Influence of semi-fermentation and drying on the phytochemical properties, antioxidant activity, and volatile compounds of Gymnema inodorum (Lour.) Decne leaves

Gymnema inodorum (Lour.) Decne (GI), an indigenous plant of northern Thailand, is rich in gymnemic acids with glucose-regulating and antioxidant potential. However, the strong grassy odor and bitterness of fresh leaves limit consumer acceptance. Fermentation and drying are promising processing techniques that can improve sensory characteristics while preserving bioactive compounds. This study evaluated how semi-fermentation and drying conditions affect the phytochemical content, antioxidant activities, and volatile compounds of GI leaves. Controlled semi-fermentation followed by drying at 80 °C significantly improved the retention of total phenolics (43.00 ± 0.10 mg GAE/g DB), flavonoids (38.40 ± 0.29 mg CE/g DB), which were approximately 6% and 34% higher levels, respectively, compared with direct drying at 60 °C (DD60). Gymnemic acid (118.12 ± 0.03 mg/g DB), antioxidant activity (46.92 ± 0.09 mg TE/g DB) and exhibited the lowest IC50 values for antioxidant activity (2.12 ± 0.01 mg/mL for DPPH assay and 1.96 ± 0.02 mg/mL for ABTS assay). Eight phenolic compounds were identified, with rutin and sinapic acid being the most abundant. Volatile analysis indicated a reduction in grassy odor and an increase in pleasant aromatic compounds such as methyl eugenol and terpinolene. Although higher extraction concentrations and temperatures enhanced bioactive yield, they simultaneously increased bitterness and reduced consumer acceptance. These findings highlight the potential of optimized semi-fermentation and drying processes to enhance both the functional and aromatic qualities of GI leaves for application in functional food development.