Temperature-responsive deep eutectic solvents as eco-friendly and recyclable media for microwave extraction of flavonoid compounds from waste onion (Allium cepa L.) skins

Polyphenols and flavonoids are abundant in spent onion skins (SOSs), being present in amounts are 2–sixfold greater times of than those in the inner bulb. In this study, an eco-friendly and efficient extraction method based on temperature-responsive deep eutectic solvents (TRDESs) was used to obtain natural flavonoids from SOSs. Microwave-assisted extraction yields of quercetin, kaempferol, luteolin, and quercetin-3-O-β-D-glucoside were evaluated to determine the efficiencies of a series of TRDESs. An extraction system composed of ethanolamine and m -cresol at ratio of 1:1 (20% water in TRDES system) was found to possess the highest efficiency. The extraction conditions were screened and optimized for efficiency using a Box-Behnken design. As a result, an extraction power of 554 W, extraction temperature of 76 °C, extraction time of 16 min, and solid–liquid ratio of 14 mg/mL were determined to be the optimal conditions. Under these extraction conditions, the total flavonoid extraction yields reached 47.83 ± 0.21 mg/g. The macroporous resin HPD 600 was selected for the separation and recovery of the TRDESs extraction mixture and the experimental measurements revealed that it allowed recovery rate more than 90%. In vitro activity testing of the flavonoid extract demonstrated excellent FRAP assay, DPPH assay and ABTS assay results for the SOSs extract prepared using a TRDES, and its inhibitory effects on S. aureus , E. carotovora , and B. subtilis were marked.