Since the early days of civilization, humans have been practicing converting hexose to ethanol with yeast, specifically Saccharomyces species for alcoholic beverages. Thus, it is only natural that this process technology is proven and quite advanced. More recently, many studies have focused on utilizing bacteria, especially Zymomonas mobilis, to produce industrial ethanol from hexose where taste is not at all a factor. In comparison, converting pentose and other sugars to ethanol is a much more recent development and still faces many unresolved challenges.
The most intensively studied fungus capable of cellulose degradation is Trichoderma reesei, which was originally named T. viride but later renamed in honor of a pioneer in cellulase research, E. T. Reese. This fungus prodigiously produces an excellent ensemble of three major classes of cellulase enzymes that function synergistically to convert cellulose all the way to glucose: (a) endoglucanases, which attack at random locations in the cellulose chain, (b) exoglucanases or glucanhydrolases, which liberate glucose dimer cellobiose from the end of the cellulose chain, and (c)
b-glucosidases, which release constituent glucose monomers from cellobiose and soluble cellodextrins. Many less suitable fungi also produce cellulases, but not necessarily in an optimal ensemble. Although T. reesei is efficient in degrading cellulose, it alone cannot convert cellulose directly to a useful final product, unless glucose happens to be the target product. In fact, only a few filamentous fungi are known to convert cellulose efficiently into useful final products. The following are among the few filamentous fungi that are capable of hydrolyzing cellulose as well as converting the sugars liberated from hydrolysis to ethanol or acetic acid: Monilia, Neurospora crassa, and Fusarium oxysporum; the following fungal species are capable of converting xylose to ethanol: Fusarium, Mucor, and Paecilomyces (Singh et al. 1992). The disadvantage of direct conversion is the slow conversion rate compared to yeast, although conversion efficiency can be as high as yeast (Krishna et al. 2001). Thus, economic considerations generally preclude converting cellulose to ethanol with these fungi. Technical feasibility does not translate to economic feasibility. Table 1 summarizes the different types of fungi responsible for converting cellulose to either ethanol or acetic acid.
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