Investigation of water adsorption and thermodynamic properties of stevia powder

Stevia rebaudiana is a natural non-caloric substitute to conventional sugar. Moisture adsorption isotherms of stevia powder, a sweet plant, were investigated at three different temperatures (30, 40 and 50 degrees C) using a gravimetric technique. The sorption isotherms were found to be typical type II sigmoid with the sorption capacity decreasing with increasing temperature. Experimental data were fitted using GAB model, the monolayer moisture content tended to decrease as temperature increased. Moreover, these experimental data curves allow us to calculate the value of the optimal water activity for the conservation and to determine the surface area of powder studied. The isosteric heat of sorption, sorption entropy and spreading pressure were determined as a function of moisture content. The net isosteric heat of adsorption and differential entropy decreased with increasing moisture contents. A plot of differential heat versus entropy satisfied the enthalpy-entropy compensation theory. The spreading pressures increased with increasing water activity but decreased with increasing temperature.