linkages of starch converting it to shorter chains such as dextrins, maltose, and
maltotriose (Pandey et al. 2000). The thermophilic Bacillus licheniformis and
recombinant Escherichia coli are usually used for production of heat-stable
α-amylase (Sanchez and Cardona 2008; Rakin et al. 2009). The saccharification of
this liquefied starch to glucose using enzyme glucoamylase is the second step after
liquefaction and usually carried out at relatively lower temperature (30–50
C)
(Plumier et al. 2015). The major source of the glucoamylase is Aspergillus niger
or Rhizopus species (Rakin et al. 2009)
7.3.2.2 Hydrolysis of Holocellulose to Fermentable Sugar for Bioethanol
Production
The pretreatment of biomass usually results in water-insoluble solid and liquid
fraction. Based on the compositional analysis of the water-soluble and -insoluble
fractions, these fractions are subjected to different hydrolysis methods to depolymerize celluloses and hemicelluloses completely to fermentable sugar (Girio et al.
2010). The acid-assisted hydrolysis is performed using dilute and concentrated acid.
The major acids used for this method are sulfuric acid certain inorganic acids such as
HCl, HNO 3 or H 3 PO 4 , and CF 3 COOH (Girio et al. 2010). The dilute acid hydrolysis
is performed at high temperature for short duration, whereas the concentrated acid
hydrolysis performed at low temperature for long reaction time (Chandel et al.
2007). The hydrolysis of hemicelluloses using dilute acid can be usually performed
at low temperature as depolymerization of hemicelluloses is maximum at low
temperature, whereas the conversion of cellulose to glucose is usually performed
at high temperature (230–240
C). For dilute acid treatment, the acid concentration
usually ranges from 0.5% to 1.5% (Balat 2011). The acid concentration for
concentrated acid hydrolysis usually ranges between 41% and 100% (Fengel and
Wegener 2011) with longer residence time and moderate process temperature as
compared to dilute acid hydrolysis (Zhang et al. 2007). The limitation associated
with acid-based hydrolysis is a requirement of specialized non-corrosive
equipments, requirement for recovery or neutralization of acid, and inhibitory
product generation which is toxic for subsequent fermentation stage. Several
advantages are associated with the enzymatic hydrolysis such as low temperature
requirement (45–50
C), no corrosion of equipment, and no or less sugar degradation
and inhibitory products (Duff and Murray 1996). Therefore, the cellulose and
hemicelluloses can be hydrolyzed using cellulase and hemicellulase enzyme, respectively. Subjecting the C. vulgaris and Scenedesmus sp. to commercial enzymatic
cocktails (Viscozyme, Celluclast, and Pectinex) resulted in 84% total sugar (Mahdy
et al. 2014).
7.3.3 Ethanol Fermentation Using Different Feedstocks
Alcoholic fermentation can be defined as the biochemical conversion of fermentable
sugars to alcohol in the presence of microorganisms (Balat and Balat 2009). A model
reaction for representing the ethanol generation is conversion of one molecule of
7 Bioethanol Production: Generation-Based Comparative Status Measurements
171
maltotriose (Pandey et al. 2000). The thermophilic Bacillus licheniformis and
recombinant Escherichia coli are usually used for production of heat-stable
α-amylase (Sanchez and Cardona 2008; Rakin et al. 2009). The saccharification of
this liquefied starch to glucose using enzyme glucoamylase is the second step after
liquefaction and usually carried out at relatively lower temperature (30–50
C)
(Plumier et al. 2015). The major source of the glucoamylase is Aspergillus niger
or Rhizopus species (Rakin et al. 2009)
7.3.2.2 Hydrolysis of Holocellulose to Fermentable Sugar for Bioethanol
Production
The pretreatment of biomass usually results in water-insoluble solid and liquid
fraction. Based on the compositional analysis of the water-soluble and -insoluble
fractions, these fractions are subjected to different hydrolysis methods to depolymerize celluloses and hemicelluloses completely to fermentable sugar (Girio et al.
2010). The acid-assisted hydrolysis is performed using dilute and concentrated acid.
The major acids used for this method are sulfuric acid certain inorganic acids such as
HCl, HNO 3 or H 3 PO 4 , and CF 3 COOH (Girio et al. 2010). The dilute acid hydrolysis
is performed at high temperature for short duration, whereas the concentrated acid
hydrolysis performed at low temperature for long reaction time (Chandel et al.
2007). The hydrolysis of hemicelluloses using dilute acid can be usually performed
at low temperature as depolymerization of hemicelluloses is maximum at low
temperature, whereas the conversion of cellulose to glucose is usually performed
at high temperature (230–240
C). For dilute acid treatment, the acid concentration
usually ranges from 0.5% to 1.5% (Balat 2011). The acid concentration for
concentrated acid hydrolysis usually ranges between 41% and 100% (Fengel and
Wegener 2011) with longer residence time and moderate process temperature as
compared to dilute acid hydrolysis (Zhang et al. 2007). The limitation associated
with acid-based hydrolysis is a requirement of specialized non-corrosive
equipments, requirement for recovery or neutralization of acid, and inhibitory
product generation which is toxic for subsequent fermentation stage. Several
advantages are associated with the enzymatic hydrolysis such as low temperature
requirement (45–50
C), no corrosion of equipment, and no or less sugar degradation
and inhibitory products (Duff and Murray 1996). Therefore, the cellulose and
hemicelluloses can be hydrolyzed using cellulase and hemicellulase enzyme, respectively. Subjecting the C. vulgaris and Scenedesmus sp. to commercial enzymatic
cocktails (Viscozyme, Celluclast, and Pectinex) resulted in 84% total sugar (Mahdy
et al. 2014).
7.3.3 Ethanol Fermentation Using Different Feedstocks
Alcoholic fermentation can be defined as the biochemical conversion of fermentable
sugars to alcohol in the presence of microorganisms (Balat and Balat 2009). A model
reaction for representing the ethanol generation is conversion of one molecule of
7 Bioethanol Production: Generation-Based Comparative Status Measurements
171