Green Extraction of Phenolic Compounds from Lotus (Nelumbo nucifera Gaertn) Leaf Using Deep Eutectic Solvents: Process Optimization and Antioxidant Activity

Natural deep eutectic solvents (NDESs) were used to extract flavonoids and polyphenols from lotus (Nelumbo nucifera Gaertn.) leaves at the same time, and the extraction process was optimized to provide reference for the effective development and utilization of lotus leaves. The deep eutectic solvents (DESs) with the highest yield of flavonoids and polyphenols were screened out from 19 different NDES combinations. The response surface method was employed to optimize the extraction process. After a rational design, a lactic acid/glycerol (molar ratio 1:2) DES was chosen as the optimal extraction solvent, and the optimum extraction parameters were as follow: water content (29%), liquid–solid ratio (37:1 mL/g), extraction time (61 min), and extraction temperature (53 °C). Compared with traditional water extraction or ethanol extraction, it improved the yield of flavonoids (126.10 mg/g) and polyphenols (126.10 mg/g). By LC–MS analysis, 19 flavonoids or organic acid compounds with known compound structural formulae were identified in the DES extract of lotus leaves. By comparing the free radical scavenging ability and total reducing ability, the extraction of lotus leaves using the NDES method was superior to both ethanol extraction and water extraction. It is a green, environmentally friendly, and efficient extraction method for antioxidants from leaves of Nelumbo nucifera Gaertn.