Single-Step Process for Isolation of Pure Quercetin from Aqueous Extract of Waste Onion Peels

Onion peel is a rich source of quercetin. Toward developing green methods to obtain pure quercetin, water-based extraction of quercetin followed by a simple single-step liquid–liquid extraction (LLE) method for the efficient isolation of quercetin is reported in this study. The developed method (LLE) obviates the need for commonly reported column chromatography. The purity of solid quercetin recovered was established using thin-layer chromatography (TLC), ultraviolet–visible spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR), high-performance liquid chromatography (HPLC), gas chromatography (GC), high-resolution mass spectroscopy (HRMS), and nuclear magnetic resonance (NMR). The purity of quercetin obtained from LLE was better at 93 ± 2% than the purity obtained from column chromatography (86 ± 2%). Optimization of the developed LLE process resulted in ∼600-fold reduction of the solvent used per milligram of quercetin purified. The maximum yield of quercetin was 10.88 ± 0.2 mg/g dry scales of onion, obtained by LLE, compared with 2.12 ± 0.3 mg/g dry scales of onion obtained from column chromatography. The LLE method thus becomes more sustainable and easy to scale up with improved process efficiency and enhanced product quality.