Ethanol cellulose hydrolysis

As mentioned in the biological—biochemical section, another approach to improve alcoholic fermentation combines saccharification and fermentation, ie, simultaneous saccharification and fermentation (SSF). Enzyme-catalyzed cellulose hydrolysis and fermentation to alcohol takes place in the same vessel in the presence of enzyme and yeast (50). Reduced fermenter pressures and enzyme and yeast recycling result in 70 to 80% ethanol yields. These process modifications, coupled with more energy-efficient distillation and heat exchanger improvements, are projected to make fermentation ethanol from low value biomass competitive with industrial ethanol (51). 

When cellulose is used as a raw material, the activity of cellulase (the enzyme catalyzing cellulose hydrolysis) is inhibited by glucose and short cellulose chains. One way to overcome this inhibition is to combine enzymatic hydrolysis with glucose fermentation to ethanol, as the accumulation of ethanol in fermenter does not inhibit cellulase. 

An approach to the production of ethylene from biomass that does not involve pyrolysis is ethanol dehydration. The catalytic conversion of syngas to ethanol from low-grade biomass (or fossil) feedstocks, and fermentation ethanol via advanced cellulose hydrolysis and fermentation methods, which make it possible to obtain high yields of ethanol from low-grade biomass feedstocks as well, are both expected to be commercialized in the United States (Chapter 11). Which technology becomes dominant in the market place has... 

Aden A, Foust T. (2009). Technoeconomic analysis of the dilute sulfuric add and enzymatic hydrolysis process for the conversion of com stover to ethanol. Cellulose, 16(4), 535—545. 

Most cellulases are inhibited by the products of cellulose hydrolysis (i.e, glucose and cellobiose). End-product inhibition resistant cellulases can produce more concentrated mixtures of sugars, which will obviate the need to concentrate them before bioconversion into bioproducts such as ethanol. 

Tsai SL, Oh J, Singh S, Chen R, Chen W. (2009). Functional assembly of miniceUulosomes on the Sacchromyces cerevisiae cell surface for cellulose hydrolysis and ethanol production. Appl Environ Micmbiol, 75, 6087-6093. 

Tsai SL, Goyal G, Chen W. (2010). Surface display of a functional minicellulosome by intracellular complementation using a synthetic yeast consortium and its apphcation to cellulose hydrolysis and ethanol production. Appl Envimn Microbiol, 76, 7514—7520. 

Ethanol production by F. oxysporim F3 was considerably affected by pH of both aerated and non-aerated cultures [64], Optimum values were obtained when the pH of the aerated and non-aerated culture of cellulose were 5.5 and 6.0, respectively. It could be due to the changes induced by low pH to systems involved in cellulose hydrolysis, utilization of sugars for bioethanol production, or both [64], At optimum pH, no insoluble cellulose could be detected in the culture medium. On the other hand, low pH of the aerated culture resulted in low ethanol yield. Adjustment of the initial pH in non-aerated growth to an optimal pH was established to be optimal for both P-glucosidase activity and ethanol production, as a consequence the conversion time resulted to about half [65],... 

Enzymatic hydrolysis Cellulase and P-glucosidase were used Cellulose Hydrolysis rate equivalent to 100% obtained in the absence of an inhibitor (cellobiose) Ethanol Philippidis et al. (1993)... 

Alcoholic Fermentation. Certain types of starchy biomass such as com and high sugar crops are readily converted to ethanol under anaerobic fermentation conditions ia the presence of specific yeasts Saccharomyces cerevisia and other organisms (Fig. 6). However, alcohoHc fermentation of other types of biomass, such as wood and municipal wastes that contain high concentrations of cellulose, can be performed ia high yield only after the ceUulosics are converted to sugar concentrates by acid- or enzyme-catalyzed hydrolysis ... 

Increasingly, biochemical transformations are used to modify renewable resources into useful materials (see Microbial transformations). Fermentation (qv) to ethanol is the oldest of such conversions. Another example is the ceU-free enzyme catalyzed isomerization of glucose to fmctose for use as sweeteners (qv). The enzymatic hydrolysis of cellulose is a biochemical competitor for the acid catalyzed reaction. 

More recently, interest has developed in the use of enzymes to catalyze the hydrolysis of cellulose to glucose (25—27). Domestic or forest product wastes can be used to produce the fermentation substrate. Whereas there has been much research on alcohol fermentation, whether from cereal grains, molasses, or wood hydrolysis, the commercial practice of this technology is primarily for the industrial alcohol and beverage alcohol industries. About 100 plants have been built for fuel ethanol from com, but only a few continue to operate (28). 

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