Page 97 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 73

3.2.1 Bioethanol
An excellent alternative to fossil fuels used in transportation is bioethanol,
which is used as a gasoline additive or as a pure fuel with high efficiency
and performance.
Cellulosic biomass is available in large amounts as an attractive feed-
stock, which is composed of a complex mixture of carbohydrate
polymers.
Since a significant portion of all process costs is about the feedstock, an
economical fermentation process will require rapid and efficient conver-
sion of all sugars present.

3.2.1.1 Saccharification
In the first step, called hydrolysis or saccharification, hydrolytic enzymes—
cellulase and hemicellulase—are utilized to degrade cellulose and hemicel-
lulose to monomeric sugars [3]. Some microorganisms (Clostridium,
Cellulomonas, Trichoderma, Penicillium, Neurospora, Fusarium, Aspergillus, etc.)
show a high cellulolytic and hemicellulolytic activity and are able to fer-
ment monosaccharides [22]. Moreover, there are many variations of
enzymes that are responsible for polysaccharide hydrolysis, most of which
are produced commercially by genetically modified strains of Saccharomyces
cerevisiae [23], T. reesei [24], Fusarium venenatum, Aspergillus oryzae, and
Aspergillus niger [25].
Insoluble cellulose can be hydrolyzed by cellulase enzymes into soluble
sugar monomers that can be metabolized by microorganisms. Three major
groups of enzymes are able to hydrolyze cellulose: (1) endoglucanase (EG)
or 1,4-β-D-glucan-4-glucanohydrolase; (2) exoglucanase, including 1,4-
β-D-glucan glucanohydrolase (also known as cellodextrinase) and 1,4-β-D-
glucan cellobiohydrolase (CBH); and (3) β-glucosidase (BGL) or
β-glucoside glucohydrolase. BGL present in cellulose is the key enzyme
component and completes the final step during cellulose hydrolysis by
converting the cellobiose to glucose. Since it gets inhibited by its product
glucose, one of the major challenges in the bioconversion of lignocellu-
losic biomass into liquid biofuels includes the search for a glucose tolerant
BGL [26]. In this context, intensive attention was addressed to fungi
belonging to the genus Trichoderma and Aspergillus, because of their high-
level production of secreted cellulases [27,28]. Also, Clostridiales (anaero-
bic) and Actinomycetales (aerobic) showed cellulase activity, with different
strategies for the degradation of cellulose [29].