## 3 Results and Discussion
### Biohydrogen Fermentation in 1.5 L Acbr  
In order to further assess the performance and ability of isolated _Enterobacter sp_. KBH 6958 as biohydrogen producer, the strain was used to ferment two different OPEFB-based carbon sources. The first carbon source used was AFEX OPEFB enzymatic hydrolysate and the second carbon source used was DAA OPEFB molasses. AFEX OPEFB hydrolysate was the enzymatic hydrolysate resulting from AFEX pretreatment, whereas DAA OPEFB molasses was the liquid discarded during the pretreatment of OPEFB using dilute acetic acid. Due to the difference of lignocellulosic pretreatment mechanism, the sugar composition of AFEX OPEFB hydrolysate greatly differs from DAA OPEFB molasses. The sugar composition and other soluble compounds found in both media after adjusting initial substrate concentration and pH prior to the batch fermentations in 1.5 L ACBR are summarized in Table 3.  
The final pH recorded after 72 h of fermentation with AFEX OPEFB hydrolysate was 4.67 \(\pm\) 0.2. The AFEX OPEFB hydrolysate contained approximately 84.3% sugar, resulting in a final monosaccharide concentration of 0.55 g/L and 0.39 g/L of glucose and xylose, respectively. The biogas samples were collected at predetermined times, and  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Modicine**} & \multicolumn{8}{c}{**Metabolites**} & \multicolumn{2}{c}{**Ethhydrogen**} \\  & **Burgie Acid** & **Aortic Acid** & **Aortic Acid** & **Fennic Acid** & **Ethanol** & **2-butanediol** & **Other** & **Petertial (mental)** \\ \hline RCM & + & + & - & - & - & - & + & + & Ethanol & 456.6 \(\pm\) 4.9 \\ Glucose and xylose & + & + & - & - & - & - & - & - & Act-amide & 46.1 \(\pm\) 3.5 \\ AFEX & + & - & - & - & - & - & - & Act-amide & 56.6 \(\pm\) 5.3 \\ OFFISPR  
the biohydrogen content in all samples ranged between 40 and 45%. The biogas and biohydrogen gas production profiles were fitted in the Gompertz model, which is widely used to study biogas and biohydrogen production from a variety of substrates due to its excellent ability to provide a good fit to model biohydrogen production during fermentation (\(R^{2}\geq 0.96\)) [36]. The lag phase profile, biohydrogen production potential, and maximum production rate can be determined empirically to fit well with experimentally measured biohydrogen evolution data. As illustrated in Figure 4, the biogas production was observed within the first few hours of fermentation, indicating a very short lag phase in product formation. The AFEX OPEFB hydrolysate afforded a total of 1866 \(\pm\) 50 mL (50.9 mmol/L) biohydrogen gas at the end of fermentation. The maximum biohydrogen productivity (Rmax) was estimated to be 98.1 mL H\({}_{2}\)/h (4.01 mmol H\({}_{2}\)/L/h), with a very short lag phase time of 32 min. The biohydrogen yield was 1.55 mol of H\({}_{2}\)/mol sugar, with the final concentrations of major metabolites amounting to 10.6, 11.8, and 4.56 mmol/L of butyric acid, acetic acid, and ethanol, respectively, while no propionic acid accumulation was evident in the current investigation. The butyric-to-acetic acid ratio (B/A) in this study was 0.89, as is likewise reflected in the form of higher acetic acid compared to butyric acid formation. After 72 h of biohydrogen fermentation with DAA molasses, a total of 2940 \(\pm\) 50 mL biogas was produced, while the total cumulative biohydrogen gas produced was 1275 \(\pm\) 50 mL. It was also observed that the pH fell from 9.10 to 5.1 \(\pm\) 0.2, and approximately 73% of the total sugar was depleted. While the initial pH was observed to be around 9, biohydrogen production only began to accelerate when the pH fell to around 6.6 \(\pm\) 0.3, which coincides to those reported optimum pH for _Enterobacter_ sp. [8; 31; 32].