individual lignocellulosic components in different biomass has been shown in Table
4.1.
4.2.1 Lignin
The term lignin was coined by a French chemist and botanist Anselme Payen in 1838
and was chemically defined by Schulze in 1885 (Doherty et al. 2011; Lange et al.
2013; Feofilova and Mysyakina 2016). Lignin encrusts the cellulose and hemicellulose components of plant cell wall and provides impermeability, mechanical
strength, and rigidity to the plant cell wall. Lignin provides defense barrier and
protects the plant against microbial attack. The distribution and concentration of
lignin vary in plant cell wall and between different species (Lange et al. 2013).
Lignin biosynthesis in plants occurs by phenylpropanoid pathway. Lignin monomer
synthesis starts with deamination of phenylalanine to form cinnamic acid through a
series of enzymatic reactions catalyzed by various enzymes (Bonawitz and Chapple
2010). The aromatic ring of cinnamic acid undergoes a series of reactions such as
hydroxylation, O-methylation, and side chain reduction from acid to alcohol, to form
three lignin monomers or monolignols: p-coumaryl alcohol, coniferyl alcohol, and
sinapyl alcohol. After synthesis, the monolignols are transported with the help of
ABC transporter to apoplast. The monolignols are polymerized by radical–radical
coupling generated by various oxidoreductases to form ether and C-C bonds
(Bonawitz and Chapple 2010; Guerriero et al. 2016).
These three phenylpropane units ( p-coumaryl alcohol, coniferyl alcohol, and
sinapyl alcohol) after polymerization get incorporated into lignin polymer as phydroxyphenyl (H), guaiacyl (G), and syringyl (S) units (Bonawitz and Chapple
Table 4.1 Composition of characteristic lignocellulosic feedstock (adapted from Harmsen et al.
2010; Menon and Rao 2012)
Composition of biomass (% dry wt.)
Sources
Cellulose
Hemicellulose
Lignin
Sugarcane bagasse
25–45
28–32
15–25
Corn stover
35.1–39.5
20.7–24.6
11.0–19.1
Rice straw
29.2–34.7
23–25.9
17–19
Wheat straw
35–39
22–30
12–16
Grasses
25–40
25–40
10–30
Switch grass
35–40
25–30
15–20
Eucalyptus
45–51
11–18
29
Poplar wood
45–51
25–28
10–21
Pine
42–49
13–25
23–29
Softwood stem
45–50
24–40
18–25
Water hyacinth
18.2–22.1
48.7–50.1
3.5–5.4
Bamboo
49–50
18–20
23
Cotton stalk
31
11
30
86
M. Kumar et al.
4.1.
4.2.1 Lignin
The term lignin was coined by a French chemist and botanist Anselme Payen in 1838
and was chemically defined by Schulze in 1885 (Doherty et al. 2011; Lange et al.
2013; Feofilova and Mysyakina 2016). Lignin encrusts the cellulose and hemicellulose components of plant cell wall and provides impermeability, mechanical
strength, and rigidity to the plant cell wall. Lignin provides defense barrier and
protects the plant against microbial attack. The distribution and concentration of
lignin vary in plant cell wall and between different species (Lange et al. 2013).
Lignin biosynthesis in plants occurs by phenylpropanoid pathway. Lignin monomer
synthesis starts with deamination of phenylalanine to form cinnamic acid through a
series of enzymatic reactions catalyzed by various enzymes (Bonawitz and Chapple
2010). The aromatic ring of cinnamic acid undergoes a series of reactions such as
hydroxylation, O-methylation, and side chain reduction from acid to alcohol, to form
three lignin monomers or monolignols: p-coumaryl alcohol, coniferyl alcohol, and
sinapyl alcohol. After synthesis, the monolignols are transported with the help of
ABC transporter to apoplast. The monolignols are polymerized by radical–radical
coupling generated by various oxidoreductases to form ether and C-C bonds
(Bonawitz and Chapple 2010; Guerriero et al. 2016).
These three phenylpropane units ( p-coumaryl alcohol, coniferyl alcohol, and
sinapyl alcohol) after polymerization get incorporated into lignin polymer as phydroxyphenyl (H), guaiacyl (G), and syringyl (S) units (Bonawitz and Chapple
Table 4.1 Composition of characteristic lignocellulosic feedstock (adapted from Harmsen et al.
2010; Menon and Rao 2012)
Composition of biomass (% dry wt.)
Sources
Cellulose
Hemicellulose
Lignin
Sugarcane bagasse
25–45
28–32
15–25
Corn stover
35.1–39.5
20.7–24.6
11.0–19.1
Rice straw
29.2–34.7
23–25.9
17–19
Wheat straw
35–39
22–30
12–16
Grasses
25–40
25–40
10–30
Switch grass
35–40
25–30
15–20
Eucalyptus
45–51
11–18
29
Poplar wood
45–51
25–28
10–21
Pine
42–49
13–25
23–29
Softwood stem
45–50
24–40
18–25
Water hyacinth
18.2–22.1
48.7–50.1
3.5–5.4
Bamboo
49–50
18–20
23
Cotton stalk
31
11
30
86
M. Kumar et al.