## 2 Results and Discussions
### Characterization of HOC and FHOC  
The color values of HOC was expressed in terms of L*, a* and b* as 81.4 \(\pm\) 1.9, 1.7 \(\pm\) 0.3 and 16.7 \(\pm\) 0.6, respectively. The moisture content of HOC was 8.25 \(\pm\) 0.35 % and water activity was 0.542 \(\pm\) 0.001.  
Statistical analysis (\(t\)-test, \(p\)\(\leq\) 0.05) showed that there were significant differences in all general properties of HOC and FHOC (Table 4). The fermentation process caused an increase in protein, lipid, and ash contents of the FHOC, while it caused a decrease in dry matter and carbohydrate contents. The significant changes during fermentation might be related to microbial activity because microorganisms produce the energy they need during their growth and proliferation primarily by consuming carbohydrates in the HOC and they use the monomers in the medium to synthesize macromolecules such as protein and lipid. The increase in the amount of protein can be associated with the production of enzymes and cellular components in the microorganisms ([PERSON] et al., 2020). The similar increase in the protein content of fermented rapeseed meal by SSF using _Aspergillus oryzae_ was observed in [PERSON] et al. (2019) ([PERSON] et al., 2019). It was also reported that the protein and ash content of fermented linseed meal increased, whereas the carbohydrate content decreased with fermentation process ([PERSON] et al., 2019). In addition to these, [PERSON] et al. (2007) and [PERSON] et al. (2012) observed an increase in both protein and fat content of the meal as a result of soybean meal fermentation ([PERSON] et al., 2020; [PERSON] et al., 2020).  
\begin{table}
\begin{tabular}{c c c} \hline  & **Hazelnut Oil Cake** & **Fermented Hazelnut Oil Cake** \\ \hline Dry matter (\%) & 91.08 \(\pm\) 0.02 & 85.43 \(\pm\) 0.23 \\ Water activity (25 \({}^{\circ}\)C) & 0.733 \(\pm\) 0.00 & 0.704\(\pm\) 0.00 \\ Protein (\%) & 58.02 \(\pm\) 0.77 & 64.64 \(\pm\) 0.28 \\ Lipid (\%) & 8.41 \(\pm\) 0.12 & 9.19 \(\pm\) 0.35 \\ Ash (\%) & 6.05 \(\pm\) 0.00 & 7.11 \(\pm\) 0.0 \\ Carbohydrate (\%) & 27.52 & 19.06 \\ \hline \multicolumn{3}{c}{Color values} \\ \hline L* & 77.34 \(\pm\) 0.03 & 54.83 \(\pm\) 0.38 \\ a* & 2.39 \(\pm\) 0.010 & 6.39 \(\pm\) 0.07 \\ b* & 13.18 \(\pm\) 0.05 & 16.47 \(\pm\) 0.08 \\ \hline \end{tabular}
\end{table}
Table 4: General characteristics of HOC and FHOC.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{3}{*}{**Run**} & **A: Initial** & **B: Incubation** & \multirow{3}{*}{\
\begin{tabular}{c} **Predicted Total** \\ **C** \\ \end{tabular} } & **Actual Total** & \multirow{3}{*}{
\begin{tabular}{c} **Peptide Content** \\ **(mg Typtone** \\ **eqv/g DM of Solid** \\ **Substrate)** \\ \end{tabular} } & **Error (\%)** \\ \hline
1 & 67.3 & 25.8 & 4.3 & 379.6 & 358.9 & 5.8 \\
2 & 64.4 & 20.4 & 4.6 & 391.3 & 354.1 & 10.5 \\
3 & 69.8 & 24.6 & 4.6 & 401.2 & 404.4 & 0.8 \\
4 & 70.0 & 30.0 & 5.0 & 375.4 & 366.1 & 2.5 \\
5 & 60.8 & 24.3 & 4.5 & 370.0 & 425.9 & 13.1 \\ \hline \end{tabular}
\end{table}
Table 3: Predicted and experimental values of TPC for model validation.  
At the end of the fermentation process, darkening in the color of the FHOC was observed. The main reasons for this color formation might be the presence of spores and mycelial structures formed during the development of molds and the presence of color pigments as the secondary metabolites required for microbial development. In addition, the tyrosinase activity shown by _Aspergillus oryzae_ can catalyze enzymatic browning reactions and phenolic oxidation, leading to the formation of melanin and other color pigments [83]. These color changes are common in mold-used SSF systems, and studies suggesting microbial growth monitoring based on color change are also available in the literature [84].