2.8.6 Physicochemical Method
Different physicochemical pretreatment methods are steam explosion, liquid hot
water, ammonia fibre explosion, and carbon dioxide explosion.
Steam explosion is one of the most sustainable methods among all physicochemical methods as drawback with other methods is that it leads to lignin solubilization.
Steam explosion method involves two steps: auto-hydrolysis which allows the
formation of acetic acid at high temperatures and de-pressurization will break the
bonds present in the complex structure. Major drawback of this method is that due to
incomplete hydrolysis of lignin carbohydrate complex it leads to the formation of
inhibitory compounds.
2.8.7 Chemical Method
Chemical pretreatment methods have been useful lignocellulosic biomass. The
various methods are acid pretreatment, alkali pretreatment, and organosolv
pretreatment. Acid pretreatment method employs the use of dilute acid. This method
is most preferred for non-cellulosic feedstock with no lignin content. Alkali
pretreatment can be used for biomass with high lignin content, however it requires
longer time for release of sugars. Organosolv process involves the use of organic
solvent which aids in hydrolysis of lignin (Kushwaha et al. 2019).
2.8.8 Production Process
Acetone-butanol-ethanol (ABE) fermentation process takes place in two phases:
acidogenesis and solventogenesis (Fig. 2.7).
In the first phase of acidogenesis, Clostridia which is a solvent-producing
heterofermentative ananerobic organism exponentially grows and produces different
end products such as butyrate, acetate, carbon dioxide and hydrogen with minute
quantities of acetoin and lactate. Pyruvate is produced from the sugar which acts as
the carbon source via glycolytic pathway and converted to acetyl Co-A and carbon
dioxide using pyruvate:ferredoxin oxidoreductase enzyme. Two molecules of acetyl
Co-A will combine together to form butyl Co-A with the use of different enzymes
thiolase, 3-hydroxybutyryl-CoA dehydrogenase, crotonase, and butyryl-CoA
dehydrogenase.
Acetyl Co-A and butyl Co-A are important intermediate compounds in the
formation of acid and solvents during the acidogenesis and solventogenesis. During
glycolytic pathway, ATP and NADH are generated. ATP which is produced is
essential for the growth and for recontinuation of the glycolytic pathway for the
continuous cycle NADH is reoxidized to NAD+.
2 Application of Microorganisms for Biofuel Production
63
Different physicochemical pretreatment methods are steam explosion, liquid hot
water, ammonia fibre explosion, and carbon dioxide explosion.
Steam explosion is one of the most sustainable methods among all physicochemical methods as drawback with other methods is that it leads to lignin solubilization.
Steam explosion method involves two steps: auto-hydrolysis which allows the
formation of acetic acid at high temperatures and de-pressurization will break the
bonds present in the complex structure. Major drawback of this method is that due to
incomplete hydrolysis of lignin carbohydrate complex it leads to the formation of
inhibitory compounds.
2.8.7 Chemical Method
Chemical pretreatment methods have been useful lignocellulosic biomass. The
various methods are acid pretreatment, alkali pretreatment, and organosolv
pretreatment. Acid pretreatment method employs the use of dilute acid. This method
is most preferred for non-cellulosic feedstock with no lignin content. Alkali
pretreatment can be used for biomass with high lignin content, however it requires
longer time for release of sugars. Organosolv process involves the use of organic
solvent which aids in hydrolysis of lignin (Kushwaha et al. 2019).
2.8.8 Production Process
Acetone-butanol-ethanol (ABE) fermentation process takes place in two phases:
acidogenesis and solventogenesis (Fig. 2.7).
In the first phase of acidogenesis, Clostridia which is a solvent-producing
heterofermentative ananerobic organism exponentially grows and produces different
end products such as butyrate, acetate, carbon dioxide and hydrogen with minute
quantities of acetoin and lactate. Pyruvate is produced from the sugar which acts as
the carbon source via glycolytic pathway and converted to acetyl Co-A and carbon
dioxide using pyruvate:ferredoxin oxidoreductase enzyme. Two molecules of acetyl
Co-A will combine together to form butyl Co-A with the use of different enzymes
thiolase, 3-hydroxybutyryl-CoA dehydrogenase, crotonase, and butyryl-CoA
dehydrogenase.
Acetyl Co-A and butyl Co-A are important intermediate compounds in the
formation of acid and solvents during the acidogenesis and solventogenesis. During
glycolytic pathway, ATP and NADH are generated. ATP which is produced is
essential for the growth and for recontinuation of the glycolytic pathway for the
continuous cycle NADH is reoxidized to NAD+.
2 Application of Microorganisms for Biofuel Production
63