Ultrasonic-Assisted Deep Eutectic Solvent Extraction of Quercetin from Onion Peel: Experimental Optimization and Mechanistic Insights

Abstract This study developed an ultrasound-assisted deep eutectic solvent (DES) platform for the green recovery of quercetin from onion peels. It elucidated the extraction mechanism through process optimization and molecular simulations. Among the screened systems, betaine-malonic acid (Bet-MLN) exhibited the strongest synergistic extraction capability. Single-factor tests and Box–Behnken design optimized the conditions at extraction temperature 50 °C, extraction time 40 min, DES water content 34.11%, liquid–solid ratio 26.98:1, and ultrasonic power 376.26 W, yielding 21.83 mg/g quercetin, 30.27 mg RE/g flavonoids, and 77.33 mg GAE/g polyphenols, consistent with model predictions. LC-MS identified quercetin diglucoside, monoglucoside, and aglycone, with monoglucoside predominating. C18 reversed-phase solid-phase extraction enabled stepwise purification and enrichment and markedly enhanced DPPH and ABTS scavenging. Quantum chemistry and molecular dynamics simulations confirmed that hydrogen bonding, electrostatic complementarity, van der Waals synergy, and diffusion collectively drove flavonoid/polyphenol desorption, solvation, and migration, thereby supporting the valorization of agricultural by-products.