Quercetin recovery from apple biowaste using ultrasound and conventional techniques, natural deep eutectic solvents, and response surface methodology-based optimization

Globally, millions of tons of apple biowaste are generated annually, posing a significant and growing environmental challenge while remaining largely underutilized. This biomass is a rich source of high-value bioactives, particularly the potent antioxidant quercetin, highlighting its strong potential for sustainable valorization. In the current study, the extraction of quercetin from apple pomace was investigated using two deep eutectic solvents (DESs), glycerol-ammonium acetate (GAA) and glycerol-choline chloride (GCC), and two techniques, ultrasound-assisted extraction (UAE) and conventional heat-assisted extraction (HAE). Both techniques were optimized using response surface methodology (RSM). Both DESs were found to be highly effective, with the quercetin extraction yield (mg/g) as HAE-GCC 0.82, HAE-GAA 2.01, UAE-GCC 0.68, and UAE-GAA 2.38. Thus, UAE-GAA was most efficient, followed by HAE-GAA, demonstrating GAA as a more effective solvent and UAE as a more efficient technique. Optimization yielded statistically robust and well-fitted models for all extraction approaches, as validated by ANOVA and comprehensive fit diagnostics. The developed response surface models quantitatively elucidated the individual, interaction, and quadratic effects governing quercetin recovery. Comparative statistical and energy assessments identified UAE-GAA as the most technically advantageous strategy, combining superior quercetin recovery with lower energy demand and reduced environmental impact. These findings position UAE-GAA as a sustainable and industrially viable platform for the valorization of apple biowaste into high-value quercetin.