Sorption isotherms of some legumes stored in a chamber the relative humidity of which is controlled by atomiser humidifier and thermodynamic approach

The moisture sorption properties of legumes such as bean, chickpea, green lentil, cowpea and fava bean stored in a chamber the relative humidity of which is regulated by an atomising humidifier were determined at 25, 35 and 45 oC and within the range of 0.1-0.9 water activity. The sorption isotherms of legumes exhibited J-shaped type III behavior. In sorption of all legumes, it was observed that the values of equilibrium moisture content decreased with an increase in temperature. The sorption isotherms of legumes exhibited the hysteresis effect. The experimental sorption data were applied to several isotherm equations such as GAB, BET, Henderson, Oswin, Peleg, Smith, Caurie, Halsey, Ferro-Fontan, Khun, Chung-Pfost and White-Eiring models. Nonlinear regression analysis was used for the determination of equation parameters. Statistical tests such as r2, E, average residual and RMSE were applied to evaluate the fitting performance of the models. The most suitable model describing the relationship between equilibrium moisture content and water activity was found. The thermodynamic functions such as sorption isosteric heat, sorption entropy, spreading pressure, net integral enthalpy and entropy were determined by using sorption isotherm data. The sorption isosteric heats were determined by the application of the Clausius-Clapeyron equation. The sorption isosteric heat and sorption entropy decreased with increasing moisture content. The enthalpy-entropy compensation theory showed that the moisture sorption of legumes was enthalpy-controlled. The net integral enthalpy decreased with increase in moisture content. The net integral entropy was negative in value and increased with an increase in moisture content.