Tailor-made deep eutectic solvents-based green extraction of natural antioxidants from partridge leaf-tea (Mallotus furetianus L.)

Herein, high efficient extraction of natural antioxidants from partridge leaf-tea (PLT) was carried out by using eco-friendly choline chloride-based deep eutectic solvents combined with ultrasonic method. Choline chloride-malic acid (ChCl-MaA), a new type of green solvent, with the highest efficient for extracting the natural antioxidants from PLT (DPPH-756.70 ± 12.13 μmol TE/g DW; FRAP-4139.17 ± 65.79 μM Fe(Ⅱ)SE/g DW) was screened for further optimization. The key parameters for simultaneously maximizing total phenolic content (TPC), total flavonoid content (TFC), 2,2-diphenyl-1-picrylhyldrazyl (DPPH) and ferric reducing antioxidant activity (FRAP) were optimized by single-factor experiments along with response surface methodology (RSM). Results exhibited that L/S ratio and water content in DES had significant effects on TPC and antioxidant activities, while ultrasonic power indicated a significant negative effect on all responses. Under the optimal conditions (ultrasonic power at 210 W, ultrasonic time at 30 min, L/S ratio at 27.50:1, water content in DES at 45%), experimentally obtained results were obtained as follows: 42.20 ± 2.17 mg GAE/g DW for TPC, 30.33 ± 2.16 mg RE/g DW for TFC, 838.64 ± 12.81 μmol TE/g DW for DPPH, and 5958.33 ± 153.52 μM Fe(Ⅱ)SE/g DW for FRAP, which were well in agreement with the predicted values. Furthermore, ChCl-MaA-based ultrasonic assisted extraction (UAE) yielded higher TPC, TFC and antioxidant activities compared with traditional solvents-based UAE. Strong antioxidant activities of PLT extracts were correlated with high levels of individual antioxidants including salvianic acid A, 3-chlorogenic acid, gallic acid, catechin and rutin. The functional groups analysis by Fourier transform infrared spectroscopy (FT-IR) were used to compare the solvents characterization before and after extraction. Scanning electron microscope (SEM) results further verified ChCl-MaA intensified the destruction of cell wall structure, and thereby allowed greater release of antioxidants from PLT. This work provides an eco-friendly ChCl-MaA-based UAE technique for efficiently extraction and valorization of phenolic compounds from PLT, which can be further used in pharmaceutical, food, and cosmetic industries.