utilize both pentoses and hexoses generated after hydrolysis of hemicelluloses and
cellulose, respectively. Third, the fermentation usually occurs at high temperature,
so thermotolerant microbial strains are required. In addition, the enzyme production
cost results in increase in overall ethanol generation cost. Therefore, keeping in view
of these limitations, several researches have been attempted to develop different
modified or recombinant strains using technological advancements in field of biotechnology. The different approaches used are altering the natural genetic makeup
with the desired traits (Hahn-Hägerdal et al. 2007) or evolutionary microbial engineering techniques (Wisselink et al. 2009) for developing strains which can utilize
the hexoses and pentoses or even capable of directly utilizing carbohydrate
polymers.
A series of attempts have been made by different research groups for modifying
yeast which can directly utilize starch for production of ethanol by cell surface
engineering. The bioengineered yeast was capable of producing glucoamylase and
α-amylase along with ethanol generation ability thus a single step conversion of
starch to bioethanol can be performed (Shigechi et al. 2002, 2004; Aydemir et al.
2014). Several strains capable of tolerating high ethanol concentration have been
isolated, or the already existing microbes are modified. Different thermotolerant
yeasts have been isolated and adapted for the simultaneous saccharification and
fermentation (Thammasittirong et al. 2013; Tikka et al. 2013).
7.3.6 Factors Affecting Bioethanol Production
Microorganism plays an important role in the generation of enzyme for hydrolysis
and fermentation of the biomass to bioethanol. Therefore, the factors affecting the
growth of microbes are physical (pH temperature, incubation time) and nutritional
parameters (carbon and nitrogen source, metal ions, etc.), which are common for
most of the microorganism (Ramirez et al. 2016). The major factors which are
specific to ethanol generation are discussed below, i.e., initial solid load, microbial
load, and accumulation of soluble by-product which sometime slow down the
fermentation process due to end product inhibition.
7.3.6.1 Initial Solid/Substrate Load Concentration Affecting Bioethanol
Production
The overall biochemical conversion efficiency of biomass to bioethanol is often
affected by initial substrate (sugars, cellulose, starch, and hemicellulose) concentration (Modenbach and Nokes 2013). As discussed above, the excessive substrate load
can inhibit the enzymes during enzymatic hydrolysis of the complex polymers, thus
effecting the overall sugar yield due to incomplete conversion of carbohydrates
(Mojović et al. 2006). Due to high substrate loading, gelatinization of starch occurs
which increases the viscosity, leading to incomplete starch conversion because of
poor mixing (Uthumporn et al. 2010). The initial solid loading of 12–38% is
suggested for optimum hydrolysis (Foerster 2010). The increase in sugar concentration enhances the ethanol yield to a certain level after which high sugar concentration
176
B. Kumar et al.
cellulose, respectively. Third, the fermentation usually occurs at high temperature,
so thermotolerant microbial strains are required. In addition, the enzyme production
cost results in increase in overall ethanol generation cost. Therefore, keeping in view
of these limitations, several researches have been attempted to develop different
modified or recombinant strains using technological advancements in field of biotechnology. The different approaches used are altering the natural genetic makeup
with the desired traits (Hahn-Hägerdal et al. 2007) or evolutionary microbial engineering techniques (Wisselink et al. 2009) for developing strains which can utilize
the hexoses and pentoses or even capable of directly utilizing carbohydrate
polymers.
A series of attempts have been made by different research groups for modifying
yeast which can directly utilize starch for production of ethanol by cell surface
engineering. The bioengineered yeast was capable of producing glucoamylase and
α-amylase along with ethanol generation ability thus a single step conversion of
starch to bioethanol can be performed (Shigechi et al. 2002, 2004; Aydemir et al.
2014). Several strains capable of tolerating high ethanol concentration have been
isolated, or the already existing microbes are modified. Different thermotolerant
yeasts have been isolated and adapted for the simultaneous saccharification and
fermentation (Thammasittirong et al. 2013; Tikka et al. 2013).
7.3.6 Factors Affecting Bioethanol Production
Microorganism plays an important role in the generation of enzyme for hydrolysis
and fermentation of the biomass to bioethanol. Therefore, the factors affecting the
growth of microbes are physical (pH temperature, incubation time) and nutritional
parameters (carbon and nitrogen source, metal ions, etc.), which are common for
most of the microorganism (Ramirez et al. 2016). The major factors which are
specific to ethanol generation are discussed below, i.e., initial solid load, microbial
load, and accumulation of soluble by-product which sometime slow down the
fermentation process due to end product inhibition.
7.3.6.1 Initial Solid/Substrate Load Concentration Affecting Bioethanol
Production
The overall biochemical conversion efficiency of biomass to bioethanol is often
affected by initial substrate (sugars, cellulose, starch, and hemicellulose) concentration (Modenbach and Nokes 2013). As discussed above, the excessive substrate load
can inhibit the enzymes during enzymatic hydrolysis of the complex polymers, thus
effecting the overall sugar yield due to incomplete conversion of carbohydrates
(Mojović et al. 2006). Due to high substrate loading, gelatinization of starch occurs
which increases the viscosity, leading to incomplete starch conversion because of
poor mixing (Uthumporn et al. 2010). The initial solid loading of 12–38% is
suggested for optimum hydrolysis (Foerster 2010). The increase in sugar concentration enhances the ethanol yield to a certain level after which high sugar concentration
176
B. Kumar et al.