Modelling of water sorption isotherms of macaroni stored in a chamber under controlled humidity and thermodynamic approach

The moisture sorption isotherm data of macaroni stored in a chamber the relative humidity of which is regulated by an atomizing humidifier were determined at 25, 35 and 45 °C and relative humidity ranging from 10% to 90%. The sorption isotherms were sigmoidal in shape (Type II). At a given water activity, moisture content decreased with increasing temperature. Hysteresis between adsorption and desorption isotherms was shown over almost the range of 0.1-0.9 water activity. The experimental sorption data were applied to various isotherm equations (GAB, BET, Henderson, Iglesias and Chirife, Oswin, Peleg, Smith and Caurie). Several statistical tests such as r2, P%, average residual and RMSE were adopted to evaluate the fitting performance of the models. The Peleg equation was best for characterizing the sorption behaviour of macaroni for the whole range of temperatures and relative humidities studied. The surface area of monolayer and the water activities corresponding to the monolayer values were determined. The thermodynamic functions determined in this study included isosteric heat of sorption, sorption entropy, spreading pressure, net integral enthalpy and entropy. The Clausius-Clapeyron equation facilitated satisfactory determination of the isosteric heats of sorption, which were found to increase with decreasing moisture content. The spreading pressures increased with increasing water activity. The net integral enthalpy decreased with moisture content. Net integral entropy was negative in value and increased with increase in moisture content.