Extraction of antioxidant compounds from onion peels grown in various regions of Türkiye using deep eutectic solvents

The aim of this thesis was to extract antioxidant compounds from the peels of certain endemic onion varieties grown in Türkiye—namely, Erdek purple, Çorum red, Karacabey brown, and Erdek white onions—using green solvents. Therefore, in this study, deep eutectic solvents, which comply with green extraction principles, were preferred over traditional organic solvents. These solvents enable both a reduction in environmental impact and a more sustainable valorization of food wastes. The effectiveness of deep eutectic solvents in the extraction of antioxidant compounds from purple, red, brown, and white onion peels was investigated. The peels of different onion varieties were compared in terms of their extracted antioxidant contents. Initially, the most effective deep eutectic solvents were identified based on total antioxidant capacity values determined by the CUPRAC method. All deep eutectic solvents used were characterized by FTIR spectroscopy, and measurements of viscosity, density, pH, and refractive index were conducted. For the optimization of the extraction processes, ultrasound-assisted extraction and microwave-assisted extraction methods were employed. The main common advantage of both methods is their ability to enable extraction within a shorter period, with less solvent, and with higher yields compared to conventional extraction techniques. Moreover, when combined with green solvents, these methods offer sustainable and innovative extraction processes with minimal environmental impact. Response surface methodology was applied to determine optimal conditions in both methods. In purple onion peels, the highest total antioxidant capacity value was obtained using the Choline chloride:1,4-butanediol:H2O combination as the deep eutectic solvent. This value was approximately twice that obtained with 70% ethanol. Furthermore, analyses of total antioxidant capacity, total phenolic content, total flavonoid content, and radical scavenging activities (DPPH and ABTS) were performed for all onion peels. The results demonstrated that extracts obtained with deep eutectic solvent, especially in purple and red onion peels, exhibited higher antioxidant, phenolic, and flavonoid contents compared to those extracted with ethanol. HPLC analysis revealed significant differences in the levels of phenolic and flavonoid compounds such as quercetin and myricetin among the onion varieties. While purple and red onion peels were found to have high antioxidant potential, very low values were observed in white onion peels. In conclusion, three factors were found to play a critical role in the extraction of valuable phenolic and flavonoid compounds from onion peels: the variety and color of the onion, the extraction technique used, and the choice of solvent. It was demonstrated that more biologically valuable extracts can be obtained when all these factors are optimized together. In addition, this research contributes to converting onion peels from waste into value-added products and ensures effective waste management. Thus, the applicability of natural and environmentally friendly extraction methods was demonstrated, and the meaningful utilization of onion peels, which represent a significant waste potential in the food industry, was targeted. The findings indicate that colored onion peels can be considered as natural and sustainable sources of antioxidants, and that deep eutectic solvents are environmentally friendly and effective solvents for this purpose.