Valorization of Garlic (Allium sativum L.) Peel Waste Using Natural Deep Eutectic Solvent (NADES) Formulations: Storage Stability and Bioaccessibility of Phenolic Extracts

Garlic peel is an underutilized by-product with potential as a source of phenolic compounds. This study evaluated garlic peel waste valorization using ultrasound-assisted extraction with eight natural deep eutectic solvent (NADES) formulations, compared with ethanol and water, in terms of phenolic composition, antioxidant capacity, storage stability, in vitro gastrointestinal behavior, bioaccessibility, and chemometric differentiation. Total phenolic content (TPC), antioxidant capacity, and individual phenolics were analyzed, while extract stability was monitored over 60 days at 4 °C and 25 °C. Glycerol:lactic acid (1:3) and glucose:lactic acid (1:5) exhibited the highest TPC among the NADES formulations, whereas ethanol showed stronger CUPRAC and FRAP responses. HPLC–PDA analysis revealed five flavonoids, including rutin, quercetin, two cyanidin derivatives, and one pelargonidin derivative, and seven phenolic acids, including gallic, protocatechuic, vanillic, chlorogenic, p-coumaric, ferulic, and sinapic acids. Selected NADES favored anthocyanin recovery and post-digestion retention of flavonols and hydroxycinnamic acids. Most choline chloride-based NADES enhanced TPC bioaccessibility (105–293%) compared with ethanol and water (86–126%). Storage stability depended on solvent composition and temperature, with 4 °C better preserving phenolic and antioxidant properties. Chemometric analyses confirmed solvent effects on extract composition. NADES-based extraction offers a green strategy for converting garlic peel waste into phenolic-rich functional extracts.