Drying Kinetics and Quality Analysis of Coriander Leaves Dried in an Indirect, Stand-Alone Solar Dryer

In this study, coriander leaves were subjected to three different drying techniques; direct sun, shade, and using an indirect solar dryer. In the developed dryer, hot air obtained from a black-body solar collector was pushed by a blower powered by a solar panel, and sent to the drying chamber with multiple trays for thin-layer drying. Experiments were conducted for summer and winter seasons, and temperature and relative humidity variations in the drying chamber were measured using a data acquisition system. Indirect solar dryer performance was evaluated and compared with sun drying and shade drying for drying kinetics, moisture diffusivity, and product quality. The drying rate curves show a linear falling rate throughout the drying process. The drying kinetic models are well-fitted with the Midilli and Kucuk thin-layer drying model. The effective moisture diffusivity of the dried coriander shows a decreasing trend, sun drying (2.63 × 10 ⁻¹⁰ m ²/s and 1.05 × 10 ⁻¹⁰ m ²/s) followed by solar dryer (1.31 × 10 ⁻¹⁰ m ²/s and 6.57 × 10 ⁻¹⁰ m ²/s), and shade drying (6.57 × 10 ⁻¹¹ m ²/s and 3.94 × 10 ⁻¹¹ m ²/s) for winter and summer seasons, respectively. Green color changes from −7.22 to −0.056, −7.22 to 3.15, and −7.22 to −0.35 in indirect solar, direct sun, and shade drying, respectively. The hue angle and Chroma are reduced by 12% and 32% in indirect solar drying, respectively. The total phenol content (TPC) value increases with drying, with summer showing the highest values (365 to 852 mg caffeic acid/100 g dry weight) while the antioxidant capacity reaches 3.41 and 3.53 in winter and summer, respectively from 0.22 μmol Trolox/g dry matter of fresh leaves.