Natural Deep Eutectic Solvent-Based Extraction of Flavonoids from Citrus Aurantium L. By-Products: Process Optimization and Physicochemical Characterization of Optimal System

Citrus aurantium L. by-products are a valuable source of bioactive compounds with potential industrial applications. This study evaluated the use of natural deep eutectic solvents (NADESs), formulated with choline chloride as the hydrogen bond acceptor (HBA) and fructose as the hydrogen bond donor (HBD), combined with ultrasound-assisted extraction to optimize the recovery of flavonoids from Citrus aurantium L. industrial by-products. A central composite design (CCD) was employed to optimize total flavonoid content (TFC). The molar ratio (MR; 1 and 2 mol of HBD per mol of HBA) and the percentage of added water (AW; 40 and 60%) were optimized. In addition to TFC, total phenolic content (TPC), total ascorbic acid (TAA), antioxidant capacity (Ax) and phenolic profiles were also evaluated. The optimal extraction conditions, as obtained by Response Surface Methodology (RSM) for TFC, corresponded to an HBD molar ratio of 1.33 per 1 mol of HBA and 47.48% added water, resulting in an optimal value of 1576.63 ± 4.9 mg quercetin equivalents (QE)/100 g dry mass (DM). Maximum values of 4276.94 ± 25.4 mg gallic acid equivalents (GAE)/100 g DM for TPC, 2077.66 ± 21.3 mg/100 g DM for TAA and 88.09 ± 0.1% inhibition for Ax were also obtained. The optimal NADES was characterized by density, viscosity, pH, Fourier-transform infrared (FTIR) and Raman spectroscopy. Additionally, phenolic profiling revealed high concentrations of hesperidin (2747.1 mg/100 g DM), naringenin (1251.93 mg/100 g DM), and quercetin + luteolin (660.65 mg/100 g DM).