Polyphenols from Onion Skin Waste: A Natural Antioxidant Source with Beneficial Health Applications

Onion (Allium cepa L.) is one of the most important vegetable crops worldwide. However, its processing generates significant amounts of waste, primarily consisting of peels and skins. Despite being rich in bioactive compounds, these residues are typically discarded in landfills, raising environmental challenges.
This study aims to valorize onion skin (OS) wastes by producing flavonoid-rich extracts with significant bioactive potential. The specific objectives include analyzing the chemical composition of the extracts, specifically their phenolic content, assessing their antioxidant, antidiabetic, and anti-inflammatory activities, and their antimicrobial effects, along with their cytotoxicity on human cells to ensure their safety.
The primary components in the OS extracts were phenolics and flavonoids, with quercetin and quercetin-4’-O-glucoside being the major identified phenolics, while sugars and proteins were present at less than 11%. The extracts exhibited strong ^●NO radical scavenging activity and even greater efficiency against O₂^●- anion radical (IC₅₀=26-27.7µg/mL). In terms of antidiabetic activity, the extracts completely inhibited aldose-reductase and α-glucosidase while preserving α-amylase activity, thus avoiding side effects associated with its inhibition. Anti-inflammatory effects were demonstrated by inhibiting 5-lipoxygenase (IC₅₀=30.2-47µg/mL) and reducing NO production in IFN-activated BV-2 cells to basal levels, indicating anti-neuroinflammatory potential, without showing cytotoxic effects on human cell lines. Additionally, the antimicrobial properties of the extracts were highlighted by their effective anti-dermatophytic activity. In summary, OS has been shown to be a natural source of flavonoids, mainly quercetin and its glucosides, exhibiting diverse biological activities and a favorable safety profile. These findings support their potential application in food supplements, functional foods, and nutraceuticals.