## 3 Results and Discussion
### Optimization of the Extraction Process Parameters
It is apparent in the FRAP assay contour plot that the antioxidant activity of the SCG extracts was not negatively influenced either by the organic solvent concentration or by SMR. These findings match those observed in the work carried out by [PERSON] et al. [44]. These researchers used the conventional solid-liquid extraction of antioxidant phenolic compounds from SCG using methanol as a solvent at different concentrations and determined the antioxidant activity in the range from 0.040 to 0.102 mM Fe(II)/g SCG. The results of their central composite design for extraction technique optimization are in accordance with the FRAP assay results presented in the current study. It is also interesting to note that in most cases, the antioxidant activity of SCG extracts measured by the FRAP assay reached higher values compared with the ABTS assay results. Likewise, [PERSON] et al. [45] observed discrepancies between the FRAP assay and DPPH assay results of antioxidant activity of coffee silverskin extracts. A possible explanation for these findings may be that the antioxidant activity procedures are based on different reaction mechanisms, antioxidants, target sample types and reaction conditions. In general, the FRAP assay is considered to be a SET-based non-radical method, but some authors have speculated that the FRAP assay has a low relationship with the radical quenching process and indicate the poor correlation with other commonly applied antioxidant activity measurements. For this reason, it is rec \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Tested** & & & & & \\
**RSM** & **Model** & **Simplified Fitted Equation** & **R\({}^{2}\)** & **Model** & **Lack-of-Fit** \\
**Response** & & & & & \\ \hline ABTS & Reduced & & & & \\ assay & quadratic & & & & \\ FRAP & Linear & & & & \\ assay & & & & & \\ TPC & Reduced & & & & \\ assay & quadratic & & & & \\ Caffeine & Quadratic & & & & \\ (HPLC) & Quadratic & & & & \\ (HPLC) & Quadratic & & & & \\ (HPLC) & Quadratic & & & & \\ (HPLC) & Quadratic & & & & \\ BI & Linear with & & & & \\ (Abs420) & interactions & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of the models of the second stage of the extraction optimization. Coded units as follows: A—organic solvent concentration (%), B—SMR (mL/g SCG d.m.). ommended to use it in combination with other antioxidant activity assays to discriminate the predominant mechanisms of various antioxidant phenolic compounds [42]. Figure 2: _Cont._  
Regarding the TPC content of extracts, the effects of all the linear (A, B), as well as quadratic (A\({}^{2}\), B\({}^{2}\)) terms were statistically significant. Maximization of TPC levels (36.38 mg GAE/g SCG d.m.) could be obtained with 65% ethanol aqueous solution and 56 mL/g SCG d.m. extracts. The total phenolics content levels in the tested SCG extracts were higher than those reported in other articles. The model proposed by [PERSON] et al. [19] suggested a maximum predicted value for TPC of 19.85 mg GAE/g SCG d.m. with the extraction parameters of 50% hydroethanolic solution, 35 mL/g SCG d.m., and extraction without the use of ultrasound assistance for 120 min at 65 \({}^{\circ}\)C, which is approx. 1.8 times lower than in this study. According to [PERSON] et al. [34], who used a Soxhlet extraction system for 6 min at 90 \({}^{\circ}\)C, the TPC in SCG ranged between 1.36 and 24.60 mg GAE/g SCG d.m. depending on the coffee species (Arabica, Robusta) and coffee preparation method (filter, espresso, plunger, mocha). As can be seen from these results, the differences in the amount of phenolic compounds could be attributed to the species and geographical origin of the coffee beans, the parameters applied during the roasting process and the extraction process conditions. Concerning the caffeine content in the extracts, the effects of all examined parameters (A, B) and their interactions (AB), as well as both quadratic terms (A\({}^{2}\), B\({}^{2}\)), were observed to be statistically significant.