Dynamic vapor sorption isotherms of medium grain rice varieties

It is known that the two popular medium rice varieties, namely M202 and M206, in California have different fissuring resistances. Therefore, the main goal of this study was to investigate the sorption behavior of these two varieties by a new approach using dynamic vapor sorption (DVS) method for elucidating the differences in fissuring resistance. The moisture sorption isotherms of rough, brown and white rice and husk were determined at 25 °C over a water activity range of 0-0.98. Although it was found that the sorption isotherms of different forms of M202 and M206 were similar, M202 absorbed and desorbed moisture faster than M206 which might make it more susceptible to fissuring. All obtained moisture sorption isotherms exhibited the sigmoid (Type II) shape and hysteresis was observed for all forms of rice. Absorption curves obtained using DVS method were compared with the traditional saturated salt solution (SSS) method. For all forms of rice, the maximum difference for equilibrium moisture content value between two methods was observed at 0 and 98% relative humidities, which was approximately 8-11% and 7-9%. Although at a single relative humidity point equilibrium with DVS was attained much faster than SSS method, obtaining a full isotherm with more than 10 data points might make SSS method more feasible for low diffusion materials. Among the select equations to describe sorption behavior of different forms of rice, Peleg equation gave the best fit for all forms of rice.

► M202 and M206 have similar sorption isotherms due to similar chemical composition. ► The Peleg equation adequately described the sorption isotherms of M202 and M206. ► Results of the dynamic method were in close agreement with the traditional method.