10.10 Bioethanol Industry
The most exciting application of enzymes is producing biofuels from agricultural
residues (e.g., sawdust and switchgrass) and forest residues (Lantana camara and
Prosopis juliflora) using cellulases (Sukumaran et al. 2005; Kuhad et al. 2010b;
Ghosh and Singh 1993). Lignocellulosic biomass to useful products undergoes
several processes, pretreatment, hydrolysis, bioconversion, separation, and purification, to obtain the value-added products (Wyman et al. 2005; Sun and Cheng 2002;
Kuhad et al. 1997). The cost associated with enzymatic hydrolysis is lower compared to that of acid or alkali hydrolysis as enzymatic hydrolysis is performed at
relatively mild conditions (45–50
C temperature and 4–6 pH). Additionally, corrosion is avoided as no alkali or acid is present during the process (Kuhad et al. 2010a;
Ghosh and Singh 1993).
Currently, available bioconversion technologies of lignocellulosic biomass
should be improved to give renewable biofuels and useful chemical by-products to
compete with conventional methods (Kuhad and Singh 1993; Mosier et al. 2005;
Baker et al. 2005; Lee et al. 1995). Two features are most widely practiced to lessen
the cost associated with fuel ethanol production through enzymatic bioconversion of
biomass: one is to optimize the production of cellulase and second is to develop a
catalyst system based on cellulase which will be more effective. Protein engineering
and aimed progression may facilitate to develop better and efficient thermophilic
cellulases. Reusing and recycling of enzymes are other options for reducing the
hydrolysis cost (Kuhad and Singh 1993; Lee et al. 1995; Singh et al. 1991;
Bernardez et al. 1993; Yang and Wyman 2004). Enzymes become deactivated
when they are adsorbed on the substrate, especially in lignin which greatly influence
the recovery of these enzymes. The adsorption process is nonspecific and irreversible when cellulase is adsorbed in lignin (Yang and Wyman 2006; Kumar and
Wyman 2009).
Nevertheless, compounds that attract lignin are being investigated to restrain the
cellulases to get adsorbed in lignin (Tu et al. 2007; Dourado et al. 2002). Several
strategies have been practiced to reuse and recycle cellulases. One is to use the
ultrafiltration method to separate cellulose fraction from sugars and other tiny
compounds which may suppress the enzyme activity, and the other is to recycle
incapacitated enzymes which facilitate the segregation of enzymes from the system
(Lee et al. 1995; Acharya and Chaudhary 2012; Lynd et al. 2005).
Figure 10.5 shows the bioethanol production flow sheet from biomass (Licht
2006). Cellulase-based ethanol production is advantageous over acid-based ethanol
production as it is economically feasible and environmentally benign and has higher
conversion of biomass, zero substrate loss, and neutral and noncorrosive operating
conditions of the conversion process (Kotaka et al. 2008; Huang et al. 2008). It is
reported that crude cellulase (combination of endoglucanase and aspergillus oryzae
β-glucosidase) is used to produce ethanol from barley directly (Bhanja et al. 2009).
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M. S. H. K. Tushar and A. Dutta
The most exciting application of enzymes is producing biofuels from agricultural
residues (e.g., sawdust and switchgrass) and forest residues (Lantana camara and
Prosopis juliflora) using cellulases (Sukumaran et al. 2005; Kuhad et al. 2010b;
Ghosh and Singh 1993). Lignocellulosic biomass to useful products undergoes
several processes, pretreatment, hydrolysis, bioconversion, separation, and purification, to obtain the value-added products (Wyman et al. 2005; Sun and Cheng 2002;
Kuhad et al. 1997). The cost associated with enzymatic hydrolysis is lower compared to that of acid or alkali hydrolysis as enzymatic hydrolysis is performed at
relatively mild conditions (45–50
C temperature and 4–6 pH). Additionally, corrosion is avoided as no alkali or acid is present during the process (Kuhad et al. 2010a;
Ghosh and Singh 1993).
Currently, available bioconversion technologies of lignocellulosic biomass
should be improved to give renewable biofuels and useful chemical by-products to
compete with conventional methods (Kuhad and Singh 1993; Mosier et al. 2005;
Baker et al. 2005; Lee et al. 1995). Two features are most widely practiced to lessen
the cost associated with fuel ethanol production through enzymatic bioconversion of
biomass: one is to optimize the production of cellulase and second is to develop a
catalyst system based on cellulase which will be more effective. Protein engineering
and aimed progression may facilitate to develop better and efficient thermophilic
cellulases. Reusing and recycling of enzymes are other options for reducing the
hydrolysis cost (Kuhad and Singh 1993; Lee et al. 1995; Singh et al. 1991;
Bernardez et al. 1993; Yang and Wyman 2004). Enzymes become deactivated
when they are adsorbed on the substrate, especially in lignin which greatly influence
the recovery of these enzymes. The adsorption process is nonspecific and irreversible when cellulase is adsorbed in lignin (Yang and Wyman 2006; Kumar and
Wyman 2009).
Nevertheless, compounds that attract lignin are being investigated to restrain the
cellulases to get adsorbed in lignin (Tu et al. 2007; Dourado et al. 2002). Several
strategies have been practiced to reuse and recycle cellulases. One is to use the
ultrafiltration method to separate cellulose fraction from sugars and other tiny
compounds which may suppress the enzyme activity, and the other is to recycle
incapacitated enzymes which facilitate the segregation of enzymes from the system
(Lee et al. 1995; Acharya and Chaudhary 2012; Lynd et al. 2005).
Figure 10.5 shows the bioethanol production flow sheet from biomass (Licht
2006). Cellulase-based ethanol production is advantageous over acid-based ethanol
production as it is economically feasible and environmentally benign and has higher
conversion of biomass, zero substrate loss, and neutral and noncorrosive operating
conditions of the conversion process (Kotaka et al. 2008; Huang et al. 2008). It is
reported that crude cellulase (combination of endoglucanase and aspergillus oryzae
β-glucosidase) is used to produce ethanol from barley directly (Bhanja et al. 2009).
292
M. S. H. K. Tushar and A. Dutta