This study aimed at investigating desorption isotherms, thermodynamic properties, drying kinetics, energy parameters, and specific energy in convective solar drying of fresh stevia leaves. Moisture desorption isotherms of stevia were determined at 30, 40 and 50°C. Thermodynamic properties such as differential enthalpy and entropy of sorption were calculated, these properties decreased with increasing moisture content. The experimental data showed that enthalpy-entropy compensation theory was applicable to the moisture sorption behavior of stevia. An indirect forced convection solar dryer was used in the thin-layer drying of stevia at air drying temperatures of 50, 60, 70 and 80 °C and air flows drying of 300 and 150 m3/ h. Increasing temperature and air flow resulted to a decrease of the total time of drying and increasing of drying rates. The drying data are used to determine the characteristic curve drying and then were fitted to six different mathematical models.

Nadia Hidar, Agro-Food Engineer, titular of a Diploma of University Studies in Biotechnology of Plants of Agricultural Interest and a Diploma of State Engineer in Industry and Food Safety at the Faculty of Sciences and Technology of Marrakech. Currently, she is a doctoral student in Food Biotechnologies in Cadi Ayyad University, Morocco.