Modeling the thin-layer drying process of Granny Smith apples: Application in an indirect solar dryer

The thin-layer drying kinetics of Granny Smith apples is determined by thermogravimetric analysis of the drying process at constant temperatures ranging from 20 degrees C to 50 degrees C, using intervals of 5 degrees C. The experimental drying curves obtained in the TGA were fitted to the Wang-Singh equation, which was found to describe precisely the drying process occurring at constant drying temperature. A novel model, capable of predicting the evolution of the moisture ratio of Granny Smith apples during the drying process with variable drying temperatures, was proposed. The model was validated with experimental TGA measurements of the drying of apples at variable temperatures, typical of solar drying, obtaining maximum deviations for the drying time less than 1.5%. Once validated, the model proposed was also applied to the drying of Granny Smith apples in an indirect solar dryer. The comparison of the model prediction with the experimental measurements of the drying of apples at variable drying conditions conducted in a lab-scale solar dryer showed a proper agreement, with low deviations (less than 10%) associated to the thermal inertia of the samples. (C) 2016 Elsevier Ltd. All rights reserved.