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

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.