## 3 Results and Discussion
### Determination of Antioxidant Capacity  
The ABTS values for each extraction are shown in Table 5. The antioxidant activity from the head extracts ranged from 9.37 to 129.38 uM TE, obtaining the highest values after 30 min of UAE extraction at 20 \({}^{\circ}\)C and pH 8.5. The highest activity observed from the skin extracts was found after UAE at 30 min, 20 \({}^{\circ}\)C, and pH 5.5, whereas for the bone extracts, the values ranged from 28.94 to 276.23 uM TE, achieving the maximum value at 30 min, 20 \({}^{\circ}\)C, and pH 5.5. Lastly, the uppermost activity (516.02 uM TE) from the viscera extracts was obtained after applying UAE 30 min, at 50 \({}^{\circ}\)C, and pH 8.5. Figure 3A,B shows the main effects observed for the antioxidant capacity of the extracts obtained from head at different temperatures and extraction times at a constant pH of 8.5. It is clearly observed how increased extraction times significantly increased the antioxidant capacity of the extracts (\(p=0.0006\)). Besides, neither the pH nor the temperature affected antioxidant capacity (\(p=0.2855\) and \(p=0.1469\), respectively). Regarding the skin (Figure 3C,D), all the studied parameters affected the antioxidant capacity of the extracts with different degrees, obtaining \(p\) values of 0.0001, 0.0034, and 0.0045 for extraction time, temperature, and pH, respectively. As shown in Figure 3E-H for both bone and viscera, a significant increase in the antioxidant activity was observed with augmented extraction times (\(p<0.001\)). On the other hand, as in the case of head, no significant effect was observed regarding the temperature and pH. Accordingly, the optimal conditions for the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Extraction Time (min)**} & \multicolumn{2}{c}{**Temperature (\({}^{\circ}\)C)**} & \multicolumn{2}{c}{**pH**} & \multicolumn{2}{c}{**Antioxidant Capacity (ABTS, uM TE)**} \\ \hline  & & & & **Head** & **Skin** & **Bone** & **Viscera** \\ \hline
1 & 30 & 50 & 8.5 & 126.51 & 74.84 & 161.99 & 516.02 \\
2 & 0.5 & 20 & 8.5 & 11.01 & 31.62 & 34.46 & 213.51 \\
3 & 0.5 & 35 & 7 & 9.91 & 43.59 & 28.94 & 137.87 \\
4 & 30 & 35 & 7 & 43.91 & 125.86 & 134.31 & 450.35 \\
5 & 30 & 20 & 5.5 & 98.22 & 285.96 & 276.23 & 432.54 \\
6 & 15.25 & 35 & 7 & 21.78 & 164.85 & 173.23 & 492.30 \\
7 & 30 & 20 & 8.5 & 129.38 & 207.65 & 291.99 & 427.19 \\
8 & 15.25 & 35 & 8.5 & 36.51 & 90.85 & 210.89 & 487.78 \\
9 & 15.25 & 50 & 7 & 37.86 & 124.63 & 160.61 & 439.34 \\
10 & 0.5 & 50 & 8.5 & 9.76 & 42.04 & 57.83 & 186.63 \\
11 & 0.5 & 50 & 5.5 & 9.37 & 48.75 & 45.61 & 253.96 \\
12 & 15.25 & 35 & 7 & 19.81 & 154.81 & 164.39 & 496.85 \\
13 & 15.25 & 20 & 7 & 93.63 & 214.01 & 349.63 & 487.57 \\
14 & 30 & 50 & 5.5 & 74.17 & 124.78 & 217.88 & 347.36 \\
15 & 15.25 & 35 & 5.5 & 29.63 & 156.27 & 197.19 & 442.86 \\
16 & 0.5 & 20 & 5.5 & 28.93 & 13.46 & 42.25 & 171.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Antioxidant capacity values obtained by ABTS assay (uM TE) from sea bass side streams extracts using UAE at different extraction times (min), temperature (\({}^{\circ}\)C), and pH. antioxidant activity of the extracts obtained from the studied side streams measured with ABTS assay are shown in Table 6.  
Figure 3: Plots shown in (**A**,**C**,**E**,**G**) indicate the response surface plot for the percentage of antioxidant capacity as a function of the extraction time (min) and temperature (°C) at a fixed pH. The plots in (**B**,**D**,**F**,**H**) show the influence of the different parameters (extraction time, temperature, and pH) on the antioxidant capacity determined as \(\upmu\)M trolox equivalent using ABTS assay. Three-dimensional response surface plots and the graphs of influence of the studied parameters are presented in Figure 4. As shown in Figure 4A,B, the extraction time is the only parameter that significantly increased the antioxidant activity (\(p\) = 0.157) for head.