## 3 Results and Discussion
### Optimization of the Extraction Process Parameters  
For selection of the optimal extraction conditions that maximize the bioactive compound recovery yield and the level of the antioxidant activity, a central composite response surface design was used. As was evident in the first-stage results, the SMR and organic solvent concentration were considered as continuous factors with an extended range in comparison with the first step of the extraction optimization. The ultrasound-assisted extraction time was set to 30 min. Additionally, the range of analysis was extended to include the ABTS assay, FRAP assay, TPC determination, caffeine content, chlorogenic acids (3-CQA) content and browning index (BI). The results obtained in all of the runs are presented in Table 5.  
Table 6 shows statistical information on the mathematical models obtained in the second stage of the extraction optimization. These models show changes in responses in individual analyses depending on the ethanol concentration and SMR. All the models were simplified by elimination of statistically non-significant terms. The lowest coefficient of determination (0.83) was found in the response model of TPC. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **ABTS** & **TPC** \\ \hline Regression equation & ABTS = 46.40 \(-\) 0.32 \(\times\) A + 1.32 \(\times\) C + 7.01 \(\times\) D & TPC = 21.60 \(-\) 0.11 \(\times\) A + 0.45 \(\times\) C - 2.63 \(\times\) D \\ Model \(p\)-value & 7.1 \(\times\) 10\({}^{-6}\) & 1.5 \(\times\) 10\({}^{-7}\) \\ R\({}^{2}\) & 80.23\% & 87.84\% \\ R\({}^{2}\) (adjusted) & 76.53\% & 85.56\% \\ R\({}^{2}\) (predicted) & 72.04\% & 81.44\% \\ Fitted value & 72.94 & 35.75 \\ (A:60; C:40; D:Yes) & \multicolumn{1}{c}{} & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of first-stage extraction optimization models. Coded units as follows: A—organic solvent concentration (%), C—SMR (mL/g SCG d.m. ), and D—ultrasound assistance. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Solvent**} & \multirow{2}{*}{**SMR**} & \multirow{2}{*}{**ABTS**} & \multirow{2}{*}{**FRAP**} & \multirow{2}{*}{**TPC**} & \multirow{2}{*}{**Caffeine**} & \multirow{2}{*}{**3-CQA**} & \multirow{2}{*}{**BI**} \\ \cline{2-2} \cline{6-9}
**Run** & & & & & & & & & \\ \cline{2-2}  
The obtained regression models were visualized by the response surface plots and their corresponding contour plots (Figure 2), which provide a better understanding in the behavior of the response at various levels of the extraction process variables. The evaluation of the SCG extract antioxidant activity was performed using the ABTS and FRAP assays, which involve a mechanism reaction based on the transfer of a single electron, namely SET. These assays determine the capacity of the antioxidant compounds to transfer an electron and cause the reduction of free radicals or metallic ions, respectively [42]. In regard to the ABTS assay, the effects of all linear (A, B) and quadratic (A\({}^{2}\), B\({}^{2}\)) terms were found to be statistically significant. According to the optimization results, maximum ABTS levels (74.96 mg Trolox/g SCG d.m.) should be obtained with 62% hydroethanonic solvent solution and 47 mL/g SCG d.m. [PERSON] et al. [33] obtained similar results with the use of 60% hydromethanonic solvent solution and 40 mL/g SCG d.m. extracts. In contrast, the ABTS levels achieved were higher in comparison to those reported by [PERSON] et al. [43]. The discrepancies between the results outlined in the present research and those described in the related literature could be due to the use of different SCG matrices, types of solvent or types of extraction method. Considering the FRAP assay, only the effects of linear terms (A, B) were observed to be statistically significant. Based on the optimization findings, the FRAP assay achieved the maximum value of 94.5 umol Fe(II)/g SCG d.m. with 88% ethanol and 68 mL/g SCG d.m.