linear effect terms; β~11~, β~22~, and β~33~ are the quadratic effect terms; and β~12~, β~13~, and β~23~ are the interaction effect terms, respectively.[@bib0130] The adequacy of the model was determined by evaluating the lack of fit, coefficient of determination (*p*-value), and the Fisher test value (*F*-value) obtained from the analysis of variance (ANOVA) that was generated by the software. Statistical significance of the model and model parameters were determined at the 5% probability level.[@bib0135]

3. Results {#sec0040}
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3.1. HPLC analysis {#sec0045}
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The major compound of licorice, glycyrrhizic acid (GA) was detected at 254 nm and representative HPLC chromatograms are shown in [Fig. 1](#fig0005){ref-type="fig"}. For GA concentrations from 12.5 to 200 μg/mL, the regression equation was Y = 17842X − 4192.5, which presented good linearity (r^2^ = 1.0000).Fig. 1HPLC chromatogram of glycyrrhzic acid in standard (a) and sample (b).Fig. 1

3.2. Effect of solvent type on the extraction method {#sec0050}
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The effect of solvent type on the extraction method was investigated at the beginning of the study. The choice of an appropriate solvent is the most important parameter to be optimized for the extraction of specific constituents.[@bib0060] The glycyrrhizic acid (GA) molecule has several hydroxyl groups, which renders it easily soluble when extracted by polar solvents.[@bib0140] In previous studies, different solvents such as methanol, ethanol, and water have been tested.[@bib0040], [@bib0045], [@bib0140], [@bib0145] To select the appropriate extraction solvent, we measured the yields of GA in different solvents. [Table 3](#tbl0015){ref-type="table"} shows significant differences in the extraction yields for the three solvents used (*p \<* 0.001). The yield of GA was highest with methanol (0.845 ± 0.030), and therefore methanol was chosen as the extraction solvent for the design of experiments.Table 3Contents