Impact of Thermal and Solar Drying on the Bioactive Profiles, Color Parameters, and Antioxidant Capacity of calyx leaves (Hibiscus sabdariffa L.)

• Ban Hussein Ali jasem

  Author

     The study was conducted in January 2026 in the laboratories of the College of Education at Diyala University (Diyala/Baghdad, Iraq). Drying is a crucial post-harvest stage for hibiscus (Hibiscus sabdariffa L.) leaves, directly affecting the stability of their physicochemical and biologically active properties. This study investigated the comparative effects of conventional drying under direct sunlight and forced thermal drying via a temperature gradient (30°C, 40°C, and 50°C) on the chemical composition, CIELAB color parameters, and antioxidant performance of hibiscus cultivated in Diyala, Iraq. The experimental matrix evaluated moisture absorption, ash, crude fat, protein, and fiber dynamics, along with total phenolic content (TPC) and free radical scavenging capacity using DPPH and ABTS assays. The experimental results showed statistically significant differences (p ≤ 0.05) between treatments. Convection drying at moderate temperatures (30–40°C), combined with drying under direct sunlight, optimally preserved the structural integrity of dietary fibers and nutrients, while achieving superior illumination (c*) and balanced color coordinates. In contrast, raising the heat treatment temperature to 50°C resulted in severe thermal degradation of heat-sensitive polyphenols, leading to a significant decrease in antioxidant activity and undesirable surface browning due to non-enzymatic Maillard reactions and phenol oxidation. Pearson correlation analysis revealed a strong positive correlation between total phenolic content (TPC) and DPPH radical inhibition (r = 0.812), compared to a moderate correlation for the ABTS matrix (r = 0.941), confirming the essential role of mono-polyphenols in promoting plant bioactivity. These results underscore the need for precise thermodynamic regulation during post-harvest processing and recommend moderate temperatures (30–40°C) as the optimal technical basis for preserving the nutritional, aesthetic, and therapeutic value of hibiscus calyx in functional food and biopharmaceutical applications. Keywords: Antioxidant capacity; Polyphenols; Hibiscus (Hibiscus sabdariffa); Thermal drying; Color properties.

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