## 3 Results and Discussion
### Protein Extraction
Figure 2A,C,E,G represents the estimated response surface by plotting the protein recoveries from sea bass head, skin, bone, and viscera versus the extraction time, temperature, and a fixed pH, for each side stream, while Figure 2B,D,F,H represents the influence of the studied parameters on the protein recovery. As can be observed in Figure 2A,B, under the tested treatment conditions, the protein recovery from head extracts increased with the elapse of extraction time from 0.5 to 15.25 min and increased temperature (from 20 to 35 \({}^{\circ}\)C), respectively. However, when both extraction time and temperature increased up to 30 min and 50 \({}^{\circ}\)C, the protein recovery reached a plateau and slowly decreased. Nevertheless, the effects of these parameters are not statistically significant (\(p>0.05\)). On the other hand, the pH significantly (\(p<0.05\)) affected the recovery of proteins, where a lower pH lead to a higher recovery (\(p=0.0091\)). According to RSM, the optimal conditions for the recovery of proteins (40.65%) from head extracts are 15 min of UAE at 35 \({}^{\circ}\)C and 5.5 pH. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Extraction Time (min)** & **Temperature (\({}^{\circ}\)C)** & **pH** & \multicolumn{4}{c}{**Protein Recovery \%**} \\ \hline  & & & **Head** & **Skin** & **Bone** & **Viscera** \\ \hline
1 & 30 & 50 & 8.5 & 17.45 & 25.07 & 75.07 & 93.21 \\
2 & 0.5 & 20 & 8.5 & 12.51 & 19.15 & 45.36 & 70.10 \\
3 & 0.5 & 35 & 7 & 15.78 & 13.64 & 23.63 & 80.48 \\
4 & 30 & 35 & 7 & 12.83 & 31.68 & 44.26 & 93.66 \\
5 & 30 & 20 & 5.5 & 24.15 & 14.11 & 56.50 & 85.35 \\
6 & 15.25 & 35 & 7 & 17.85 & 12.54 & 31.87 & 85.93 \\
7 & 30 & 20 & 8.5 & 14.96 & 12.49 & 63.91 & 82.26 \\
8 & 15.25 & 35 & 8.5 & 33.46 & 18.95 & 37.94 & 90.03 \\
9 & 15.25 & 50 & 7 & 17.47 & 20.26 & 33.52 & 86.42 \\
10 & 0.5 & 50 & 8.5 & 21.87 & 19.23 & 54.75 & 92.50 \\
11 & 0.5 & 50 & 5.5 & 31.12 & 17.12 & 35.09 & 84.81 \\
12 & 15.25 & 35 & 7 & 20.32 & 17.19 & 36.04 & 93.01 \\
13 & 15.25 & 20 & 7 & 25.32 & 12.41 & 42.56 & 81.38 \\
14 & 30 & 50 & 5.5 & 28.10 & 24.95 & 38.66 & 99.37 \\
15 & 15.25 & 35 & 5.5 & 39.89 & 24.78 & 38.28 & 84.73 \\
16 & 0.5 & 20 & 5.5 & 31.11 & 18.79 & 36.68 & 77.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Percentage of protein recovered from sea bass side streams extracted using UAE at different extraction times (min), temperature (\({}^{\circ}\)C), and pH. Figure 2: Plots shown in (**A**,C,E,G**) indicate the response surface plot for the percentage of recovered protein as a function of the extraction time (min) and temperature (\({}^{\circ}\)C) at fixed pH. The plots in (**B,D,F,H**) show the influence of the different parameters (extraction time, temperature, and pH) on the recovery of protein (%) from sea bass side streams. As shown in Figure 2C,D, increasing extraction times from 15 to 30 min with simultaneous increase of temperature up to 35 \({}^{\circ}\)C, progressively increased the recovery of proteins from skin. However, none of the studied parameters had a statistically significant impact (\(p>0.05\)) on the recovery of proteins from skin extracts. The optimal conditions generated by RSM were extraction time 30 min, temperature 37 \({}^{\circ}\)C, pH 5.5 with a 28.13% protein recovery. For the bone and viscera (Figure 2E-H), the percentage of protein recovery significantly increased as the extraction time increased (\(p\leq 0.01\)). Higher pHs had a positive effect on the recovery of protein from bone (\(p=0.0125\)). On the other hand, higher temperatures strongly affected the recovery from the viscera extracts (\(p=0.0072\)). Consequently, under the optimal conditions of UAE (30 min, 50 \({}^{\circ}\)C, and 8.5 pH), 70.25% of proteins were recovered from bone extracts. Likewise, 96.07% of proteins from viscera extracts were recovered under optimal UAE (30 min, 50 \({}^{\circ}\)C, pH 5.5).