## 3 Results
### RSM for Optimization of A. tegmentosum  
The goal of RSM is to establish the optimal conditions for extracting compounds with high antioxidant activities from _A. tegmentosum_. To establish optimal extraction conditions, the central point of the CCD was applied to the optimal values of 2 h (extraction time), 85 \({}^{\circ}\)C (extraction temperature), and 100 (solvent-to-solid ratio) based on the previously acquired results (Table 1). The independent variables of extraction temperature, extraction time, and solvent-to-solid ratio were set at 2-6 h, 63-97 \({}^{\circ}\)C, and 100 in the CCD range, respectively. Twenty experimental points were selected by Minitab and applied to the RSM design. The extraction conditions from the CCD consist of six central points, six axial points, and eight factorial designs. Each extract condition, the TPC, and the DPPH results are shown in Table 4. The three independent variables were as follows: X\({}_{1}\), extraction temperature; X\({}_{2}\), extraction time; X\({}_{3}\), solvent-to-solid ratio. The RSM model for TPC estimation was calculated using the following equation:  
\[\text{TPC (mg\,GAE/g)}=-570.0513+18.9147\,\text{X}_{1}-3.2803\,\text{X}_{2}-1.1768 \,\text{X}_{3}+0.1692\,\text{X}_{1}\times\text{X}_{2}+0.0022\text{X}_{1}\times \text{X}_{3}+0.0452\,\text{X}_{2}\times\text{X}_{3}-0.1305\,\text{X}_{1}^{2}-1. 3943\,\text{X}_{2}^{2}-0.0010\,\text{X}_{2}^{2} \tag{1}\]  
The RSM plots for maximal TPC were calculated using three variables (Figure 3). Figure 3A-F were plotted based on the interactions between two variables (extraction time and temperature (Figure 3A/B), extraction temperature and solvent-to-solid ratio (Figure 3C/D), and extraction temperature and solvent-to-solid ratio (Figure 3E/F)). The TPC values showed significant interactions based on extraction temperature and time (Figure 3C/D). This result suggests that the TPC value is maximized by increasing the extraction time and temperature in the controlled experimental ranges. Among the extraction conditions of _A. tegmentosum_, the extraction temperature (X\({}_{1}\), \(p\) = 0.034; X\({}_{1}\)\({}^{2}\), \(p\) = 0.003) and the solvent-to-solid ratio (X\({}_{3}\), \(p\) < 0.001) significantly affected the TPC (Table 5). Therefore, the extraction temperature was within the tested conditions; however, the solvent-to-solid ratio was outside the experimental range. The extraction temperature and the solvent-to-solid ratio had significant effects on the TPC of _A. tegmentosum_ (X\({}_{1}\)\(\times\) X\({}_{3}\), \(p\) = 0.006; Table 5). Figure 2: Measurement of the DPPH (2,2-diphenyl-1-pictylhydrazyl) activity by changing the extraction time, the extraction temperature, and the solvent-to-solid ratio for hydrothermal extraction of _Acer tegmentosum_. (**A**) Extraction temperature varied from 45–95 \({}^{\circ}\)C, and extraction time and solvent-to-solid ratio were fixed at 2 h and 80, respectively. (**B**) Extraction time varied from 0.5–8 h, and extraction temperature and solvent-to-solid ratio were fixed at 85 \({}^{\circ}\)C and 80, respectively. (**C**) Solvent-to-solid ratio varied from 10–320, and extraction temperature and extraction time were fixed at 85 \({}^{\circ}\)C and 2 h, respectively. Data are presented as the mean \(\pm\) standard deviation (_n_ = 3). Labeled averages without a common letter differ. \(p\) < 0.05. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Independent Variables**} & \multicolumn{2}{c}{**Dependent Variables**} \\ \cline{2-5}
**Experiment** & \multirow{2}{*}{**Temperature**} & \multirow{2}{*}{**Time (h)**} & \multicolumn{2}{c}{**Solvent-to-**} & \multirow{2}{*}{**TPC (mg**} & \multirow{2}{*}{**DPPH 2 (\%)**} \\  & & (\({}^{\circ}\)C) & & **(mL/g)** & & **GAE 1/g** \\ \hline  
\hline \hline \end{tabular} \({}^{1}\) GAE: gallic acid equivalent; \({}^{2}\) DPPH: activity.  
\end{table} 
Table 4: The effect of three independent variables for total polyphenol content (TPC) and DPPH activities.