EFFECT OF INITIAL MOISTURE ON THE ADSORPTION AND DESORPTION EQUILIBRIUM MOISTURE CONTENTS OF POLISHED RICE

The moisture adsorption and desorption properties for polished rice have been measured using a dynamic ventilatory method. Air temperatures of 10,20,30 and 40 degrees C, relative humidities of 50,60,70,80 and 90%, and five levels of initial moisture contents ranging approximately from 8% to 19% d.b. were used to obtain moisture content data. The value of equilibrium moisture content for each initial moisture content at the range of air condition was determined by a method of nonlinear least squares. Results revealed that the equilibrium moisture content depends on the initial moisture content at each temperature/r.h. condition. Observed data were fitted to the moisture transfer equation. The sorption was in the first falling rate period and the sorption coefficient (k) followed the Arrhenius type temperature dependency. An attempt was made to relate the behavior of equilibrium moisture content with initial moisture content by introducing two basic parameters, the percentage deviation of equilibrium moisture content (DM(e)) and the percentage deviation of initial moisture (DM(o)). Equilibrium moisture content variation due to initial moisture may be supposed as an effect of hysteresis. But the effect has to be studied further to confirm it. Increasing relative humidity tended to decrease equilibrium moisture content deviation due to initial moisture content.