## 2 Results and Discussions  
### Optimization of SSF Conditions  
The formation of bioactive components by fermentation depends on process conditions and determining the right conditions is very important for fermentation [21]. The literature survey showed that the initial moisture content (MC) and water activity of the environment, the selected microorganism, incubation time, and incubation temperature are the factors that have significant effects on the final product [63; 64; 65; 66]. For this reason, the effects of initial moisture content, incubation temperature, and incubation time on total peptide content (TPC) of the fermented hazelnut oil cake (FHOC) after the SSF process were investigated using a Box-Behnken experimental design with 17 runs. The experimental results from all factor combinations and responses of each run are given in Table 1.  
The second-order polynomial regression equation showing the empirical relationship between the TPC and the independent variables in terms of coded levels of SSF conditions is given in Equation (1). \[ \begin{split} 14.1\cdot\chi_{1}\chi_{2}+52.9\cdot X_{1}X_{3}-47.6 \cdot\chi_{2}X_{3}-33.3\cdot\chi_{1}{}^{2}-34.7\cdot\chi_{2}{}^{2}-67.6\cdot \chi_{3}{}^{2}\end{split}  \tag{1}\]  
Analysis of variances (ANOVA) based on the TPC was carried out to evaluate the significance and fitness of the model coefficients is summarized in Table 2. A large value of F and a small value of \(p\) (i.e., \(p<0.05\)) indicate that the model is statistically significant when examining the results of ANOVA [67]. Results given in Table 2 show that the constructed model is significant (\(p<0.05\)). In addition, main terms of initial moisture content and incubation time, their interaction, and quadratic term of incubation time were found to have significant (\(p<0.05\)) effect on TPC response, whereas the incubation temperature did not have a significant (\(p<0.05\)) impact on the TPC. A relatively high R\({}^{2}\) (0.916) value indicated a good correlation between the experimental and predicted responses. Moreover, the predicted R\({}^{2}\) (0.665) was found to be in reasonable agreement with the adjusted R\({}^{2}\) (0.809) since the difference is less than 0.2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Run**} & \multicolumn{3}{c}{**Actual and Coded Levels of Factors**} & \multicolumn{2}{c}{**Response**} \\  & **Initial Moisture** & **Incubation** & **Tremeture (\(\chi_{2}\), \({}^{\circ}\)C)** & **Incubation Time (\(\chi_{3}\), Day)** & **Total Peptide Content (mg** \\  & **Content (\(\chi_{1}\), \%)** & **Temperature (\(\chi_{2}\), \({}^{\circ}\)C)** & **Incubation Time (\(\chi_{3}\), Day)** & **Tryptone eqv/g DM of Solid** \\  & & & & **Substrate)** \\ \hline
1 & 70 (+1) & 30 (0) & 5 (+1) & 369.8 \(\pm\) 27.9 \\
2 & 30 (\(-1\)) & 20 (\(-1\)) & 3.5 (0) & 158.1 \(\pm\) 24.5 \\
3 & 50 (0) & 30 (0) & 3.5 (0) & 346.3 \(\pm\) 84.4 \\
4 & 70 (+1) & 40 (+1) & 3.5 (0) & 235.8 \(\pm\) 81.8 \\
5 & 30 (\(-1\)) & 30 (0) & 5 (+1) & 113.9 \(\pm\) 9.5 \\
6 & 30 (\(-1\)) & 40 (+1) & 3.5 (0) & 146.3 \(\pm\) 77.0 \\
7 & 50 (0) & 20 (\(-1\)) & 5 (+1) & 344.0 \(\pm\) 59.2 \\
8 & 50 (0) & 30 (0) & 3.5 (0) & 212.9 \(\pm\) 38.5 \\
9 & 50 (0) & 30 (0) & 3.5 (0) & 280.9 \(\pm\) 9.8 \\
10 & 50 (0) & 30 (0) & 3.5 (0) & 247.2 \(\pm\) 66.1 \\
11 & 30 (\(-1\)) & 30 (0) & 2 (\(-1\)) & 132.1 \(\pm\) 1.6 \\
12 & 70 (+1) & 30 (0) & 2 (\(-1\)) & 136.5 \(\pm\) 84.5 \\
13 & 50 (0) & 40 (+1) & 2 (\(-1\)) & 104.6 \(\pm\) 45.1 \\
14 & 50 (0) & 20 (\(-1\)) & 2 (\(-1\)) & 66.3 \(\pm\) 32.3 \\
15 & 50 (0) & 40 (+1) & 5 (+1) & 191.8 \(\pm\) 115 \\
16 & 70 (+1) & 20 (\(-1\)) & 3.5 (0) & 304.0 \(\pm\) 139.1 \\
17 & 50 (0) & 30 (0) & 3.5 (0) & 307.9 \(\pm\) 107.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Box–Behnken design of factors with experimental TPC values. The preparation of 3D plots as a function of two parameters while maintaining all other parameters at fixed levels is useful for comprehending the interaction effects of the two parameters [67].