## 5 Results and Discussion
### Response Surface Interaction Analysis  
To better understand the interactions between these variables, the response surface diagram of the model was produced. When the feed-liquid ratio (X2) was constant, the theabrownins content gradually increased as the concentration of the bacterial solution (X3) increased (Figure 1A). The maximum theabrownins content was reached when the concentration of the bacterial solution was about \(1\times 10^{7}\) CFU/mL. A higher or lower concentration of the bacterial solution resulted in lower theabrownins content. At the same time, when the concentration of the bacterial solution was constant, the content of theabrownins gradually increased with the material-liquid ratio. The theabrownins content reached the maximum when the solid-to-liquid ratio was 1:30. A higher or lower ratio resulted in lower theabrownins content. The ovality in the contour line was smaller, which proved that the interaction between the two factors had no significant effect on the theabrownins content. It can be seen from Table 6 that the \(p\)-value of the AB interaction = 0.3678 > 0.05, indicating an insignificant impact. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Serial Number** & **FLR** & **BC** & **RS** & **Theabrownins** \\  & **(g/mL)** & **(CFU/mL)** & **(r/min)** & **Content (\%)** \\ \hline
1 & 0 (1:30) & 1 (\(1\times 10^{8}\)) & \(-1\) (150) & 25.01 \\
2 & \(-1\) (1:25) & 0 (\(1\times 10^{7}\)) & 1 (210) & 19.43 \\
3 & \(-1\) (1:25) & \(-1\) (\(1\times 10^{6}\)) & 0 (180) & 18.23 \\
4 & 0 (1:30) & 0 (\(1\times 10^{7}\)) & 0 (180) & 29.5 \\
5 & 1 (1:35) & 0 (\(1\times 10^{7}\)) & 1 (210) & 24.15 \\
6 & 0 (1:30) & 0 (\(1\times 10^{7}\)) & 0 (180) & 28.53 \\
7 & 0 (1:30) & 0 (\(1\times 10^{7}\)) & 0 (180) & 28.97 \\
8 & 0 (1:30) & 1 (\(1\times 10^{8}\)) & 1 (210) & 27.98 \\
9 & \(-1\) (1:25) & 1 (\(1\times 10^{8}\)) & 0 (180) & 18.87 \\
10 & 0 (1:30) & \(-1\) (\(1\times 10^{6}\)) & \(-1\) (150) & 25.67 \\
11 & 1 (1:35) & 0 (\(1\times 10^{7}\)) & \(-1\) (150) & 23.01 \\
12 & 0 (1:30) & 0 (\(1\times 10^{7}\)) & 0 (180) & 28.44 \\
13 & 0 (1:30) & 0 (\(1\times 10^{7}\)) & 0 (180) & 28.86 \\
14 & 0 (1:30) & \(-1\) (\(1\times 10^{6}\)) & 1 (210) & 24.11 \\
15 & \(-1\) (1:25) & 0 (\(1\times 10^{7}\)) & \(-1\) (150) & 17.67 \\
16 & 1 (1:35) & \(-1\) (\(1\times 10^{6}\)) & 0 (180) & 23.42 \\
17 & 1 (1:35) & 1 (\(1\times 10^{8}\)) & 0 (180) & 24.89 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Box–Behnken optimized design and results. Figure 1: Contour line and response surface of the effect of the interaction between the feed-liquid ratio (X\({}_{2}\)) and bacterial concentration (X\({}_{3}\)) on the theabrownins content (**A**). Contour lines and response surface of the effect of the interaction of feed–liquid ratio (X\({}_{2}\)) and rotation speed (X\({}_{6}\)) on the theabrownins content (**B**). Contour lines and response surface of the effect of the interaction of bacterial concentration (X\({}_{3}\)) and rotation speed (X\({}_{6}\)) on the theabrownins content (**C**). When the rotational speed was constant, the the unknowns content gradually increased with the feed-liquid ratio (X\({}_{2}\)) (Figure 1B). When the ratio was 1:30, the theabrownins content reached the maximum. When the feed-liquid ratio continued to increase, the theabrownins content gradually decreased. When the feed-liquid ratio remained unchanged, the theabrownins content gradually decreased upon increasing the rotation speed. When the rotation speed (X\({}_{6}\)) was 180 r/min, the theabrownins content reached the maximum value, and then upon further increasing the rotation speed, the theabrownins content gradually decreased again. The ovality in the contour line was smaller, which proved that the interaction of the two factors had no significant effect on the theabrownins content. It can be seen from Table 6 that the _p_-value of the AC interaction = 0.4954 > 0.05, and the effect was not significant.