Optimization of ultrasonic-assisted extraction to attain tannins and flavonoids from spent tea leaves using natural deep eutectic solvents

This research targeted recovering tannins and flavonoids from spent tea leaves using natural deep eutectic solvents (NADES) combined with ultrasonic-assisted extraction (UAE). NADES were produced by organic acids with choline chloride, polyols, and sugars, then the extraction performance of NADES was investigated. NADES from acetic acid-glycerol (NADES3) expressed the highest extraction efficiency of flavonoids and tannins. The single-factor experiments played a role in investigating the influence of UAE conditions on the extraction efficiency of tannins and flavonoids before the Box-Behnken design model was utilized to optimize the UAE processes. The optimum extraction efficiency of flavonoids and tannins was 20.4 mg RE/g db and 185.9 mg CE/g db, respectively, at 900 W of ultrasonic power, 41 mL/g of liquid-to-solid ratios, 69 °C, 20 min, and 10% of water content in NADES. After that, the biological activity of extracts acquired at optimal conditions was quantified. This research illustrates that NADES-based UAE is a green and sustainable method for efficiently recovering tannins and flavonoids from spent tea leaves.