Green Extraction of Lotus Leaf (Nelumbo nucifera Gaertn) Polyphenols: Unraveling the Mechanism of Ultrasound-Assisted Deep Eutectic Solvents

Deep eutectic solvents (DESs) have attracted considerable attention in recent years because of their cost-effectiveness, safety, and sustainability. In this study, we developed 19 DESs for the extraction of antioxidant polyphenolic compounds from lotus leaves, utilizing ultrasound-assisted extraction (UAE). Among the DESs examined, choline chloride (ChCl) and lactic acid (ChCl: lactic acid) exhibited the highest extraction efficiency. The optimal conditions were established as follows: molar ratio of 1:2.6, solid-to-liquid ratio of 1:20 g/mL, water content of 8%, and ultrasound time of 65 min, which proved to be more efficient than conventional extraction methods such as water and ethanol. Under the optimal conditions, the total phenolic content (TPC) was 187.23 ± 14.67 mg GAE/g DW, and the extracts exhibited high antioxidant activity (DPPH IC50: 0.92 ± 0.23 mg/mL; FRAP: 21.56 ± 3.05 mg Trolox/g DW). This superiority arises from the formation of robust hydrogen bonds between ChCl and lactic acid, in conjunction with improved mass transfer efficiency. This study provides a green alternative method for polyphenol extraction from lotus leaves.