## 3 Results and Discussion  
### Selection of the Crucial Operational Extraction Process Variables  
In the first stage of extraction optimization of bioactive compounds from coffee silverskin, the influence of the type of used solvent and its concentration, SMR, extraction duration, and the applied extraction equipment was assessed to characterize CS extracts properties. As RSM responses, ABTS assay and TPC determination were performed. The results obtained in the preliminary analysis are summarized in Table 3. Ethanol, methanol, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Type of Sample**} & \multicolumn{3}{c}{**Methanol/Ethanol**} & \multicolumn{3}{c}{**Water**} \\ \cline{2-7}  & **Solvent Concentration [\%]** & **Time** & **SMR** & **US \({}^{*}\)** & **Time** & **SMR** & **US \({}^{*}\)** \\  & **[min]** & **[cm\({}^{3}\)/g]** & **Us \({}^{*}\)** & **[min]** & **[cm\({}^{3}\)/g]** & **Us \({}^{*}\)** \\ \hline
1 & 60 (\(-\)1) & 30 (\(-\)1) & 10 (\(-\)1) & w (+1) & 30 (\(-\)1) & 10 (\(-\)1) & w (+1) \\
2 & 100 (+1) & 30 (\(-\)1) & 10 (\(-\)1) & w (+1) & 90 (+1) & 10 (\(-\)1) & w (+1) \\
3 & 60 (\(-\)1) & 90 (+1) & 10 (\(-\)1) & w (+1) & 30 (\(-\)1) & 40 (+1) & w (+1) \\
4 & 100 (+1) & 90 (+1) & 10 (\(-\)1) & w (+1) & 90 (+1) & 40 (+1) & w (+1) \\
5 & 60 (\(-\)1) & 30 (\(-\)1) & 40 (+1) & w (+1) & 30 (\(-\)1) & 10 (\(-\)1) & w (\(-\)1) \\
6 & 100 (+1) & 30 (\(-\)1) & 40 (+1) & w (+1) & 90 (+1) & 10 (\(-\)1) & w (\(-\)1) \\
7 & 60 (\(-\)1) & 90 (+1) & 40 (+1) & w (+1) & 30 (\(-\)1) & 40 (+1) & wo (\(-\)1) \\
8 & 100 (+1) & 90 (+1) & 40 (+1) & w (+1) & 90 (+1) & 40 (+1) & wo (\(-\)1) \\
9 & 60 (\(-\)1) & 30 (\(-\)1) & 10 (\(-\)1) & wo (\(-\)1) & - & - & - \\
10 & 100 (+1) & 30 (\(-\)1) & 10 (\(-\)1) & wo (\(-\)1) & - & - & - \\
11 & 60 (\(-\)1) & 90 (+1) & 10 (\(-\)1) & wo (\(-\)1) & - & - & - \\
12 & 100 (+1) & 90 (+1) & 10 (\(-\)1) & wo (\(-\)1) & - & - & - \\
13 & 60 (\(-\)1) & 30 (\(-\)1) & 40 (+1) & wo (\(-\)1) & - & - & - \\
14 & 100 (+1) & 30 (\(-\)1) & 40 (+1) & wo (\(-\)1) & - & - & - \\
15 & 60 (\(-\)1) & 90 (+1) & 40 (+1) & wo (\(-\)1) & - & - & - \\
16 & 100 (+1) & 90 (+1) & 40 (+1) & wo (\(-\)1) & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental design layout applied in the first stage of extraction optimization of bioactive compounds from coffee silverskin. The extraction process variables are shown as real and (coded) values. US \({}^{*}\)—ultrasound assisted extraction, extraction conducted with ultrasound bath (w) or laboratory shaker with water bath (wo). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Sample** & **Solvent Concentration [\%]** & **SMR [cm\({}^{3}\)/g]** \\ \hline
1 & 20 (\(-\)1) & 20 (\(-\)1) \\
2 & 80 (+1) & 20 (\(-\)1) \\
3 & 20 (\(-\)1) & 60 (+1) \\
4 & 80 (+1) & 60 (+1) \\
5 & 7.58 (\(-\)1.414) & 40 (0) \\
6 & 92.42 (+1.414) & 40 (0) \\
7 & 50 (0) & 11.72 (\(-\)1.414) \\
8 & 50 (0) & 68.28 (+1.414) \\
9 & 50 (0) & 40 (0) \\
10 & 50 (0) & 40 (0) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental design layout applied in the second stage of extraction optimization of bioactive compounds from coffee silverskin. The extraction process variables are shown as real and (coded) values. and their aqueous solutions, as well as distilled water, have been used in this stage of extraction process optimization. These chemicals are known as the typical solvents, ensuring high level of recovery of bioactive compounds from raw plant materials and their by-products matrix [36; 51]. It is apparent from Table 3 that in all examined extraction conditions CS extracts indicated the antioxidant potential and the CS ethanolic extracts had the highest level of the antioxidant activity (93.0 \(\upmu\)mol Trolox/g d.m.). ABTS assay revealed that methanolic and ethanolic CS extracts were characterized by the significantly higher antioxidant activity in comparison to the CS extracts produced with distilled water as an extracting solvent. These findings are in agreement with the results presented by [PERSON] et al. [37].