Green extraction of active ingredients from finger citron using subcritical water and assessment of antioxidant activity

In this study, subcritical water extraction (SWE) was used to extract active compounds, such as total phenols, total flavonoids, and essential oil, from finger citron. Response surface methodology (RSM) was applied to determine the optimal extraction conditions (total phenols: liquid-to-solid ratio (LSR) 40.9 mL/g, extraction time (ET) 2.7 h, subcritical water temperature (SWT) 183 °C; total flavonoids: LSR 38.8 mL/g, ET 2.4 h, SWT 176 °C; essential oil: LSR 42.3 mL/g, ET 2.5 h, SWT 182 °C). Under these conditions, the extraction yields of total phenols, total flavonoids, and essential oil were 45.65 ± 0.21 mg/g, 33.12 ± 0.19 mg/g, and 4.36 %, respectively. The extracts had remarkable antioxidant activity in vitro, especially those from SWE, which outperformed other methods. For the extracts, the IC50 values for 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, 2,2′-Azinobis-(3-ethylbenzthiazoline-6-sulphonat) (ABTS) radical scavenging, and (2-Phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl) (PTIO) radical scavenging were 0.32 mg/mL, 0.05 mg/mL, and 6.14 mg/mL, respectively. Finger citron essential oil (FCEO) was also analyzed by GC-MS and 37 constituents were identified. The major component was D-limonene at 23.59 %. The essential oil extracted by subcritical water also showed notable antioxidant activity, with IC50 values of 0.97 mg/mL, 0.08 mg/mL, and 5.35 mg/mL for DPPH radical scavenging, ABTS radical scavenging, and PTIO radical scavenging, respectively. These findings show that SWE has a promising potential to extract active compounds from natural products and enhance their biological activity.