Second-Generation Bioethanol: Advancement of Ethanologenic …
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Generally, plants are consists of three biopolymers: cellulose, hemicellulose, and
lignin. Together, these three constituents form a highly complex lignocellulosic
matrix that is unique in composition and degree of complexity with different plant
species, its age, and stage of growth. Cellulose is a linear, unbranched homopolysaccharide consist of β-D-glucose units joined by β–1,4 glycosidic linkage. There is the
formation of hydrogen bonds between hydroxyl groups and oxygen atoms within the
repeated single glucose chain and adjacent glucose chains. In addition, the formation of intermolecular and intramolecular Van der Waals forces attach each cellulose
chain together and promote parallel stacking of cellulose microfibrils (Somerville
2006). Cellulose exists in two forms, namely crystalline and amorphous form. Former configuration requires more specialized enzyme and less susceptible to enzyme
degradation compared with latter configuration (Pérez et al. 2002; Hall et al. 2010).
In the economical perspective, lignocellulosic biomass with high content of cellulose
is preferred because cellulose content is directly proportional with bioethanol yield
(van der Weijde et al. 2013).
Hemicellulose is a complex and diverse heteropolysaccharide comprises hexose (β–D–glucose, α–D–galactose and β–D–mannose), pentose (β–D–xylose
and α–L–arabinose) and uronic acids (α–D–glucuronic, α–D–galacturonic and
α–D–4–O–methylgalacturonic acid). Trace amount of sugars such as α–L–rhamnose
and α–L–fructose may also present in this biopolymer. Hemicellulose is more soluble and relatively easy to hydrolyze compared to cellulose due to their amorphous
form with short lateral chains and lower molecular weight (Saha 2003). The main
structure of hemicellulose is either short branches homopolymer or a heteropolymer
linked by β–1,4 glycosidic linkage and sometimes β–1,3 glycosidic linkage. Hemicelluloses in hardwoods, municipal wastes, and agricultural residues, are typically
xylan while softwoods are highly in glucomannan (Bajpai 2016). Due to sugars
diversity, hemicellulose requires various enzymes to hydrolyze the biopolymer into
fermentable sugars. However, the formation of unwanted products such as furfurals
and hydroxymethyl furfurals must be avoided to prevent inhibition of fermentation
process (Palmqvist and Hahn-Hägerdal 2000).
Lignin is a unique amorphous complex biopolymer that does not saccharify into fermentable sugar. The monomers of lignin are three hydroxycinnamyl
alcohols of p-coumaryl, coniferyl and sinapyl that forms respective aromatic unit phydroxyphenyl, guaiacyl, and syringyl units (Feofilova and Mysyakina 2016). It has
been studied that, lignin from different sources has different ratios of these aromatic
units which in turn affect enzymatic hydrolysis of cellulose (Studer et al. 2011).
Lignin functions as a supportive polymer that strengthens cell walls within xylem
tissue, forming dense structure that binds cellulose microfibrils and other cell walls
components, thus giving rigid support and prevents collapsing of vascular plants
(Martone et al. 2009). Among lignocellulosic biomass, softwoods have highest lignin
content with 30–60% (dry weight), followed by hardwoods with 30–55%. Grasses
and agricultural residues have lower lignin content with 10–30% and 3–15%, respectively (Limayem and Ricke 2012). Typical composition inside the woody biomass is
presented in Fig. 2. Currently, most lignin residue is burned for the source of heat and
power for the processing plant, despite many potential applications (Yuan et al. 2013).
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Generally, plants are consists of three biopolymers: cellulose, hemicellulose, and
lignin. Together, these three constituents form a highly complex lignocellulosic
matrix that is unique in composition and degree of complexity with different plant
species, its age, and stage of growth. Cellulose is a linear, unbranched homopolysaccharide consist of β-D-glucose units joined by β–1,4 glycosidic linkage. There is the
formation of hydrogen bonds between hydroxyl groups and oxygen atoms within the
repeated single glucose chain and adjacent glucose chains. In addition, the formation of intermolecular and intramolecular Van der Waals forces attach each cellulose
chain together and promote parallel stacking of cellulose microfibrils (Somerville
2006). Cellulose exists in two forms, namely crystalline and amorphous form. Former configuration requires more specialized enzyme and less susceptible to enzyme
degradation compared with latter configuration (Pérez et al. 2002; Hall et al. 2010).
In the economical perspective, lignocellulosic biomass with high content of cellulose
is preferred because cellulose content is directly proportional with bioethanol yield
(van der Weijde et al. 2013).
Hemicellulose is a complex and diverse heteropolysaccharide comprises hexose (β–D–glucose, α–D–galactose and β–D–mannose), pentose (β–D–xylose
and α–L–arabinose) and uronic acids (α–D–glucuronic, α–D–galacturonic and
α–D–4–O–methylgalacturonic acid). Trace amount of sugars such as α–L–rhamnose
and α–L–fructose may also present in this biopolymer. Hemicellulose is more soluble and relatively easy to hydrolyze compared to cellulose due to their amorphous
form with short lateral chains and lower molecular weight (Saha 2003). The main
structure of hemicellulose is either short branches homopolymer or a heteropolymer
linked by β–1,4 glycosidic linkage and sometimes β–1,3 glycosidic linkage. Hemicelluloses in hardwoods, municipal wastes, and agricultural residues, are typically
xylan while softwoods are highly in glucomannan (Bajpai 2016). Due to sugars
diversity, hemicellulose requires various enzymes to hydrolyze the biopolymer into
fermentable sugars. However, the formation of unwanted products such as furfurals
and hydroxymethyl furfurals must be avoided to prevent inhibition of fermentation
process (Palmqvist and Hahn-Hägerdal 2000).
Lignin is a unique amorphous complex biopolymer that does not saccharify into fermentable sugar. The monomers of lignin are three hydroxycinnamyl
alcohols of p-coumaryl, coniferyl and sinapyl that forms respective aromatic unit phydroxyphenyl, guaiacyl, and syringyl units (Feofilova and Mysyakina 2016). It has
been studied that, lignin from different sources has different ratios of these aromatic
units which in turn affect enzymatic hydrolysis of cellulose (Studer et al. 2011).
Lignin functions as a supportive polymer that strengthens cell walls within xylem
tissue, forming dense structure that binds cellulose microfibrils and other cell walls
components, thus giving rigid support and prevents collapsing of vascular plants
(Martone et al. 2009). Among lignocellulosic biomass, softwoods have highest lignin
content with 30–60% (dry weight), followed by hardwoods with 30–55%. Grasses
and agricultural residues have lower lignin content with 10–30% and 3–15%, respectively (Limayem and Ricke 2012). Typical composition inside the woody biomass is
presented in Fig. 2. Currently, most lignin residue is burned for the source of heat and
power for the processing plant, despite many potential applications (Yuan et al. 2013).