BACKGROUND 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. RESULTS 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. CONCLUSION Major flavonoids – quercetin, kaempferol, and myricetin – were identified and quantified in the extracts. Moreover, high reducing power (PR) 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

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The present study investigates recovery of polyphenolic compounds from ripe mango ( Mangifera indica L.) peel using deep eutectic solvents based on microwave-assisted extraction method. Lactic acid/sodium acetate/water (3:1:4) screened out from eight different types of deep eutectic solvent systems was used as extractant. A Box–Behnken design along with response surface methodology was applied to optimize the effect of microwave power (W), time (min), and liquid-to-solid ratio (mL g−1) on polyphenol extraction. The optimized conditions determined were power of 436.45 W, time of 19.66 min, and liquid-to-solid ratio of 59.82 mL g−1. Under the optimal conditions, the recovery of total phenolic content, ferric reducing antioxidant power, and 2,2-diphenyl-1-picrylhydrazyl scavenging activity was 56.17 mg gallic acid equivalent g−1 dw, 683.27 µmol ascorbic acid equivalent g−1 dw, and 82.64 DPPHsc%, respectively. High Performance Liquid Chromatography (HPLC) analysis revealed mangiferin as the prominent phenolic compound in the mango peel extracts. Microwave-assisted deep eutectic solvent extraction showed remarkable effects on the extraction efficiency of phenolic compounds as revealed from scanning electron microscopy analysis. Rancimat test results revealed that the oxidative stability almost doubled upon addition of purified mango peel extracts to the sunflower oil and thus paving way for the use of mango peel waste as a potential source of antioxidants.

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