Page 173 - Lignocellulosic Biomass to Liquid Biofuels

Lignocellulosic biomass to biodiesel 141

The insoluble portion of the biomass is initially broken as a result of
synergistic action of different enzymes because of multistep and heteroge-
neous reactions [81,82].
The hydrolytic enzymes can be divided into the following species:
• Endoglucanases, which hydrolyze randomly internal β-1,4-D glucosidic
linkages in the cellulose chain
• Cellobiohydrolases (CBHs, also known as exoglucanases), which prog-
ress along the cellulose line and cleave off cellobiose units from the
ends
• β-Glucosidases (BG), which hydrolyze cellobiose to glucose and also
cleave off glucose units from cello-oligosaccharides
The synergistic action of these enzymes is able to increase glucose con-
version starting from lignocellulosic biomass. In addition, the introduction
of hemicellulosic enzymes is required, such as:
• Endo-1,4-β-D-xylanases to make xylan chain
• 1,4-β-D-xylosidases to release xylose
• Endo-1,4-β-D-mannanases to break internal bonds to obtain mannans
• 1,4-β-D-mannosidase that cleave molecules of manno-oligosaccharide
in mannose.
Cellulases and hemicellulases described above can be produced by bac-
teria, such as Clostridium, Ruminococcus, Streptomyces, Cellulomonas, Bacillus
and Erwinia, and fungi such as Trichoderma, Penicillium, Fusarium, and
Humicola [36,83]. Among the cellulases produced by various microorgan-
isms, those derived from Trichoderma reesei or from Trichoderma viride have
been widely studied and better characterized. The stability of their enzy-
matic activity and the resistance to the presence of inhibitors are positive
properties, though these enzymes show a reduced BG activity. On the
other hand, cellulases from Aspergillus are able to overcome the inefficien-
cies related to Trichoderma. In many studies, it has been observed a synergy
between enzymes derived from two microbial strains to improve the effi-
ciency of the hydrolysis process [84 88] (Fig. 4.4).
The obstacles still preventing the implementation of this process on an
industrial scale are associated to the cost of the enzymes. In the last years
the intense research activity of different companies, such as Novozymes
and Genencor, that focused on the process operating-cost minimization,
has increased the use of these enzymes [89].
Since industrial processes require high product concentrations, the
inhibition phenomena significantly reduce the reaction rate and the effi-
ciency of enzymatic hydrolysis reactions, both in batch and in continuous