Antioxidant potential and antimicrobial activity of coloured onion peels extracted using ultrasound-assisted deep eutectic solvents

In this study, the most effective deep eutectic solvent (DES) for extracting antioxidants from purple onion peel was identified using total antioxidant capacity (TAC) values obtained by the CUPRAC assay. The DES systems used in this study consisted of choline chloride (ChCl) combined with ethylene glycol (EG), diethylene glycol (DiEG), triethylene glycol (TriEG), tetraethylene glycol (TetEG), 1,4-butanediol (1,4-but), and 1,2-propanediol (1,2-pro) at a molar ratio of 1 : 2 with 30% water content. All DESs were characterised in terms of Fourier transform infrared (FTIR) spectra, viscosity, density, and pH. Ultrasound-assisted extraction was optimised using response surface methodology, resulting in a maximum TAC of 582.96 mg Trolox equivalent (TE).g-1 dry sample (DS) for ChCl : 1,4-but : H2O, which was almost twice the amount achieved with 70% ethanol. Antioxidant parameters, including TAC, total phenolic content, total flavonoid content, and radical scavenging activities [2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)], were evaluated for purple, red, brown, and white onion peels. Antimicrobial activity was assessed using the broth microdilution method. HPLC analysis was employed to quantify phenolic and flavonoid compounds, revealing significant compositional differences among the onion peels. Overall, the results demonstrate that purple and red onion peels exhibit superior antioxidant and antimicrobial properties, supporting their potential as sustainable sources of natural antioxidants.