Ultrasound-Assisted Deep Eutectic Solvent Extraction of Total Alkaloids from Eupatorium lindleyanum: Process Optimization and Preservation Application in Postharvest Grapes

A green efficient deep eutectic solvent (DES)-based system was developed for total alkaloid extraction from Eupatorium lindleyanum DC. and evaluated for postharvest grape preservation. Among 18 DESs, choline chloride–citric acid (2:1, molar ratio) showed the highest extraction efficiency. Box–Behnken design and response surface methodology optimized the process to 52 min, 42 °C, and a liquid-to-solid ratio of 39:1 mL∙g−1, yielding 277.96 mg∙g−1 total alkaloids. SEM showed pronounced structural disruption of E. lindleyanum tissues after UAE-DES treatment. The DES extract exhibited DPPH and ABTS radical scavenging activities of 72.74% and 77.35%, respectively, significantly higher than those of the water extract. In postharvest grapes, the DES extract reduced MDA accumulation and maintained higher VC levels. In Botrytis cinerea-inoculated grapes, it reduced visible gray mold-associated deterioration and alleviated oxidative damage. This study extends UAE-DES research from extraction optimization to evaluation in a real postharvest fruit system and supports the potential of E. lindleyanum extracts with high total alkaloid content for natural antioxidant-based preservation.