Non-edible onion skin waste as a source of bioactive agents for functional foods development: Chemical composition and multifunctional bioactivity

Onion skin, often discarded during processing, remains rich in bioactive compounds, particularly flavonoids. This study assessed phenolic extraction (37 °C, 60 min) from Horcal and Red cultivars, using 70:30 ethanol:water. Extracts were rich in phenolics (103-155 mg/g), mainly quercetin and quercetin-4'-O-glucoside (61-67 %), and showed strong O₂ ^●- (IC₅₀ = 26-27.7 μg/mL) and ^●NO (IC₅₀ = 53-56 μg/mL) scavenging. They also inhibited α-glucosidase (IC₅₀ = 2.2-2.3 μg/mL) and aldose reductase (IC₅₀ = 37-44 μg/mL), while preserving α-amylase activity (IC₅₀ = 932-1126 μg/mL). Both extracts demonstrated anti-inflammatory effects via 5-LOX inhibition (IC₅₀ = 30.2-47 μg/mL) and anti-neuroinflammatory effects by restoring NO in BV2 cells. They further inhibited tyrosinase (IC₅₀ = 14.9-15.5 μg/mL) and xanthine oxidase (IC₅₀ = 15.1-19.2 μg/mL) above acetylcholinesterase (IC₅₀ = 301-532 μg/mL), with Horcal showing greater potential due to higher phenolic content. Extracts were non-cytotoxic and displayed anti-dermatophytic activity, particularly against T. rubrum and E. floccosum. These results suggest onion skin as a promising resource for bioactive compounds, with potential applications in supplements, functional foods, and nutraceuticals.