Ethanol, fermentation Zymomonas bacteria

As previously mentioned and in the earlier discussion of fermentation methanol, bacteria of the genus Zymomonas such as Z. mobilis are known to convert hexoses to ethanol at high yields and short residence times. These bacteria are facultative anaerobes that have fermentative capacity and convert only glucose, fructose, and sucrose to equimolar quantities of ethanol and CO2 the pentoses are not converted. The Entner-Doudoroff pathway is utilized instead of the Embden-Meyerhof pathway, and a net yield of 1 mol of ATP is generated, not 2 mol as in bakers yeast. But pyruvate is the same key intermediate. In Z. mobilis, it is decarboxylated by pyruvate decarboxylase to yield acetaldehyde which is then reduced to ethanol by alcohol dehydrogenase. 

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

Alcoholic fermentation is the anaerobic transformation of sugars, mainly glucose and fructose, into ethanol and carbon dioxide. This process, which is carried out by yeast and also by some bacteria such as Zymomonas mobilis, can be summarised by this overall reaction. 

Metabolic engineering [39, 40] has been used to impart the capacity for ethanol production and xylose fermentation in E. coli [41-45], Klebsiella oxytoca [A6,A7],Zymomonas mobilis [48,49] andS. cerevisiae [50-53]. In general, attempts at metabolic engineering have been more successful in bacteria than in yeasts. Although the reasons are not entirely clear, the smaller genomes and fewer feedback regulatory factors found in bacteria make these organisms much easier to work with. 

Enzymes can convert lignocellulosic biomass into a suitable fermentation feed-stock for biofuel production. Different yeast strains are used for ethanol production, such as S. diastaticus, Candida sp., S. cerevisiae and K. marxianus, as well as different bacteria such as Zymomonas mobilis. The employment of distillation is desirable for food grade purity of applications other than that of biofuel. In fact, batch fermentation was coupled with a membrane distillation process developed with the application of a membrane distillation bioreactor for ethanol production. Meanwhile,... 

Many bacteria may be used to catalyze the fermentation reaction, and depending on the choice of microorganism and the process conditions, products other than ethanol may be obtained. Microorganisms such as Saccharomyces cerevisae, Zymomonas sp., and Candida sp. are well-known fermentation agents. However, Saccharomyces cerevisae, commonly known as baker s yeast, can be used either in its natural form or as a genetically modified form. 

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