Deep eutectic solvent-based extraction of polyphenolic antioxidants from onion (Allium cepa L.) peel

Biomass waste, including fruit and vegetable residues, poses a serious challenge to the environment. This demands novel approaches for handling and disposing of it in a safe manner. However, such waste is extremely rich in bioactive components, which are considered to have a beneficial impact on health. This has led to the recovery of high-value compounds from biomass waste before finally disposing of it. The present investigation reveals the use of green solvents such as eutectic mixtures composed of choline chloride with three different hydrogen bond donors like sucrose (4:1), urea (1:2), and sorbitol (3:1) as extractants for recovering valuable phenolic antioxidants from onion peel.

A deep eutectic solvent system consisting of choline chloride:urea (ChCl:urea) showed the best result for total phenolic content from onion peel while the other two eutectic mixtures showed relatively lower extraction efficiency. Experiments were performed with different molar ratios of ChCl:U (1:1-1:3) and liquid-solid ratios (10:1-60:1) under a temperature range (50-90 °C) and a time range (60-150 min). Taguchi's method was employed to maximize the extraction of polyphenolic antioxidants from onion peel, resulting in optimal conditions: extraction temperature 60 °C, time 120 min, liquid to solid ratio 50:1 and molar ratio of ChCl:U as 1:2. Under optimum conditions, the corresponding TPC value of the extract was found to be highest at 222.97 mg gallic acid equivalent (GAE) g^-1 dw.

Major flavonoids - quercetin, kaempferol, and myricetin - were identified and quantified in the extracts. Moreover, high reducing power (P_R ) value was reported in the case of ChCl:U extract, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity was found to be higher in ChCl:So extract. © 2018 Society of Chemical Industry.