present in cell wall of lignocellulosic and algal biomass (Pirwitz et al. 2016). Each
method has several advantages and disadvantages; thus while choosing an ethanol
generation process from a particular biomass, these measures must be evaluated for
maximum sugar/ethanol yield and minimum cost. Different pretreatment methods
used for generation of ethanol from 1G, 2G, and 3G are tabulated in Table 7.3
(1G and 2G feedstock) and Table 7.4 (3G feedstock) and are explained as mentioned
below
7.3.2 Hydrolysis of Carbohydrate Polymer to Fermentable Sugar
for Bioethanol Production
The hydrolysis is one of the primary requirements of converting the polymeric
carbohydrates (starch, cellulose, and hemicelluloses) into fermentable sugar which
can be converted to ethanol by fermentation. However, ethanologenic organism such
as yeast cannot utilize these carbohydrate polymers in their native state (Liu et al.
2016). Therefore, there is a need to find a method to hydrolyze the polymeric
structure to simpler form. Different acid- or enzyme-based methods are suggested
for the breakdown of starch, hemicelluloses, and cellulose.
7.3.2.1 Hydrolysis of Starch to Fermentable Sugar for Bioethanol
Production
Starch is the major component in the food crops and in some algal biomass used as
feedstock for bioethanol generation. Chemical (acid)-based hydrolysis and physical
and enzymatic methods are usually used for breaking down the starch into soluble
sugars. During starch hydrolysis, the major constituents of starch, i.e., amylose and
amylopectin, are broken down by biological or chemical agent to generate soluble
sugars such as maltose, glucose, and maltotriose. The acid hydrolysis is an efficient
method for conversion of complex polymers to fermentable sugars. Due to severity
of the acid-assisted hydrolysis, a need for acid recovery and generation of undesirable inhibitory products (such as 5-hydroxymethylfurfurals (5-HMF), levulinic and
formic acid) minimizes the utilization of acid-based hydrolysis (Yang et al. 2011).
The inhibitory by-products are toxic for growth of microbial cells (yeast), negatively
affecting the overall ethanol generation (Loow et al. 2016). Physical method such as
high pressure and temperature-based extrusion method were also tried for the
hydrolysis of starch. However, results show that due to catalytic inhibition effect
on α-amylase under high pressure, rate of starch to sugar conversion is slow resulting
in low fermentable sugar yield (Buckow et al. 2007).
The enzyme-based hydrolysis is the most sorted method for conversion of starch
to fermentable sugars due to its biological origin, simple, highly specific and
eco-friendly nature, and high efficiency (Duvernay et al. 2013). The thermostable
α-amylase and glucoamylase regulate two major steps of starch to fermentable sugar
conversion, i.e., liquefaction and saccharification, respectively (Lamsal et al.
2011; Zabed et al. 2016a). Liquefaction is performed at higher temperature
(85–165
C) where “α-amylase,” an endoenzyme, cleaves the α-1 ! 4 glycosidic
170
B. Kumar et al.
method has several advantages and disadvantages; thus while choosing an ethanol
generation process from a particular biomass, these measures must be evaluated for
maximum sugar/ethanol yield and minimum cost. Different pretreatment methods
used for generation of ethanol from 1G, 2G, and 3G are tabulated in Table 7.3
(1G and 2G feedstock) and Table 7.4 (3G feedstock) and are explained as mentioned
below
7.3.2 Hydrolysis of Carbohydrate Polymer to Fermentable Sugar
for Bioethanol Production
The hydrolysis is one of the primary requirements of converting the polymeric
carbohydrates (starch, cellulose, and hemicelluloses) into fermentable sugar which
can be converted to ethanol by fermentation. However, ethanologenic organism such
as yeast cannot utilize these carbohydrate polymers in their native state (Liu et al.
2016). Therefore, there is a need to find a method to hydrolyze the polymeric
structure to simpler form. Different acid- or enzyme-based methods are suggested
for the breakdown of starch, hemicelluloses, and cellulose.
7.3.2.1 Hydrolysis of Starch to Fermentable Sugar for Bioethanol
Production
Starch is the major component in the food crops and in some algal biomass used as
feedstock for bioethanol generation. Chemical (acid)-based hydrolysis and physical
and enzymatic methods are usually used for breaking down the starch into soluble
sugars. During starch hydrolysis, the major constituents of starch, i.e., amylose and
amylopectin, are broken down by biological or chemical agent to generate soluble
sugars such as maltose, glucose, and maltotriose. The acid hydrolysis is an efficient
method for conversion of complex polymers to fermentable sugars. Due to severity
of the acid-assisted hydrolysis, a need for acid recovery and generation of undesirable inhibitory products (such as 5-hydroxymethylfurfurals (5-HMF), levulinic and
formic acid) minimizes the utilization of acid-based hydrolysis (Yang et al. 2011).
The inhibitory by-products are toxic for growth of microbial cells (yeast), negatively
affecting the overall ethanol generation (Loow et al. 2016). Physical method such as
high pressure and temperature-based extrusion method were also tried for the
hydrolysis of starch. However, results show that due to catalytic inhibition effect
on α-amylase under high pressure, rate of starch to sugar conversion is slow resulting
in low fermentable sugar yield (Buckow et al. 2007).
The enzyme-based hydrolysis is the most sorted method for conversion of starch
to fermentable sugars due to its biological origin, simple, highly specific and
eco-friendly nature, and high efficiency (Duvernay et al. 2013). The thermostable
α-amylase and glucoamylase regulate two major steps of starch to fermentable sugar
conversion, i.e., liquefaction and saccharification, respectively (Lamsal et al.
2011; Zabed et al. 2016a). Liquefaction is performed at higher temperature
(85–165
C) where “α-amylase,” an endoenzyme, cleaves the α-1 ! 4 glycosidic
170
B. Kumar et al.