Moisture desorption isotherms of rough rice at high temperatures

Abstract

Moisture desorption isotherms of medium-grain rough rice ( Lido cultivar) at temperatures from 40 to 80 °C and water activity from 0.14 to 0.90 were determined using the static gravimetric method. The moisture isotherms were sigmoid shaped and showed a clear effect of temperature. The most commonly used models, modified Henderson, modified Halsey, modified Chung–Pfost, modified Oswin and modified GAB, were used to fit experimental values using a non-linear regression analysis. The models were compared using the standard error of estimate, relative percent error, coefficient of correlation and residuals plot. Results probed that the modified GAB model was more appropriated to predict desorption equilibrium moisture content of rough rice for the range of temperatures and water activities studied. Modified Chung–Pfost model and modified Henderson model gave an acceptable fitting, while modified Halsey model and modified Oswin model did not seem appropriate for the description of the desorption moisture isotherms of rough rice at drying temperatures. Monolayer moisture content of rough rice for the range of temperatures and moisture contents studied ranged from 2.905% to 4.870% (d.b.). Isosteric heat of desorption ranged from 2.510 to 18.387 kJ/mol for moisture contents ranging from 0.04% to 24% (d.b.), and an expression for predicting this thermal property was developed.