A hydrothermal process using subcritical water and ethanol as an extraction solvent has been studied to extract high-value flavonoid compounds from Quercus leaves. The univariate analysis showed that the maximum quercetin yield was 34.6 mg/g obtained at the liquid-solid ratio of 30:1 ml/g, concentration of ethanol 60% (V/V), extraction temperature of 180 C, and extraction duration of 5 h. Based on the Box-Behnken experiment design, the response surface methodology (RSM) was used to optimize the liquid-to-solid ratio, extraction time, extraction temperature, ethanol concentration, and other operating conditions of hydrothermal extraction of quercetin. The regression equation showed that the maximum yield of quercetin was 36.1 mg/g under the conditions of solvent ethanol concentration of 65% (V/V), liquid-solid ratio of 40:1 ml/g, extraction temperature of 189 C, and extraction duration of 5.5 h. The quercetin yield of 36.1 mg/g obtained by the hydrothermal method was 5.6 times the 6.4 mg/g obtained by the traditional ethanol reflux extraction at the same concentration of ethanol 65% (V/V), liquid-solid ratio of 40:1 ml/g, and extraction duration of 5.5 h, but at the boiling point of the solvent.

Mass and heat transfer models of Quercus leaf particles during hydrothermal extraction were developed in COMSOL Multiphysics. The main physical properties used in the model were measured. The results showed that the average particle size was 116.7 μm, and after soaking in water for 24 h, the average particle size increased to 127.3 μm. However, the average particle size increased from 116.7 to 132.8 μm after soaking in a 60% (v/v) ethanol solution for 40 min, and decreased to 127.3 μm after 60 min. The Quercus leaf particles have a higher ability to absorb ethanol solution than water. The simulations showed that it took 3.2 s for the ethanol concentration and 0.5 s for the temperature in the particles to tend stably after soaking in a 60% (v/v) ethanol solution at 40°C. From the edge of the particle to the centre, the increasing rates of ethanol concentration and temperature gradually slowed down, while the changing rates of the centre accelerated with the increase in solution concentration and temperature, respectively. When the particles soaked at the lowest concentration, the central concentration first reached stability, and at the lowest temperature, the centre temperature first stabilized.

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