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Cellulosic Materials- The cellulose is first converted into glucose and other fermentable sugars, which are then converted into alcohol using yeast. The current process is based on the following three microorganisms:

(i) Trichoderma reisei (aerobic fungus producing cellulase converts cellulose into glucose),

(ii) S. cerevisiae ferments hexoses to produce alcohol and

iii) recombinant E. coli to convert pentoses since yeasts do not utilize pentoses.

The organisms which utilize pentoses since yeast do not utilize pentoses. The organisms, which utilize pentoses, e.g., E. coli and Klebsiella oxytoca, are poor producers of ethanol.

The genes encoding pyruvate decarboxylase and acetaldehyde dehydrogenase have been transferred from Zymomonas mobilis into E. coli and K. oxytoca. If all these functions were performed by a single organism with high production rates, the cost of production will be markedly reduced.

Thus conversion of cellulose to ethanol involves the following four steps:

(i) production of cellulase,

(ii) hydrolysis of cellulose,

(iii) hexose fermentation and

(iv) pentose fermentation.

 

The hexoses are obtained from hydrolysis of cellulose in form of glucose and cellobiose; hemicellulose digestion gives rise to pentoses. The cellulosic biomass must be suitably pretreated, e.g., a brief exposure to high temperature in combination with acid or ammonia and/or explosive decompression, to make the cellulose available for digestion by cellulase.

The areas for further improvements to reduce cost of production are

(i) pretreatment and

(ii) biological conversion.

 

 

Biological conversion can be improved by the following approaches:

(i) by making efficient ethanol producers like yeast and Z. mobilis more capable of utilizing the substrate, and

(ii) by increasing the ethanol producing ability of organisms, which have the ability to utilize cellulose and hemicellulose, e.g., thermophilic bacteria. It is expected that in near future, enzymic hydrolysis and fermentation of cellulose to obtain ethanol would become an economically viable activity.
 

 
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