China

Equilibrium moisture content (EMC) data for the grains of thirteen rice cultivars were obtained by gravimetric method at 11% to 96% equilibrium relative humidity (ERH) and 10, 20, 25, 30, and 35°C above nine saturated salt solutions. Six commonly used mathematic models, namely, BET, Modified- Guggenheim–Anderson-deBoer (MGAB), Modified-Chung-Pfost (MCPE), Modified-Henderson (MHE), Modified-Oswin (MOE), and Strohman-Yoerger (STYE), were fitted to the data with evaluating the coefficient of determination, residue sum-of-squares, standard error of estimate, mean relative percent error, and residual plots. The best fitted equations to the EMC/ERH data were MCPE and STYE, but MCPE is three-parameter, readily transformed equation and adopted in this study. The isosteric heats for both rice desorption and adsorption, and for both the sorption of Japonica and Indica rice, decreased rapidly with an increase in seed moisture content until the moisture content of 20% d.b. (dry basis) was reached, and thereafter they decreased smoothly with increasing moisture content. The isosteric heats of rice desorption were higher than those of adsorption below 22.5% d.b. EMC, but thereafter there was no difference found between desorption and adsorption. The sorption isosteric heats of Indica rice were slightly higher than those of Japonica rice under all moisture contents at a constant temperature.