5.5 Use of Ionic Liquid
Less hazardous solvents such as ionic liquids (ILs) can
facilitate hydrolysis of LC biomass. ILs are salts with low
melting points and show promising ability as a catalyst for
chemical derivatization (Zhu et al. 2006; El Seoud et al.
2007) and nonwoven fiber production (Hermanutz et al.
2006). ILs act on the non-covalent interactions in the LC
without any significant degradation. ILs cleave the linkages
of hemicellulose and lignin (Bhaumik and Dhepe 2016).
Hemicelluloses recovered by IL pretreatment showed
enhanced enzymatic digestibility (Binder and Raines 2010).
The sugars released from ILs treated hemicelluloses are
easily recovered and act as efficient feedstock for ethanol
production and microbial growth (Binder and Raines 2010).
However, high cost of ILs renders them less competitive for
large-scale production (Yang and Wyman 2008).
6 Limitations of Chemical Pretreatment
Chemicals hydrolyze LC biomass by cleaving cellulose and
hemicellulose into individual sugar molecules. As discussed
earlier, strong acids act by disrupting intra and inter-chain
hydrogen bond network and decrystallize cellulose to make
it more accessible to the reagents. However, the adoption of
this technology also has some drawbacks due to the hazardous nature of concentrated acids (Binder and Raines
2010), high cost of materials required for construction of
corrosion resistant reactors (Jönsson and Martín 2016), and
flammability and explosiveness. The formation of furfural,
levulinic acid (LA), and formic acid from LC degradation is
another impediment as these compounds act as inhibitory
by-products and affect the enzymatic hydrolysis or fermentation (Ussiri and Lal 2014).
7 Enzymatic Hydrolysis of Biomass
Enzymatic hydrolysis is a process in which biochemical
conversion of LC biomass (lignin, cellulose, and hemicelluloses) takes place followed by the release of monomeric
sugars. The effectiveness of enzymatic hydrolysis is significant for the proficient transformation of biomass to the ideal
products (Kucharska et al. 2018). Recent reports suggest that
enzymatic hydrolysis has become more suitable because it
offers many advantages over chemical hydrolysis. Enzymatic hydrolysis is environment friendly process as there is
no problem with the handling of enzymes as compared to
acids. It is also not necessary to use costly corrosive resistant
materials. Enzymes are highly selective and exhibit high
specificity to form a single product from their substrates.
Enzymatic processes require normal temperature and pressure to perform, hence, are considered as lower
energy-intensive process. Formation of fewer undesirable
by-products is yet another advantageous factor over acidic or
alkaline hydrolysis (Brummer et al. 2014; Chen 2015).
However, in order for it to work properly, several factors
affecting enzymatic hydrolysis need to be considered and
optimized (Azmi et al. 2017).
7.1 Enzymes Involved in Biomass Hydrolysis
Cellulose and hemicelluloses are the carbohydrate polymers
which are enzymatically hydrolyzed through multistep process by the synergistic action of cellulases and hemicellulases. Table 5 illustrates the major enzymes involved in the
LC biomass degradation. There are four cellulase enzymes
(Endoglucanase, b-glucosidase, cellobiohydrolases, and
exoglucohydrolases) that work together for complete
degradation of cellulose (Wahlström and Suurnäkki 2015).
It is imperative to note that hemicellulose is easily
hydrolyzed than cellulose (Maitan-Alfenas et al. 2015) yet a
more complex group of enzymes called hemicellulases is
involved in its degradation. These include endoxylanases or
Endo-b-1,4-xylanase (Enzyme Commission [EC] number
3.2.1.8), a-glucuronidase (EC 3.2.1.139), b-xylosidase (EC
3.2.1.37),
a-glactosidase
(EC
3.2.1.22),
a-L-arabinofuranosidase (EC 3.2.1.55), ferulic acid esterase
(EC 3.1.1.73), acetyl xylan esterase (EC 3.1.1.72), and
endo-1,4-b-D-mannanase (EC 3.2.1.78) (Kumar and Murthy
2013; Van Dyk and Pletschke 2012). The accessory
enzymes are supplemented during biomass saccharification
to enhance the sugar yield (Robl et al. 2013).
7.1.1 Endo-b-1, 4-Xylanase
Endo-b-1, 4-xylanase, (EC 3.2.1.8), is one of the noteworthy
hydrolytic enzymes among various xylanases that
de-polymerize the xylan to xylobiose and xylooligomers.
b-1, 4-glycosidic internal bonds in the polymer of xylan are
cleaved by these endo-b-1, 4-xylanase (Cha et al. 2014).
Xylanases, in general, were classified into different categories or families of glycosyl hydrolase (GH) number (3, 5,
7, 8, 10, 11, 30, 39, 43, 52, and 54) based on the protein’s
primary structure; whereas, endo-b-1,4-xylanases are included in the GH family 10 and 11 (Bhardwaj et al. 2019).
Several reviews have documented many bacterial and fungal
isolates for endo-b-1, 4-xylanase production (Dhiman and
Mukherjee 2018; Maheshwari et al. 2000; Subramaniyan
and Prema 2002; de Vries and Visser 2001; Manju and
Singh Chadha 2011) among which thermophilic strains are
of biotechnological significance (Manju and Singh Chadha
2011).
