Moisture sorption isotherms, isosteric heat, and Gibbs free energy of stevia leaves

There is scientific evidence that Stevia leaves, commonly used as a sugar substitute, can reduce the risk of obesity, diabetes, hypertension, or cardiovascular diseases. In this study, the moisture sorption isotherms of stevia leaves at 35 degrees C, 45 degrees C, and 55 degrees C were analyzed by the static gravimetric method within a relative humidity range between 0.06 and 0.92. The experimental data obtained exhibits a J-shaped Type II sorption isotherms. The data were fitted to six mathematical models: Iglesias & Chirife, Lewicki, Oswin, Hasley, GAB, and Khun. GAB model was the one best-fitted to the isotherms obtained. The monolayer value was between 0.0479 and 0.0589 g water/g dry matter. The isosteric heat of adsorption and Gibbs free energy were obtained at different temperatures using the Clausius-Clapeyron equation. The isosteric heat of adsorption exponentially decreased from 2850 J/g to 2480 J/g as the equilibrium moisture content increased from 0.062 to 0.522 g water/g dry matter. The chemical potential of adsorption increased at high moisture content values. These results suggest an exothermic, spontaneous process. Novelty Impact Statement The importance of studying adsorption isotherms, and its fitting to a mathematical model, allows us to know the potential of the water that influences on the reactions of physical-chemical deterioration correlated with the stevia leaves stability and quality. The thermodynamic properties derived from moisture adsorption isotherms provide relevant information about the water chemical potential and the energy amount required in moisture adsorption processes, and its relationship with stevia stability in different storage conditions.

Automated summary

Scientific evidence supports the potential health benefits of Stevia leaves in reducing the risk of obesity, diabetes, hypertension, and cardiovascular diseases. The study on moisture sorption isotherms of Stevia leaves revealed a J-shaped Type II sorption curve, best fitted by the GAB model. The isosteric heat of adsorption decreased with increasing moisture content, while the chemical potential of adsorption increased at high moisture content levels.