## 3 Results and Discussion
### Biohydrogen Fermentation in 1.5 L Acbr
The time-course profile of biohydrogen production was fitted to the Gompertz model and is depicted in Figure 5. Based on the Gompertz computation, the maximum biohydrogen production rate, \(R_{\text{max}}\), was 81.1 mL H\({}_{2}\)/h (3.07 mmol H\({}_{2}\)/L/h), the lag time was 2.14 h, and the biohydrogen yield was 1.19 mol H\({}_{2}\)/mol sugar. Figure 4: The cumulative biogas and biohydrogen produced from fermentation of AFEX OPEFB hydrolysate in 1.5 L ACBR fitted in the Gompertz equation. The soluble metabolite products (SMP) found at the end of batch fermentation with DAA OPEFB molasses were butyric acid, acetic acid, and ethanol with concentrations of 8.64, 6.01, and 8.04 mmol/L, respectively. The butyric acid-to-acetic acid ratio (B/A) was 1.44, indicating that butyrate-type acidogenesis is the predominant pathway in this fermentation. Higher B/A ratio, lower biohydrogen yield, and the presence of ethanol and propionic acid (2.43 mmol/L) in the fermentation broth suggested that the potential of hydrogen production from OPEFB molasses has yet to be fully realised. As can be seen in Table 3, both media represented two extremes of fermentation medium conditions for biohydrogen production potential. Earlier biohydrogen production tests in serum bottles clearly demonstrated that AFEX OPEFB hydrolysate had the highest biohydrogen potential when used as the carbon source, while DAA OPEFB molasses had the lowest biohydrogen potential. AFEX OPEFB hydrolysate was found to be a more favorable medium for biohydrogen production because it was rich in glucose (hexose) and contained no sugar-degraded compounds, such as furfural and HMF. DAA molasses, on the other hand, represented the other extreme of a less favorable medium for microbial growth due to the significant amount of furfural. Furthermore, the major soluble sugars found in DAA molasses were made of pentose and oligosaccharides. The maximum theoretical biohydrogen yield from pentose with butyric acid as major metabolite was 1.67 mol H\({}_{2}\)/mol pentose, which is lower than maximum theoretical biohydrogen yield of 2.0 mol H\({}_{2}\)/mol hexose with butyric acid as major metabolite [37; 38]. Hence, a pentose enriched media will theoretically yield lower biohydrogen gas as compared with hexose-enriched media. As for the performance of both carbon sources' utilization, the biohydrogen yields in the current study were lower than that reported by [PERSON] et al. [31], who attained up to 1.8 mol H\({}_{2}\)/mol glucose using mutant _Enterobacter aerogenes_ IIU-101. [PERSON] et al. [8] reported 1.8 mol H\({}_{2}\)/mol glucose yield in batch biohydrogen fermentation using _Enterobacter cloace_ KBH3. In both cases, however, the researchers used refined, synthetic glucose-enriched  
Figure 5: Gompertz curve fitting graph of biogas and biohydrogen evolved at initial pH 9 and substrate level 7.0 g/L of DAA molasses in 1.5 L ACBR. medium, as opposed to the complex media, i.e., AFEX OPEFB hydrolysate and DAA molasses used in this study. Nonetheless, as stated by previous researchers, one possible strategy for increasing biohydrogen production from biomass is through the genetic recombination of microbes and decoupling microbial growth from product formation [39].