In opposition to cellulose, hemicelluloses are not chemically homogeneous and
its compositions present great variation in proportion and content, depending on
species, tissue kind, growth phase, and environmental and physiological conditions
(Fengel and Wegner 1984; Brienzo et al. 2010; Brienzo et al. 2014). There is a great
variety in content and composition of hemicelluloses in stalks, leaves, roots, and
rinds. For example, the wheat hemicelluloses present different degrees of substitution between their tissues. The relation arabinose/xylose equals 1, when the external
part of the grain (pericarp) is analyzed, while the internal part presents low substitution degree (0.3) (Brillouet and Joseleau 1987). Xylan is the main polysaccharide
found in hemicelluloses from hardwoods, while mannan prevails in softwoods (Gao
et al. 2013).
Hemicelluloses present an amorphous and hydrophilic structure and, thus, can be
removed from cell walls with more ease than cellulose. Xylose and xylooligomers
are frequently the main products obtained by pretreating and by enzymatic hydrolysis of hemicelluloses, being able to be used as fermentable sugars for bioethanol and
organic acids generation, for example (Gao et al. 2013).
Other products of aggregated value of intermediary compounds, such as xylitol,
furfural, and levulinic acid, used as producing chemicals and polymers, can also be
generated from hemicelluloses through appropriate catalytic approaches (Werpy
et al. 2004; Alonso et al. 2010). Other authors have suggested that the hemicellulose
content and composition may also affect the recalcitrance of the cell wall (York and
O’neill 2008).
It is considered that interactions between microfibers from hemicelluloses and
cellulose and the lignin-carbohydrate connections can stop enzyme attack
(Chundawat et al. 2011; HSU 1996). The transgenic Arabidopsis, with less content
of methyl groups in the lateral glucuronoxylan chains, has released more xylose than
the type of wild control in less severe conditions after the enzymatic hydrolysis
(Urbanowicz et al. 2012).
2.7
Biomass Quality
Biomass quality can be classified by its physical properties and its component
characteristics, such as density, particle size, calorific value, specific gravity, moisture content, extractive content, ash (as mineral content), and lignocellulosic composition (Kenney et al. 1990). To exemplify some of these properties, let us start with
cellulose crystallinity. More organized regions of cellulose chain are called crystalline regions, differing them from the less systematic amorphous regions. Formed by
microfibrils, crystalline regions present a more coordinated cellulose chain than the
amorphous regions (Nakamura et al. 2014). These crystalline regions provide a
higher recalcitrance to the biomass, making it harder for enzymes to reach cellulose
and diminishing digestibility and accessibility (Zhao et al. 2012). Moving on, the
polymerization degree impacts accessibility and can raise recalcitrance.
Carbohydrates, more specifically glucose units, form cellulose. The quantity of
these units in a cellulose chain determines the degree of polymerization. If cellulose
46
F. L. Shimizu et al.
its compositions present great variation in proportion and content, depending on
species, tissue kind, growth phase, and environmental and physiological conditions
(Fengel and Wegner 1984; Brienzo et al. 2010; Brienzo et al. 2014). There is a great
variety in content and composition of hemicelluloses in stalks, leaves, roots, and
rinds. For example, the wheat hemicelluloses present different degrees of substitution between their tissues. The relation arabinose/xylose equals 1, when the external
part of the grain (pericarp) is analyzed, while the internal part presents low substitution degree (0.3) (Brillouet and Joseleau 1987). Xylan is the main polysaccharide
found in hemicelluloses from hardwoods, while mannan prevails in softwoods (Gao
et al. 2013).
Hemicelluloses present an amorphous and hydrophilic structure and, thus, can be
removed from cell walls with more ease than cellulose. Xylose and xylooligomers
are frequently the main products obtained by pretreating and by enzymatic hydrolysis of hemicelluloses, being able to be used as fermentable sugars for bioethanol and
organic acids generation, for example (Gao et al. 2013).
Other products of aggregated value of intermediary compounds, such as xylitol,
furfural, and levulinic acid, used as producing chemicals and polymers, can also be
generated from hemicelluloses through appropriate catalytic approaches (Werpy
et al. 2004; Alonso et al. 2010). Other authors have suggested that the hemicellulose
content and composition may also affect the recalcitrance of the cell wall (York and
O’neill 2008).
It is considered that interactions between microfibers from hemicelluloses and
cellulose and the lignin-carbohydrate connections can stop enzyme attack
(Chundawat et al. 2011; HSU 1996). The transgenic Arabidopsis, with less content
of methyl groups in the lateral glucuronoxylan chains, has released more xylose than
the type of wild control in less severe conditions after the enzymatic hydrolysis
(Urbanowicz et al. 2012).
2.7
Biomass Quality
Biomass quality can be classified by its physical properties and its component
characteristics, such as density, particle size, calorific value, specific gravity, moisture content, extractive content, ash (as mineral content), and lignocellulosic composition (Kenney et al. 1990). To exemplify some of these properties, let us start with
cellulose crystallinity. More organized regions of cellulose chain are called crystalline regions, differing them from the less systematic amorphous regions. Formed by
microfibrils, crystalline regions present a more coordinated cellulose chain than the
amorphous regions (Nakamura et al. 2014). These crystalline regions provide a
higher recalcitrance to the biomass, making it harder for enzymes to reach cellulose
and diminishing digestibility and accessibility (Zhao et al. 2012). Moving on, the
polymerization degree impacts accessibility and can raise recalcitrance.
Carbohydrates, more specifically glucose units, form cellulose. The quantity of
these units in a cellulose chain determines the degree of polymerization. If cellulose
46
F. L. Shimizu et al.