Fermentation Technologies
 Glucose Utilizing Ethanol Fermentation

Ethanol fermentation with glucose substrate is the most studied of all the sugar fermentation reactions. Actually, even the fermentation processes delineated in the fermentation feed processes are all based on the fermentation of glucose. For these processes, and generally for the fermentation of glucose two types of microbes have been determined to effectively perform the fermentation and are as shown in Tables 1 and 2: The first is for yeasts and the second is for bacteria.

Fermentation by Yeast
The types of yeasts suited for effecting the reaction are as listed. Returning first again to the biophysics of the fermentation mechanism, a first  consideration is the factors that impact the choice of the yeast. Obviously they are quite many and it stands to reason that each one will have a different metabolic reaction and consequently generate a different product mix for the fermentation.

However, even though several microorganisms, including Clostridium sp., are capable of utilizing glucose and fermenting ethanol, and have been considered as ethanologenic microbes, only a few of the yeasts: Saccharomyces cerevisiae (Baker's yeast), Saccharomyces monacensis, and S. uvarum; are actually of primary interest to industrial operations in the fermentation of glucose into ethanol.

The glucose metabolic reaction leading to ethanol fermentation has been studies extensively over the years. The metabolic reaction - or fermentation reaction - begins with the processing of glucose primarily by way of the Embden-Meyerhof pathway followed by the biochemical fermentation reaction.

These studies have also help determine that generally yeasts metabolize glucose to ethanol under anaerobic conditions, none the less, small concentration of oxygen must be provided to the fermenting yeast being a necessary component in

the biosynthesis of polyunsaturated fats and lipids needed by the yeasts: Typical amounts of Oxygen maintained in the broth are 0.05 – 0.10 mm Hg oxygen tension. Fermentation however, does not necessarily have to be carried out in an anaerobic environment; even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as large quantity of sugars are readily available for consumption. Clearly then the abundance of oxygen together limited sugar availability will result in cell mass growth instead of fermentation.

In any case, the conversion of the glucose to ethanol is also never 100 percent, rather the maximum conversion is about 95%, because of the need to partly utilize some nutrients in synthesizing new biomass and other cell maintenance related reactions. The relative requirements for nutrients not utilized in ethanol synthesis are in proportion to the major components of the yeast cell.

Fermentation by Bacteria
The types of bacteria suited for effecting the reaction are as listed. Even though several bacterium have been considered as ethanologenic microbes, the facultative bacterium Zymomonas mobilis are better candidates for industrial alcohol production.  Zymomonas mobilis is a bacterium belonging to the genus Zymomonas.  Z. mobilis possesses advantages over even S. cerevisiae as an ethanologenic microbe with respect to productivity and in tolerating ethanol concentration up to 13 percent.

The metabolic reaction including fermentation reaction in the Z mobilis begins with the processing of glucose primarily by way of the Entner–Deudoroff pathway followed by  the fermentation reaction to produce ethanol and carbon dioxide as the only products analogous to the yeast.

 

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Fermentation Process Design Issues
The concentrations of the various substances can impact the metabolic rate of consumption of other substances. In a detailed analysis, such as is usually required for engineering design needs, all these interactions need to be accounted for, and are being elicited in course of the evolution of the planned analysis.

Even then, deciding on the type of microbe  to use for reactor design is not so simple and each case has to be carefully and thoroughly analyzed in the context of the design objective as well as by-products that result from the fermentation reaction. However, the deployment of these yeast can be carried out in one of two ways: homogenous suspension and immobilized suspension.

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