Development of sodium propionate-based deep eutectic solvents for polyphenol extraction from onion solid wastes

The objective of the study presented herein was the optimisation of onion solid waste (OSW) polyphenol extraction, using sodium propionate-based deep eutectic solvents (DESs) and process variables optimised through response surface methodology. Initial screening of several novel DESs composed of either L-lactic acid or glycerol as hydrogen bond donors, and sodium propionate as the hydrogen bond acceptor, showed that the DES with glycerol/sodium propionate at a molar ratio 8:1 was the highest-performing system. Extraction optimisation showed that theoretical optimal settings were 85% (w/w) aqueous DES, 100 mL g−1 liquid-to-solid ratio and a stirring speed of 900 rpm. Under these conditions, the temperature assay demonstrated that extraction may be performed at 80 °C, without compromising total polyphenol yield and antioxidant activity. Comparison with other green solvents showed that the DES tested could provide extracts with high flavonoid concentration and superior ferric-reducing power. The stability test performed over a period of 30 days suggested that OSW extract may suffer no significant alterations in the polyphenolic profile, but the antiradical activity may be enhanced. Changes in the antioxidant behaviour were not correlated with changes in the two major OSW constituents, quercetin 4′-O-glucoside and quercetin.