## 3 Results and Discussion
### The Influence of the Filling Amount and Filling Material on Fermentation
Maximum specific growth rates (max) of 0.21 h\({}^{-1}\) and 0.18 h\({}^{-1}\) have been reported for glycerol and glucose heterotrophic growth in a bioreactor when injected through a nozzle and packed bed [53]. The maximum cell dry weight was 16.60 g/L, the maximum growth rate was 0.13039 h\({}^{-1}\), and the K\({}_{\rm L}\alpha\) was 458 h\({}^{-1}\) when plastic filler material was used; the maximum cell dry weight was 17.70 g/L, the maximum growth rate was 0.09460 h\({}^{-1}\), and the K\({}_{\rm L}\alpha\) was 420 h\({}^{-1}\) when no filler was used; and the maximum CDW was 23.30 g/L, the maximum growth rate was 0.13572 h\({}^{-1}\), and the K\({}_{\rm L}\alpha\) was 800 h\({}^{-1}\) with stainless steel (Table 3). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Unfilled** & **Plastics** & **Stainless Steel** \\ \hline Shape & - & Saddle & 0.9760 \\ Size (mm) & - & \(5\times 9\) & \(5\times 9\) \\ Bed volume (L) & - & \(1\) & \(1\) \\ Total bed height (cm) & - & \(6\) & \(5\) \\ (K\({}_{\rm L}\alpha\)) O\({}_{2}\) (h\({}^{-1}\)) & 420 & 458 & 700 \\ \(\mu\)max (h\({}^{-1}\)) & 0.09460 & 0.13039 & 0.13572 \\ Maximum cell density (CDW, g/L) & 17.70 & 16.60 & 23.30 \\ Average growth rate from 48 h to 120 h (h\({}^{-1}\)) & 0.01431 & 0.01342 & 0.01885 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The overall properties of the filler materials. Figure 4: Stainless steel (**A**) and plastic (**B**) fillers. The stainless steel has a saddling shape, and the shape of the plastic is cylindrical; both materials’ shapes are 9 mm long and 5 mm wide. Studies have shown that excessive filling material can lead to a decrease in fluidity in the reactor, resulting in dead zones, which affect the growth and metabolism of microorganisms and ultimately lead to a decrease in fermentation efficiency or even no effect. A small amount of filler material does not provide sufficient surface area, resulting in a decrease in the growth and metabolic rate of microorganisms, which prolongs the fermentation cycle. During gas fermentation, filler materials can help improve the distribution and exchange of gasses. Small amounts of filler material may cause an uneven gas flow, affecting the dissolution and transfer of oxygen, which in turn can affect the growth of microorganisms. Stainless steel filler material offers the following advantages: high-temperature resistance, corrosion resistance, and wear resistance, while also easy to clean and disinfect; although the cleaning cost is higher, its durability is strong, and its long-term use is more economical. Plastic filler material boasts the following: it is lightweight, low-cost, easy to process and mold, has good chemical stability, and is relatively simple to clean, but it has poor durability and may need to be replaced frequently. In this study, the effects of stainless steel filler and plastic on fermentation efficiency were studied. At the filling amount of C (500 cm\({}^{3}\) on the top and 500 cm\({}^{3}\) on the bottom), the filling effect of readily available commercial plastic (Figure 4B) and stainless steel (Figure 4A) was compared. After 108 h, the CDW value stabilized at 16.5 g/L for plastic and 23.0 g/L for stainless steel (Figure 5a). The CDW value of the stainless steel filler material is 39.39% higher than that of the plastic filler material. The K\({}_{\mathrm{L}}\alpha\) of oxygen determined during fermentation in the presence of stainless steel filler material was 280 h\({}^{-1}\) higher than that in the presence of plastic. Plastic surfaces can be detrimental to the adhesion of microorganisms and affect the formation of biofilms. Stainless steel generally provides a better surface for microbial attachment and promotes biofilm formation and growth. These results show that in this study, stainless steel filler material was preferred over plastic material for fermentation.