also affects the final yield and overall cost of production of bioethanol (Aditiya et al.
2016).
7.3.5 Microorganisms Used in Ethanol Generation
During conversion of the biomass to bioethanol, various microorganisms are used
during different steps such as pretreatment, hydrolysis, and fermentation. The
selection of these microorganisms usually depends upon their suitability of the
particular process, cost involved for maintaining them, and physical condition
required for the growth of microbes. Usually in processes such as pretreatment,
detoxification, and hydrolysis, biological products such as enzyme are often used.
But for the ethanol generation, ethanologenic microbes are always used. As the
ethanol (alcohol) generation is one of the oldest processes developed by human and
thus for the ethanol generation on large scale, ethanologenic microbes must have
different characteristic properties such as (i) requirement of inexpensive media,
(ii) high growth rate, (iii) tolerance to stress condition such as high ethanol (>40.0
g/L)/sugar (above 20%) concentration, (iv) very high ethanol yield (>90.0%),
(v) high productivity of ethanol (>1.0 g/L/h), and (vi) capability to minimize growth
of contaminants (Dien et al. 2003; Zabed et al. 2017). Yeast, namely, Saccharomyces cerevisiae, is the most commonly used microbes for generating fuel-grade
ethanol from a wide range of biomass. Different properties of Saccharomyces
cerevisiae make them an attractive choice for ethanol generation over other
organisms. These properties are as follows: (i) high sugar to alcohol conversion
efficiency, (ii) tolerance to high ethanol concentration (Snoek et al. 2016), (iii) floc
formation ability during fermentation, and (iv) nontoxic or safe (generally
recognized as safe GRAS) organism (Lin and Tanaka 2006). Generally, Saccharomyces cerevisiae have the capability to secrete invertase enzyme that can hydrolyze
sucrose-rich crop juices into fructose and glucose (Zabed et al. 2017). The another
well-recognized organism for production of ethanol by fermentation of starch and
lignocellulosic hydrolysate is gram-negative and facultative anaerobe Zymomonas
mobilis (Cazetta et al. 2007). The Zymomonas mobilis has several better properties
as compared to Saccharomyces cerevisiae such as higher ethanol tolerance and
better glucose uptake with higher ethanol productivity and yield (Bai et al. 2008).
One major disadvantage that limits Zymomonas mobilis from replacing the Saccharomyces cerevisiae as major bioethanol producer is its narrow substrate range. There
are different microbes reported to help in ethanol production; however limitations
are associated with these bio-agents to be used on commercial scale.
There are certain limitations associated with microorganism for ethanol generation. First, there is an inability of microorganisms to directly utilize the complex
carbohydrate polymer for ethanol generation, as they require reducing sugars (glucose, fructose, and sucrose) for ethanol generation (Hahn-Hägerdal et al. 2006).
Therefore, there is a need for an additional step converting naturally occurring
carbohydrate polymers to simple sugars requiring costly enzymes which increase
the overall cost and processing time. Second, these microbes are not capable to
7 Bioethanol Production: Generation-Based Comparative Status Measurements
175
2016).
7.3.5 Microorganisms Used in Ethanol Generation
During conversion of the biomass to bioethanol, various microorganisms are used
during different steps such as pretreatment, hydrolysis, and fermentation. The
selection of these microorganisms usually depends upon their suitability of the
particular process, cost involved for maintaining them, and physical condition
required for the growth of microbes. Usually in processes such as pretreatment,
detoxification, and hydrolysis, biological products such as enzyme are often used.
But for the ethanol generation, ethanologenic microbes are always used. As the
ethanol (alcohol) generation is one of the oldest processes developed by human and
thus for the ethanol generation on large scale, ethanologenic microbes must have
different characteristic properties such as (i) requirement of inexpensive media,
(ii) high growth rate, (iii) tolerance to stress condition such as high ethanol (>40.0
g/L)/sugar (above 20%) concentration, (iv) very high ethanol yield (>90.0%),
(v) high productivity of ethanol (>1.0 g/L/h), and (vi) capability to minimize growth
of contaminants (Dien et al. 2003; Zabed et al. 2017). Yeast, namely, Saccharomyces cerevisiae, is the most commonly used microbes for generating fuel-grade
ethanol from a wide range of biomass. Different properties of Saccharomyces
cerevisiae make them an attractive choice for ethanol generation over other
organisms. These properties are as follows: (i) high sugar to alcohol conversion
efficiency, (ii) tolerance to high ethanol concentration (Snoek et al. 2016), (iii) floc
formation ability during fermentation, and (iv) nontoxic or safe (generally
recognized as safe GRAS) organism (Lin and Tanaka 2006). Generally, Saccharomyces cerevisiae have the capability to secrete invertase enzyme that can hydrolyze
sucrose-rich crop juices into fructose and glucose (Zabed et al. 2017). The another
well-recognized organism for production of ethanol by fermentation of starch and
lignocellulosic hydrolysate is gram-negative and facultative anaerobe Zymomonas
mobilis (Cazetta et al. 2007). The Zymomonas mobilis has several better properties
as compared to Saccharomyces cerevisiae such as higher ethanol tolerance and
better glucose uptake with higher ethanol productivity and yield (Bai et al. 2008).
One major disadvantage that limits Zymomonas mobilis from replacing the Saccharomyces cerevisiae as major bioethanol producer is its narrow substrate range. There
are different microbes reported to help in ethanol production; however limitations
are associated with these bio-agents to be used on commercial scale.
There are certain limitations associated with microorganism for ethanol generation. First, there is an inability of microorganisms to directly utilize the complex
carbohydrate polymer for ethanol generation, as they require reducing sugars (glucose, fructose, and sucrose) for ethanol generation (Hahn-Hägerdal et al. 2006).
Therefore, there is a need for an additional step converting naturally occurring
carbohydrate polymers to simple sugars requiring costly enzymes which increase
the overall cost and processing time. Second, these microbes are not capable to
7 Bioethanol Production: Generation-Based Comparative Status Measurements
175