Biotechnological Applications of Hemicellulosic Derived Sugars: State-of-the-Art - Sustainable Biotechnology: Sources of Renewable Energy - page 72

fermentability of hydrolysates, substantially eliminating phenolic compounds. An
enzymatic method using laccase was developed to eliminate the impurities of phe-
nolic monomers and phenolic acids from hemicellulosic hydrolysates of sugarcane
bagasse [24].
3.1.2 Enzymatic Hydrolysis
Hemicellulases, which catalyze the hydrolysis of plant cell polysaccharides, are
multi-domain proteins generally containing structurally discrete catalytic and non-
catalytic modules [27]. The most important non-catalytic modules consist of
carbohydrate binding domains (CBD), which facilitate the targeting of the enzyme
to the polysaccharide, interdomain linkers, and dockerin modules. The dockerin
modules mediate the binding of the catalytic domain via cohesion-dockerin inter-
actions, either to the microbial cell surface or to enzymatic complexes such as the
cellulosome [27, 28].
The coordinated action of hemicellulases is necessary to obtain a satisfactory
yield of pentose sugars from lignocellulosic as summarized in Fig. 2. Therefore,
the development of low-cost and commercial hemicellulases is expected to be a
limelight research area for cellulosic ethanol production. Table 2 shows the hemicel-
lulase titers from different microorganisms and their mechanistic applications [29].
3.2 Hemicellulose Hydrolysates into Products
of Industrial Significance
3.2.1 Ethanol
Bioethanol is a clean-burning (emits less CO 2 and other green house gasses due to
availability of free O 2 ), non-petroleum liquid fuel that is considered to be a safe sup-
plement to gasoline for transportation. The production and combustion of ethanol
do not contribute to the total amount of carbon dioxide in the atmosphere [3, 21].
Ethanol can be mixed with gasoline in 10% (E10), 20% (E20), and 22% (E22)
blends without engine modifications, but higher-level blends (such as 85% or 95%)
require some engine modification. As a fuel additive, ethanol provides oxygen to
the fuel, thus improving fuel combustion and reducing tailpipe emissions of carbon
dioxide and unburned hydrocarbons.
Microorganisms
One of the main industrial uses of microorganisms has been alcoholic fermenta-
tion. The giant “microbial libraries” in current vogue can be studied for microbes
that convert cheaper carbohydrates into value-added products, which can serve as
raw materials for the fermentation of hemicellulosic-derived sugars into valuable
commercial commodities [30]. The bioconversion process holds more promise of
utilizing both hexose and pentose sugars from lignocellulosic materials. Microbial