Solar drying of Stevia (Rebaudiana Bertoni) leaves using direct and indirect technologies

Abstract

The Stevia plant has acquired great importance and has increased its consumption in the world, because it is a great natural sweetener, up to 300 times than sugar. Its higher sweetening content occurs mainly in dry leaves, being important to analyze the drying process. In a previous study they were not found works related to solar drying of the Stevia. In this work, experimental dehydration of the Stevia leaves using direct (cabinet type) operating at natural and forced convection and indirect (air heated by a solar water heating system) as solar drying technologies is presented. The experimental results demonstrated the technical feasibility for the solar drying of Stevia leaves. Indirect solar drying was observed to have superior conditions, moderate drying times, better control of the operating conditions and greater protection against the effects of temperature compared with direct exposure to solar radiation, drying kinetics were highly similar between samples, and equilibrium was reached between 240 min and 270 min. Colorimetric analyses indicate that the effect of temperature is the most significant parameter on tone degradation. Integrating the shadow-mesh cover in the direct solar dryer improved the final quality of the leaves, reduced the discoloration effect, with the mesh-shadow cover, natural convection yielded a drying time of 360 min and forced convection had a drying time between 550 min and 600 min. Without mesh, these values were 250 min and 300 min, respectively. The Weibull and Two-term exponential models for direct solar drying and Weibull model for indirect solar drying were the best fit to the experimental results. The analyzed solar drying technologies will allow to obtain important savings of conventional energy with a smaller environmental impact and a better quality of the final product in relation to the traditional methods of drying.