Water desorption isotherm and its thermodynamic analysis of glutinous rice flour

Abstract

Background: Water sorption isotherm and thermodynamic properties are necessary for food process design. Methodology: Moisture desorption isotherms of glutinous rice flour were determined using a static gravimetric method at 10, 20 and 30°C and for water activity ranging from 0.06-0.98. Results: Out of the four chosen isotherm models (Chung-Pfrost, Oswin, Dent and Lewicki), the Lewicki model was found to best describe the experimental sorption data. The best-fit model was further used to evaluate the thermodynamic functions of glutinous rice flour on the base of Clausius-Clapeyron equation and Gibbs-Helmholtz relationship. Differential enthalpy and entropy and integral enthalpy for water desorption all were positive in value and decreased with increasing equilibrium moisture content. Integral entropy with reference to pure liquid water was negative in value and increased with increasing equilibrium moisture content. All the four thermodynamic functions were adequately characterized by an exponential model. Spreading pressure decreased with increase in temperature at constant water activity and increased with increase in moisture content at constant temperature. Conclusion: The enthalpy-entropy compensation theory was proved to be valid for desorption of glutinous rice flour. Through the entropy-enthalpy theory, it was founded that the water desorption of glutinous rice flour was non-spontaneous and enthalpy-controlled.