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Dry outer onion leaves resulting from the industrial processing of onions are currently discarded as agricultural waste, although some studies have demonstrated that onion peel has beneficial biological effects. Considering the traditional applications of onion peel, the present study evaluated comparatively the chemical and biological characteristics of four types of onion peel extracts, utilizing methanol, ethanol, acetone, or ethyl acetate as the extracting solvent. The extracts were analyzed for their chemical composition, antioxidant potential, and antimicrobial activity. The chemical analysis by UHPLC-DAD-ESI/MS identified 23 compounds in the extracts, belonging to phenolic acids and flavonoids. Quercetin was the main compound in all extracts, ranging in concentrations from 14.91 mg/g DW in the ethanol extract to 48.53 mg/g DW in the methanol extract. The highest contents of total phenols and flavonoids were found in the acetone extract, and the methanol extract demonstrated the highest antioxidant activity in DPPH and ABTS assays. The antimicrobial potential of the extracts was screened using a microdilution method against a spectrum of gastrointestinal microorganisms. The results revealed that all four extracts have significant antimicrobial potential against the tested microorganisms, of which the ethanol extract demonstrated the highest antimicrobial potency.
Joković et al. (Sat,) studied this question.