Extraction of isoflavones from Puerariae lobata using subcritical water

Abstract

As an alternative to organic solvents, subcritical water was employed for the first time as an effective solvent for the extraction of isoflavones from Puerariae lobata. Optimum experimental conditions for the extraction of the four main isoflavones were established by single factor experiments, and the optimum experimental conditions for total isoflavone extraction were established further by response surface methodology. With an extraction time of 45 min and a liquid/solid ratio of 1 : 20, the extraction yields of puerarin, 3'-methoxypuerarin, and daidzin reached maxima at extraction temperatures of 120 °C, 140 °C and 200 °C, respectively. Moreover, puerarin, 3'-methoxypuerarin and daidzin were degraded and produced various byproducts due to hydrothermal reactions at higher temperatures. The maximum extraction yields of the total isoflavones were obtained by response surface methodology (extraction time of 45 min, solid/liquid ratio of 1 : 15 and extraction temperature of 120 °C). Compared to conventional solvents, subcritical water utilized less solvent and required a shorter extraction time.

Fig. 1. (a) Typical HPLC chromatogram of the Puerariae lobata extract obtained by SCWE (120 °C; 15 min; 4.50 mL water); (b) HPLC chromatogram of standard substances (puerarin, 3′-methoxypuerarin, daidzin and daidzein) mixed at concentrations of 10 μg mL⁻¹.

Fig. 2. Effect of extraction temperature on the extraction yields of isoflavones from Puerariae lobata (30 min; 4.50 mL water).

Fig. 3. Effect of extraction time on the extraction yields of isoflavones from Puerariae lobata (120 °C; 4.50 mL water).

Fig. 4. Effect of solid/liquid ratio on the extraction yields of isoflavones from Puerariae lobata (120 °C; 4.50 mL water).

Fig. 5. Comparison of experimental and predicted total isoflavones yields from Puerariae lobata.

Fig. 6. Response surface plots showing interactions between experimental parameters on the total isoflavone yields from Puerariae lobata: (a) extraction time (min) and temperature (°C); (b) solid/liquid ratio and temperature (°C); (c) solid/liquid ratio and extraction time (min).