## 3 Results and Discussion
### The Influence of the Filling Amount and Filling Material on Fermentation  
Here we will summarize some experimental data on the production of SCPs and poly(3-hydroxybutyrate) (PHB) using _C. menactor_ under different fermentation conditions (Table 2). It is worth noting that most of the literature reports provide research on the production of PHB. In fact, nutrient restriction limits cell growth by inhibiting pathways such as protein and nucleic acid synthesis, leading to a decrease in carbon flux and a shift to the synthesis of biopolymers such as PHB [49]. The protein content of the bacteria that produce SCPs is higher than that of the bacteria that produce PHB. Generally, the carbon source used is CO\({}_{2}\), the CDW for the production of PHB is 39.60 g/L at 3 bars, and the CDW for the production of PHB is 17.50 g/L at 1 bar. In this study, the CDW of the non-nitrogen-restricted production of SCPs at 1 bar was 25.17 g/L; in the literature, the CDW of PHB production with other carbon sources such as fructose is 60.94 g/L, and the CDW when using fruit waste as the carbon source is 6.20 g/L (Table 2). By filling the fermentation tank with material (Figure 4), the fermentation broth forms a liquid film on the filler material (Figure 4), which increases the surface area of the liquid-gas interface. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Entry**} & **Carbon** & **Hydrogen** & **Pressure** & **Temperature** & **Products** & **CDW (g/L)** & **References** \\  & **Source** & **Resource** & **(bar)** & **(\({}^{\circ}\)C)** & **Products** & **CDW (g/L)** & **References** \\ \hline
1 & CO\({}_{2}\) & H\({}_{2}\) & 1 & 30 & SCPs & 25.17 & this work \\
2 & CO\({}_{2}\) & H\({}_{2}\) & 3 & 30 & PHB & 39.60 & [50] \\
3 & CO\({}_{2}\) & H\({}_{2}\) & 1 & 30 & PHB & 17.50 & [31] \\
4 & Fructose & - & 1 & 20–40 & PHB & 60.94 & [51] \\
5 & Fruit waste & - & 1 & 37 & PHB & 6.20 & [52] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _C.neator_ uses different substrates to produce different products. Figure 3: Fermentation, in batches containing inoculation amounts of 10\(\%\) (\(\blacksquare\)), 20\(\%\) (\(\bullet\)), and 30\(\%\) (\(\blacktriangle\)), for 144 h in a 5 L reactor at atmospheric pressure. Optical density was measured at 620 nm (**a**) and the cell dry weight was also measured (**b**). The effect of the inoculum amount on fermentation under conditions of optimal filling was assessed. The filling material was stainless steel with a filling volume of 500 cm\({}^{3}\) on the top and 500 cm\({}^{3}\) on the bottom (**c**). All mean values were taken from three samples. In this study, we batch filled the reactor with stainless steel packing in the amounts A (0 cm\({}^{3}\) on the top and 0 cm\({}^{3}\) on the bottom), B (250 cm\({}^{3}\) on the top and 250 cm\({}^{3}\) on the bottom), C (500 cm\({}^{3}\) on the top and 500 cm\({}^{3}\) on the bottom), and D (1000 cm\({}^{3}\) on the top and 1000 cm\({}^{3}\) on the bottom). Bacterial fermentation was carried out under the fermentation conditions of 10% secondary seed liquid inoculum. As can be seen from Figure 5b, when the filling amount is C (500 cm\({}^{3}\) on the top and 500 cm\({}^{3}\) on the bottom), after 108 h, the CDW value stabilizes at about 23.30 g/L; after 132 h, the CDW stabilizes at 22.70 g/L (Figure 3b). The CDW seen with fill volume C (500 cm\({}^{3}\) on top and 500 cm\({}^{3}\) on the bottom) is 60.69%, 2.64%, and 60.69% higher compared to A (0 cm\({}^{3}\) on the top and 0 cm\({}^{3}\) on the bottom), B (250 cm\({}^{3}\) on the top and 250 cm\({}^{3}\) on the bottom), and D (1000 cm\({}^{3}\) on the top and 1000 cm\({}^{3}\) on the bottom), respectively. Stainless steel and plastic batches were therefore used for fermentation at a filling amount of C (500 cm\({}^{3}\) on the top and 500 cm\({}^{3}\) on the bottom) (Figure 4). Fermentation in packed-bed bioreactors is highly dependent on the packing material for its surface area per unit volume [31]. This has been reported in the existing literature.