Comparative analysis of antioxidant and immunostimulatory effects of Allium cepa L. skin extracts prepared by distinct extraction methods

The escalating global demand for sustainable strategies to reduce food waste has intensified interest in the valorization of Allium cepa L. (onion) by-products, particularly onion skins (OS), which are rich in bioactive compounds yet often discarded. Although previous studies have documented the antioxidant and anti-inflammatory properties of OS, systematic evaluations comparing multiple extraction methods under standardized conditions in both in vitro and in vivo models remain scarce. This study addresses that gap by providing the first integrated chemical, functional, and mechanistic comparison of OS extracts prepared by three distinct methods, hot water (HW), 30 % ethanol (30E), and 50 % ethanol (50E) with direct relevance to industrial scalability. UPLC-Q-TOF MS analysis identified quercetin derivatives as the predominant phytochemicals across all extracts. Ethanol-based extracts, particularly 30E, demonstrated superior radical scavenging activity and significantly enhanced nitric oxide production and cytokine secretion in RAW 264.7 macrophages compared with HW. In contrast, HW extract most effectively restored immune function in cyclophosphamide-induced immunosuppressed mice, as evidenced by increased serum IgG, T-cell and B-cell proliferation, COX-2 upregulation, and MAPK pathway activation. The 50E extract induced the highest phosphorylation of NF-κB. These method-specific functional profiles advance the global understanding of OS bioactivity and enable tailored applications in functional food and nutraceutical development. Beyond their mechanistic significance, the results highlight OS upcycling as a globally relevant, sustainable, and health-promoting strategy that supports circular bio-economy initiatives.