Water sorption behaviour of by-products from the rice industry

• Rice flour made from wastes was less hygroscopic than isolated starch.
• The hysteresis cycles for all systems were Type H3 following IUPAC classification.
• GAB and Chung-Pfost models nicely represents both sorption processes for tested samples.
• Net isosteric adsorption and desorption heats decrease exponentially with moisture content

The adsorption and desorption isotherms of rice flour made from by-products from the rice industry and isolated starch were determined at different temperatures (25, 35, 45 and 55 °C) using a static gravimetric method. Several saturated salt solutions were selected to generate different water activities over a range of 0.09–0.91. The obtained isotherms for all systems were of Type II following the BET classification. The observed hysteresis cycles were of Type H3 for all tested systems according IUPAC classification. GAB (R2> 0.993, ERMS < 3.9%) and Chung-Pfost (R2 > 0.992, ERMS < 3.9%) models nicely fitted all experimental isotherms. The net isosteric sorption heat, calculated by means of Clausius–Clapeyron equation, decreased when moisture content increased. The maximum values of net isosteric sorption heat (kilojoules per mole) were approximately 51 and 58 for rice flour and 34 and 42 for rice starch, for adsorption and desorption processes, respectively, in the range of temperature from 25 °C to 55 °C.