Enzyme-assisted ultrasonic-microwave synergistic extraction and UPLC-QTOF-MS analysis of flavonoids from Chinese water chestnut peels

Chinese water chestnut [Eleocharis dulcis (Burm. f.) Trin. ex Hensch] peels (CWCPs) were a kind of vegetable waste and the present work was focusing on its comprehensive exploitation and utilization. Enzyme-assisted ultrasonic-microwave synergistic extraction (EAUMSE) of flavonoids from CWCPs was investigated, along with their qualitative and quantitative analysis using a combination of ultra-performance liquid chromatography-photodiode array (UPLC-PDA) and quadrupole-time-of-flight mass spectrometry (QTOF-MS). An optimal yield of 1.48% (w/w) was achieved with a 2:1 ratio (w/w) of cellulase and pectinase, 1.5% enzyme concentration, 5.0 pH, 50 °C hydrolysis temperature, 2 h hydrolysis time, with ethanol and 0.1 mol/L NaH2PO4 buffer (2:1, v/v) mixture as solvent, ultrasound and microwave power was 50 and 200 W, respectively, and 60 s extraction time. The flavonoid yield by EAUMSE was 26.50%, 22.31% and 12.98% higher than that by solvent extraction (SE), ultrasound-assisted extraction (UAE) and microwave-assisted extraction (MAE), respectively. Sixteen flavonoids were characterised, including four new ones, with the major flavonoids being luteolin (249 mg/100 g DW), eriodictyol (97 mg/100 g DW), 6-methylluteolin (74 mg/100 g DW) and fisetin (46 mg/100 g DW). The EAUMSE was an effective and green method for flavonoid extraction in the functional food and pharmaceutical industries.