Comparative antioxidant recovery evaluation of ultrasound and supercritical CO₂ Yemeni red onion peel extractions with LC-ESI-MS/MS profiling of ultrasound-assisted extracts

Yemeni red onion ( Allium cepa L.) peels of the local Yemeni cultivar ‘Ba Fatim’ represent an underutilized agro-waste rich in bioactive compounds with significant antioxidant potential. This study aimed to comparatively evaluate ultrasonic-assisted extraction (UAE) and supercritical CO₂ extraction (SC-CO₂) for the recovery of antioxidant constituents under optimized conditions. UAE was performed using 70% aqueous ethanol (1:10, w/v) at 45 °C for 30 min, while SC-CO₂ extraction was conducted at 35 MPa and 50 °C with 10% ethanol as a co-solvent. These parameters were selected based on preliminary trials and previously published studies. The antioxidant capacity of both extracts was assessed using total phenolic content (TPC), total flavonoid content (TFC), and DPPH and ABTS radical scavenging assays. UAE exhibited significantly superior performance ( p < 0.05), yielding higher TPC (84.55 ± 0.92 mg GAE/g DW) and TFC (31.00 ± 0.55 mg QE/g DW), along with enhanced radical scavenging activity, with DPPH inhibition of 65.28 ± 0.36% (IC₅₀ = 61.29 µg/mL) and ABTS inhibition of 62.43 ± 0.82% (IC₅₀ = 66.07 µg/mL). Owing to its superior efficiency, the UAE extract was selected for LC-ESI-MS/MS analysis, leading to the identification of 28 compounds. Phytochemical profiling revealed that phenolic compounds, particularly flavonoids and phenolic acids, were predominant, alongside minor classes including organic acids, nucleotides, phosphorylated sugars, and coumarins. Notably, several metabolites were tentatively annotated in the present extract and not previously reported in onion peels, including daphnetin, sinapoyl malate, syringaldehyde, N -acetylglycine, DL -6,8-thioctic acid, and esculin, highlighting the distinctive chemical profile of this Yemeni cultivar. Overall, these findings demonstrate that UAE is a more effective and sustainable extraction technique than SC-CO₂ under the laboratory-scale conditions employed in this study, positioning Yemeni red onion peels as a promising source of bioactive compounds for nutraceutical and pharmaceutical applications.