Analysis of the interface conditions during drying of rice

Abstract

SummaryThe influence of the relative air humidity on the surface conditions of rice grain during drying was investigated. It was found that the equilibrium condition given by the desorption isotherm of the product is not valid over the whole range of relative humidity analysed. While this condition was verified with air of high relative humidity (between 43 and 59% for drying temperatures ranging from 40 to 70°C), the experimental results indicated that the surface moisture approaches the monolayer value given by the Brunauer, Emmett and Teller (BET) desorption isotherm when drying takes place with air of low relative humidity (7 and 14% for the same temperature range). An explanation of these facts is suggested taking into account the high value of the heat of desorption associated with the monolayer moisture content of the product under investigation. Finally, a method based on the Fick's second law solution when the time of drying becomes large enough is proposed to evaluate the surface moisture content of the grain independently of the conditions of the air drying.