Valorization of Onion By-Products and Assessment of Their Biological Activities

Onions represent one of the most widely consumed vegetables within the Allium genus, generating a significant amount of waste. Onion waste is mainly composed of non-edible fractions, including roots, outer dry skins, and the outer fleshy scale. This study aimed to valorize red onion waste (ROW) and assess its total phenolic content and antioxidant activity using a water bath extraction method with distilled water as the extraction solvent. response surface methodology was employed to optimize the extraction parameters, namely temperature and time. The phenolic composition of freeze-dried ROW extract was analyzed using liquid chromatography–mass spectrometry analysis. The antibacterial activity was assessed using the disk diffusion and broth dilution methods against Gram-positive and Gram-negative bacteria, and the anticancer activity was tested against H460 lung, Caco-2 colon, and HT-29 colorectal cancer cell lines. The ideal extraction parameters were 1:40 g/mL, 98 °C for 27 min. The major detected compounds were isorhamnetin (55.32%), hyperoside (19.44%), and quercetin (13.65%). ROW extract showed antibacterial activity against Bacillus cereus, Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, Pseudomonas aeruginosa, and Salmonella Typhimurium, with the highest activity observed against S. aureus. A cytotoxic effect against H460 and Caco-2 was shown by the ROW extract, with IC50 values detected within the tested concentrations. The results suggest that ROW extract has potential for effective application as an antioxidant, antibacterial, and anticancer agent, demonstrating its potential for incorporation into functional foods and nutraceutical development.