Simple presoaking modulates phytochemical recovery, antioxidant capacity, and macrophage responses of onion skin extracts

Abstract

Onion skin is a phenolic-rich by-product, but practical preprocessing strategies for directing phytochemical recovery remain insufficiently defined. This study evaluated whether brief presoaking in water, 5% vinegar, 1% citric acid, a vinegar–citric acid mixture, or 0.3% H₂O₂ for 4–12 min altered subsequent methanol-reference and hot-water extracts. Extraction yield, phenolic and flavonoid indices, quercetin markers, UHPLC–Orbitrap HRMS profiles, DPPH activity, and RAW 264.7 responses were assessed. Presoaking solution affected all major variables more consistently than duration. Across treatments, yields ranged from 5.78 to 10.67% for methanol and 6.87–10.53% for hot-water extracts, yet higher yield did not indicate greater phenolic density. Methanol preferentially recovered quercetin aglycone, whereas hot water produced a more polar profile. The 0.3% H₂O₂ condition showed favorable antioxidant performance without marked loss of metabolic viability. However, mild oxidative preprocessing requires further validation of peroxide removal, safety, and regulatory compliance before practical food application.