Green Ultrasound-Assisted Extraction of Onion Polyphenols Using a Choline Chloride-Urea Deep Eutectic Solvent: Extraction Efficiency, Solvent Selectivity, and Antioxidant Assay Compatibility

Deep eutectic solvents (DES) emerge as sustainable alternatives for antioxidant polyphenol extraction; however, their analytical performance and compatibility with antioxidant assays remain insufficiently characterized. A choline chloride-urea-water DES (1:2:4) was compared with 70% ethanol for extracting polyphenols from onion bulbs and peels from cultivars grown in Spain and North Macedonia. Extraction conditions were selected through a comparative evaluation of vortex- and ultrasound-assisted extraction and benchmarked against conventional ethanolic stirring (2 h). Two-way ANOVA identified solvent composition as the main determinant of total phenolic content (F(1,20) = 1526.28, p < 0.001), while extraction method significantly influenced recovery under DES conditions (F(2,30) = 408.52, p < 0.001). The selected UAE-DES protocol increased TPC up to 2.2-fold and reduced extraction time to <5 min. TPC ranged from 25 to 44 mg GAE/g dw in bulbs and 42–62 mg GAE/g dw in peels, with red cultivars showing the highest values. UHPLC-MS/MS revealed solvent-dependent selectivity: ethanol favored flavonols (quercetin 5–11 mg/g dw), whereas DES enhanced phenolic acids (gallic acid up to 0.3 mg/g dw; protocatechuic acid up to 7 mg/g dw). FRAP correlated with TPC (r = 0.64–0.92), while ABTS was incompatible with DES extracts. Storage reduced TPC by 45–75% but preserved cultivar ranking. These findings demonstrate that UAE-DES enables rapid and efficient polyphenol recovery while highlighting the need to validate antioxidant assays in non-conventional solvents.