Improving Postharvest Stabilization of Henna Leaves Using a Solar-Assisted Zigzag-Airflow Dryer: Drying Kinetics, Quality Attributes, and Economic Feasibility.

Henna (Lawsonia inermis L.) leaves are an important medicinal and cosmetic plant raw material, but their high moisture content makes them highly susceptible to quality deterioration during postharvest handling. This study evaluated a solar-assisted zigzag-airflow dryer (SAZA) for stabilizing henna leaves under arid-region conditions in Aswan, Egypt. Drying experiments were conducted using two airflow rates (0.09 and 0.14 m3/s), three loading layer thicknesses (2, 4, and 6 cm), and four fixed tray positions. The initial moisture content of henna leaves was 168.1% dry basis (d.b.), which decreased to below 6% d.b. under all tested conditions. Increasing the airflow rate and reducing layer thickness markedly shortened drying time, with the fastest drying achieved at 0.14 m3/s, 2-cm layer thickness, and Tray 1, requiring approximately 10 h. In contrast, 0.09 m3/s, 6-cm layer thickness, and Tray 4 required about 26 h. Apparent effective moisture diffusivity ranged from 1.76 × 10- 8 to 7.12 × 10- 8 m2/s, reflecting the combined effects of airflow rate, layer thickness, tray position, and model assumptions. Among the tested models, the Modified Midilli II model provided the best fit to the experimental drying data. Quality assessment showed that higher airflow and thinner layers improved lawsone retention, reduced water activity, and minimized total color difference. Economic analysis confirmed the feasibility of the dryer, with payback periods ranging from 0.70 to 1.85 years. Overall, the proposed zigzag-airflow solar dryer provides an effective and economically viable approach for postharvest stabilization of henna leaves in arid regions, particularly when balancing drying performance, tray-wise quality retention, and production cost.