Optimization of ultrasound-assisted extraction of total phenolics and anthocyanin from purple onion peel

Abstract

Purple onion (Allium cepa L.) peels, an abundant agro-industrial by-product, are particularly rich in flavonoids and anthocyanins with strong antioxidant potential. The effective recovery of these biologically active compounds enables the conversion of plant residues into high value-added products, supporting both clean-label ingredient development approaches and circular bioeconomy strategies. This study aimed to optimize ultrasound-assisted extraction (UAE) conditions to maximize the yield of total phenolic content (TPC) and total monomeric anthocyanins (TMAC) from purple onion peel s. A central composite design based on response surface methodology (RSM) was employed to systematically investigate the effects of four independent variables: ethanol concentration (0-100%), solvent-to-solid ratio (10-60 mL/g), extraction temperature (20-60 °C), and extraction time (30-120 min). The results indicated that solvent composition, temperature, and duration significantly influenced bioactive compound recovery. TPC ranged from 237.06 to 1230.96 mg GAE/100 g, while TMAC varied between 1645.84 and 12357.50 mg/kg. Multi-response optimization identified 95% ethanol, 55 mL/g solvent-to-solid ratio, 40 °C, and 105 min as optimal conditions, which were subsequently validated through confirmatory experiments. Extracts obtained under these optimized conditions exhibited desirable color parameters, low water activity values (0.20-0.52), indicating high stability and functional applicability in various formulations. Overall, the integration of UAE with RSM offers an efficient, scalable, and environmentally friendly approach for the extraction of natural pigments and antioxidants from purple onion peels, emphasizing their value as a sustainable ingredient source for food, nutraceutical, and related applications.