| Butyric acid is one of the short-chain fatty acids with many applications in chemical, foodstuff, and pharmaceutical industries. Currently, butyric acid is synthesized commercially via petrochemical routes. However, the demand for butyric acid from microbial fermentation increased due to a strong preference by consumers for using bio-based natural ingredients, as well as concerns about the future scarcity, cost and environmental impact of fossil fuel. It is thus of interest to produce butyric acid from the fermentation of biomass resources. In the present study, attempts to improve butyric acid fermentation in terms of its final product concentration were conducted by Clostridium tyrobutyricum immobilized in a fibrous bed bioreactor with the cheap biomass such as non-food raw materials and waste biomasses as the substrate.The fermentation results showed that glucose as well as xylose, fructose and sucrose could be well utilized by C. tyrobutyricum to produce butyric acid. In batch FBB fermentation, the optimal medium composition for butyric acid production consisted of 60 g/L carbon source and 10 g/L nitrogen source, and a higher reactor productivity of 3.40 g/(L·h) could be achieved in batch mode. Also, it was proposed that corn steep liquor could be employed as an altertive nitrogen source in the future industrial production of butyric acid. The final concentrations of butyric acid production were 45.4?53.92 g/L and a yiled of 0.41?0.46 g/g in fed-batch FBB fermentation, together with butyrate/acetate ratio reaching a maximum lever of 25.7.Cassava starch and residue should be pretreated before fermentation. Cassava starch hydrolysis with dilute HCl produced 98.81% (w/w) glucose. Butyric acid fenmentation of cassava starch hydrolysate gave a high final concentration of 49.95 g/L and a yiled of 0.44 g/g in fed-batch FBB mode. Cassava residue hydrolysis with dilute HCl at either atmospheric pressure or high pressure produced ~56% total reduding sugars. The fermentation reached a maximum butyric acid concentration of 23.20 g/L and a yield of 0.44 g/g from cassava residue hydrolysate in batch FBB mode. Cane molasses coule be utilized directly after dilution. Batch fermentations were conducted to investigate the influence of pH on the metabolism of the strain with cane molasses as the substrate, and the results showed that optimal pH for butyric acid fermentation was 6.0. It was found that sulfuric acid treatment gave better results regarding butyric acid concentration. Also, fed-batch fermentation from cane molasses pretreated with sulfuric acid was performed to further incease the butyric acid concentration and the butyrate/acetate ratio up to 55.2 g/L and 21.23, which was 59.54% and 1.39, respectively higher than in the batch mode, as well as the high yiled of 0.47 g/g.The transport and uptake of glucose by C. tyrobutyricum were investigated. The presence of a glucose phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) and ATP-dependent glucokinase were confirmed in toluene-treated cells prepared from cultures grown on glucose. Glucose phosphorylation was constitutive. Extract fractionation revealed that both soluble and membrane components were required for PEP-PTS bioactivity. Further resolution of the soluble fraction by elution chromatography demonstrated the presence of the PTS component HPr. This study also provided the evidence for phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) activity in C. tyrobutyricum. Sucrose phosphorylation was inducible and the specificity for sucrose PTS activity resides was proved within the membrane in C. tyrobutyricum. Sucrose-6-phosphate hydrolase and fructokinase activities were also detected in sucrose grown cultures. Based on these findings, a pathway of sucrose metabolism in this organism was proposed.In order to optimize cell immobilization in the FBB to achieve controlled avtivity and immobilization of productive cells for long-term stable operation of the immobilized cell fermentation, the mechanism and factors controlling cell adhesion on fibers including cell surface characteristics, cell age, pH, ionic strength and species, and composition of the media were studied. Cell adhered on the the fiber carrier chemical modified were different compared with the untreated fiber as a function of static adsorption, dynamic adsorption and desorption. Further, an optimal method of cell immobilization was proposed.Through adaptation in the FBB repeated fed-batch fermentation, a high butyric acid concentration of 86.9 g/L was obtained with glucose as the substrate, as well as a high yiled of 0.46 g/g and productivity of 1.1 g/(L·h). This concentration was the highest ever attained in butyric acid fermentation to date. The adapted strain cultured from the FBB was phenotypically and physiologically different from the original strain. Clearly, the ability of the FBB in adapting and selecting acid-tolerant strains for high-producing of butyric acid could make it to be not only an effective means but also an ideal breeding strategy. |
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