A comparative study of direct and indirect solar drying of mango

Abstract

In this work, direct and indirect solar drying parameters of two mango varieties were estimated and compared using direct and indirect solar dryers under the same meteorological conditions. For both drying methods, drying curves were established and fitted using 10 semi-empirical models, drying rate and drying efficiency curves were
determined, effective water diffusivity was estimated and quality of dry slice was evaluated. Results showed that in indirect solar drying, the tray position in the dryer did not have an influence on drying curves whereas in direct solar drying this influence was very significant. Indirect solar drying curves were suitably fitted by Approximation diffusion model (with R2. 0.99, RMSE.0.0387, E<12 % and c2<10-5) while direct drying curves were best fitted by Verma and al. and Approximation diffusion models (with R2.0.99, RMSE<0.0276, E<12 % and c2<10-3). Indirect solar drying
offered highest drying rates and water diffusivities. Its diffusivities increased with the number of drying days between 1.5 x10-10 and 2 x10-10 m2/s whereas those of direct solar drying decreased with the drying days number between 5 x10-11 and 1.85 x10-10 m2/s. With efficiency from 2 to 48 % indirect solar drying was found to be more effective than
direct solar drying with efficiency from 0 to 34 %. Indirect solar drying with an average final water content of 16.6 % (dry basis) and a final water activity of 0.57 was then the most efficient, but also the most expensive. Thus, indirect solar dryer was found to be suitable for industrial or semi industrial mango drying, whereas direct solar dryer was appropriate to a family scale traditional mango drying.