Drying-Induced Chemotypic Modulation and Structure-Volatile Interplay in Zanthoxylum oxyphyllum

Postharvest drying can differentially volatilize, oxidize, or thermally degrade plant constituents, thereby shifting the relative proportions of chemical classes within an essential oil, a phenomenon termed chemotypic modulation. This study aimed to determine how five drying methods: shade drying (SD), microwave drying (MW), freeze drying (FD), and oven drying at 40°C (OD-40) and 60°C (OD-60) differentially modulate the chemotype, physicochemical properties, antioxidant activity, volatile composition, and essential oil yield of Zanthoxylum oxyphyllum leaves. Significant variations were observed across treatments. FD resulted in the highest rehydration ratio (6.18), essential oil yield (0.156%), chlorophyll retention (1150.80 µg/g dw), and monoterpene preservation (∼10.6%), while high-temperature drying (OD-60) markedly reduced oil yield (0.106%) and antioxidant activity. Essential oil composition was dominated by 2-nonanone (39.44%-58.54%) and 2-undecanone (17.45%-32.63%), establishing a consistent ketone-dominant chemotype across treatments. Monoterpenes were highly sensitive to thermal exposure, whereas sesquiterpenes exhibited comparatively greater stability. Strong positive correlations were observed between oil yield and rehydration ratio (r ≈ 0.83) and between oil yield and monoterpene content (r ≈ 0.82), indicating that both structural preservation and chemical stability govern volatile retention. Antioxidant loss under high-temperature drying was disproportionately greater than oil loss, suggesting higher thermal sensitivity of phenolic constituents. Overall, FD best preserved structural integrity and bioactive quality, whereas oven drying at 40°C offered a practical balance between quality retention and process feasibility. These findings highlight drying temperature as a critical determinant of chemotypic modulation and functional performance in Z. oxyphyllum leaves. PRACTICAL APPLICATIONS: This study provides evidence-based postharvest guidelines for Zanthoxylum oxyphyllum, a commercially important aromatic leaf spice from Northeast India. Freeze drying is recommended for premium aroma-rich and bioactive applications, while oven drying at 40°C offers a scalable and economically feasible alternative for commercial spice production. High-temperature drying (60°C) and microwave drying compromise volatile and antioxidant quality and are less suitable for value-added botanical products. These findings can directly guide drying protocol optimization in small-to-medium spice enterprises operating in the Eastern Himalayan region.