EQUILIBRIUM MOISTURE CONTENTS OF PURELINE, HYBRID, AND PARBOILED RICE

Equilibrium moisture contents of long- and medium-grain rough rice of both pureline and hybrid cultivars, and a parboiled rough rice of unknown cultivar, were measured in a near-static air environment at five temperatures (10°C, 20°C, 30°C, 45°C, and 60°C) and relative humidities in the range of 10% to 70% using a gravimetric method. Results showed that there were no consistent significant differences between the equilibrium moisture contents of pureline and hybrid or medium- and long-grain rice cultivars. However, the equilibrium moisture content of parboiled rice was significantly less than that of non-parboiled rice for almost all air conditions. Nonlinear regression analysis was used to estimate empirical constants of five models used for describing grain sorption isotherms. The appropriateness of each model in describing the equilibrium data was evaluated using root mean square errors and residual patterns. The modified Chung-Pfost equation best described equilibrium data of non-parboiled samples, followed by the modified GAB, modified Oswin, modified Halsey, and modified Henderson equations. The modified Chung-Pfost and modified GAB equations were equally effective when describing equilibrium data of individual cultivars.