Efficient and green strategy based on pulsed electric field coupled with deep eutectic solvents for recovering flavonoids and preparing flavonoid aglycones from noni-processing wastes

The recovery of flavonoids and preparation of flavone aglycones from pomace waste are accompanied by the use of acidic organic solvents and high energy consumption, which lead to environmental pollution. This study aimed to develop an efficient and eco-friendly method based on pulsed electric field-assisted extraction (PEF-AE) combined with deep eutectic solvents (DESs) to recover flavonoids from noni pomace. The feasibility of using pulsed electric fields (PEF) combined with DESs as an alternative to conventional enzymatic, acidic, and pyrolytic methods, for the conversion of flavone glycosides to flavone aglycones was also evaluated. The results showed that, by using PEF-AE combined with DESs, the recovery efficiencies of the two main flavonoids (rutin and quercetin) in noni pomace were significantly superior to those of conventional organic extraction. Choline chloride/ethylene glycol (Ch/Eth) and choline chloride/oxalic acid (Ch/Oxa) were found to be optimal extractants for the recovery of rutin and quercetin. After response surface methodology optimisation, the highest extraction amounts of rutin and quercetin were 16.21 mg/g and 19.85 mg/g, respectively. During the extraction process, it was found that almost 100% of degraded rutin can be converted to quercetin in Ch/Oxa. Under identical treatment time and temperature, the rutin conversion efficiency of PEF was 25.4–206.4 folds higher than that of hot-water treatment (HWT). An excellent rutin conversion efficiency (8.72%/min) for PEF was achieved by adjusting the key reaction conditions. Overall, PEF-AE combined with DESs is an efficient, selective, and sustainable method for recovering flavonoids from noni pomace waste. This study also demonstrated that PEF coupled with DESs is a promising method for the efficient and eco-friendly preparation of flavone aglycones.