Biomass to Xylose
251
Less hazardous solvents such as ionic liquids (ILs) can
facilitate hydrolysis of LC biomass. ILs are salts with low
melting points and show promising ability as a catalyst for
chemical derivatization (Zhu et al. 2006; El Seoud et al.
2007) and nonwoven fiber production (Hermanutz et al.
2006). ILs act on the non-covalent interactions in the LC
without any significant degradation. ILs cleave the linkages
of hemicellulose and lignin (Bhaumik and Dhepe 2016).
Hemicelluloses recovered by IL pretreatment showed
enhanced enzymatic digestibility (Binder and Raines 2010).
The sugars released from ILs treated hemicelluloses are
easily recovered and act as efficient feedstock for ethanol
production and microbial growth (Binder and Raines 2010).
However, high cost of ILs renders them less competitive for
large-scale production (Yang and Wyman 2008).
6 Limitations of Chemical Pretreatment
Chemicals hydrolyze LC biomass by cleaving cellulose and
hemicellulose into individual sugar molecules. As discussed
earlier, strong acids act by disrupting intra and inter-chain
hydrogen bond network and decrystallize cellulose to make
it more accessible to the reagents. However, the adoption of
this technology also has some drawbacks due to the hazardous nature of concentrated acids (Binder and Raines
2010), high cost of materials required for construction of
corrosion resistant reactors (Jönsson and Martín 2016), and
flammability and explosiveness. The formation of furfural,
levulinic acid (LA), and formic acid from LC degradation is
another impediment as these compounds act as inhibitory
by-products and affect the enzymatic hydrolysis or fermentation (Ussiri and Lal 2014).
7 Enzymatic Hydrolysis of Biomass
Enzymatic hydrolysis is a process in which biochemical
conversion of LC biomass (lignin, cellulose, and hemicelluloses) takes place followed by the release of monomeric
sugars. The effectiveness of enzymatic hydrolysis is significant for the proficient transformation of biomass to the ideal
products (Kucharska et al. 2018). Recent reports suggest that
enzymatic hydrolysis has become more suitable because it
offers many advantages over chemical hydrolysis. Enzymatic hydrolysis is environment friendly process as there is
no problem with the handling of enzymes as compared to
acids. It is also not necessary to use costly corrosive resistant
materials. Enzymes are highly selective and exhibit high
specificity to form a single product from their substrates.
Enzymatic processes require normal temperature and pressure to perform, hence, are considered as lower
energy-intensive process. Formation of fewer undesirable
by-products is yet another advantageous factor over acidic or
alkaline hydrolysis (Brummer et al. 2014; Chen 2015).
However, in order for it to work properly, several factors
affecting enzymatic hydrolysis need to be considered and
optimized (Azmi et al. 2017).
7.1 Enzymes Involved in Biomass Hydrolysis
Cellulose and hemicelluloses are the carbohydrate polymers
which are enzymatically hydrolyzed through multistep process by the synergistic action of cellulases and hemicellulases. Table 5 illustrates the major enzymes involved in the
LC biomass degradation. There are four cellulase enzymes
(Endoglucanase, b-glucosidase, cellobiohydrolases, and
exoglucohydrolases) that work together for complete
degradation of cellulose (Wahlström and Suurnäkki 2015).
It is imperative to note that hemicellulose is easily
hydrolyzed than cellulose (Maitan-Alfenas et al. 2015) yet a
more complex group of enzymes called hemicellulases is
involved in its degradation. These include endoxylanases or
Endo-b-1,4-xylanase (Enzyme Commission [EC] number
3.2.1.8), a-glucuronidase (EC 3.2.1.139), b-xylosidase (EC
3.2.1.37),
a-glactosidase
(EC
3.2.1.22),
a-L-arabinofuranosidase (EC 3.2.1.55), ferulic acid esterase
(EC 3.1.1.73), acetyl xylan esterase (EC 3.1.1.72), and
endo-1,4-b-D-mannanase (EC 3.2.1.78) (Kumar and Murthy
2013; Van Dyk and Pletschke 2012). The accessory
enzymes are supplemented during biomass saccharification
to enhance the sugar yield (Robl et al. 2013).
7.1.1 Endo-b-1, 4-Xylanase
Endo-b-1, 4-xylanase, (EC 3.2.1.8), is one of the noteworthy
hydrolytic enzymes among various xylanases that
de-polymerize the xylan to xylobiose and xylooligomers.
b-1, 4-glycosidic internal bonds in the polymer of xylan are
cleaved by these endo-b-1, 4-xylanase (Cha et al. 2014).
Xylanases, in general, were classified into different categories or families of glycosyl hydrolase (GH) number (3, 5,
7, 8, 10, 11, 30, 39, 43, 52, and 54) based on the protein’s
primary structure; whereas, endo-b-1,4-xylanases are included in the GH family 10 and 11 (Bhardwaj et al. 2019).
Several reviews have documented many bacterial and fungal
isolates for endo-b-1, 4-xylanase production (Dhiman and
Mukherjee 2018; Maheshwari et al. 2000; Subramaniyan
and Prema 2002; de Vries and Visser 2001; Manju and
Singh Chadha 2011) among which thermophilic strains are
of biotechnological significance (Manju and Singh Chadha
2011).
Biomass to Xylose
251