Valorization of Quince By-Products Using Natural Deep Eutectic Solvents (NADES): Extraction and In Vitro Digestion of Phenolic Compounds

Quince (<i>Cydonia oblonga</i> Mill.) processing generates peel and core by-product fractions that are underexploited resources with untapped potential for valorization in sustainable food systems. In this study, ultrasound-assisted extraction was performed using several choline chloride-based natural deep eutectic solvents (NADES, six formulations with distinct hydrogen-bond donors) and compared with 70% (<i>v</i>/<i>v</i>) ethanol. Extracts were analyzed for total phenolic content, antioxidant capacity, and individual phenolic compounds by LC-MS/MS, and their bioaccessibility was determined through a standardized in vitro digestion model. Organic acid-based NADES, particularly ChCl:MA (2:1) and ChCl:LA (1:1), yielded significantly higher phenolic contents from the peel than ethanol (up to ~45% increase, <i>p</i> < 0.05), and ChCl:MA maintained superior antioxidant capacity after digestion. In the core fraction, glucose- and glycerol-based NADES promoted the release of bound phenolics, resulting in bioaccessibility values exceeding 100%, indicating the release of previously bound phenolics under digestive conditions. The present study provides novel insights into the effects of NADES on both extraction efficiency and digestibility of quince by-products. These findings highlight quince peel and core as promising raw materials for developing functional food and nutraceutical ingredients, thereby offering a feasible strategy for upcycling fruit-processing residues into health-promoting applications.