he glycyrrhizic acid (GA) standard was purchased from Wako Pure Chemical Industries (Osaka, Japan).

2.2. Sample preparation {#sec0020}
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Samples (0.5 g) were placed into an extraction vessel with 12 mL of the extraction solvent and sonicated (ultrasonic cleaner 8510; 250 W, 44 kHz; Branson Co., Danbury, CT, USA)[@bib0015] for various experimental durations and temperatures. The extracted liquid fraction and the residue were then collected separately, and the residue was re-extracted. After extraction, the sample volumes were made up to 25 mL in volumetric flasks and filtered through a 0.2 μm membrane filter prior to HPLC analysis.

2.3. HPLC analysis {#sec0025}
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The HPLC system consisted of a Waters e2695 liquid chromatography system (Waters, USA), equipped with a Waters 2998 photodiode array detector. Data processing was carried out with the Empower software (Waters, USA). An XBridge C~18~ column (4.6 mm × 250 mm, 5.0 μm, Waters, USA) was employed. The mobile phase was 7% acetic acid and acetonitrile (60:40, v/v) with a flow-rate of 0.8 mL/min. The injection volume for all the samples was 20 μL. A wavelength of 254 nm was used for the detection of glycyrrhizic acid (GA). The GA peak was identified by comparing its retention time with those of standards, and the concentration was calculated from the calibration curves. Results are expressed as the mean values of assays for each experiment, which were performed in triplicate.

2.4. Experimental design {#sec0030}
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To further study the interaction between the factors, we optimized the operating conditions by RSM and used the central composite design (CCD) method. The range and center point values of three independent variables presented in [Table 1](#tbl0005){ref-type="table"} were based on the results of preliminary experiments. This generated 20 treatments with six replications at the central points to estimate the repeatability of the method ([Table 2](#tbl0010){ref-type="table"