glucose to form two molecules of alcohol and two molecules of carbon dioxide.
Theoretical ethanol yield from conversion of 1 kg of glucose is around 0.51 kg, and
the remaining 0.49 kg is CO 2 . However, the actual yield is comparatively low as
microbes require glucose to meet their nutrient need (Demirbas 2005). Saccharomyces cerevisiae is the most widely used organism apart from some bacteria, i.e.,
Zymomonas mobilis and recombinant E. coli. The properties of ethanologenic
microbes are discussed later under different headings.
7.3.3.1 Solid-State Fermentation for Bioethanol Production
During solid-state fermentation, the substrate is fermented in its natural state and
does not require additional water or extraction of juice from crops, and thus it is
economically and technologically easy (Zabed et al. 2017). The solid-state fermentation has several other advantages such as low sterilization cost, easier aeration
(large surface area), and lower contamination risks, which make the process an ideal
choice for bioethanol generation (Yu et al. 2008). The limitations associated with the
solid-state fermentations are poor heat transfers, design of reactors and its operation
on large scale and difficulty in agitation of substrate (Li et al. 2013). The major
advantage of this process, i.e., low water requirement, may sometimes act as a
disadvantage which limits the microbial growth under low humidity. In order to
overcome these limitations, several technological advancements such as design of
special rotary drum bioreactors can help in proper agitation and mass heat transfer
and result in enhanced production of bioethanol under short fermentation time (Han
et al. 2010; Wang et al. 2010).
7.3.3.2 Very High Gravity Fermentation (VHG) for Bioethanol
Production
There is a need of fermentation process with high initial substrate concentration to
enhance the concentration of ethanol in fermentation broth that can result in
decreases in overall energy consumption during distillation process (Alkasrawi
et al. 2002). Therefore, normal fermentation has fixed initial solid concentration of
20–25% for starch and 10–15% for lignocellulosic biomass and thus have low
ethanol yield. Advancement of gravity fermentation that is “very high gravity”
(VHG) fermentation is a promising technology that can overcome several limitations
of the normal fermentation process. It can lead to several benefits in the form of
savings associated with water and energy requirements through high initial solid
concentration of approximately 270 g/L or more solids (Bayrock and Ingledew
2001; Puligundla et al. 2011). Several studies demonstrated that VHG can be
exploited for enhanced and prolonged growth of Saccharomyces cerevisiae under
fermentation-induced oxidative stress or low oxygen level (Burphan et al. 2018).
Other advantages associated with the VHG are high fermentation efficiency, better
utilization of fermenter space, low water requirement, low energy/power consumption, low distillation cost, reduced contamination risk, and high ethanol yield (Lim
et al. 2013). Apart from benefits, there are certain limitations associated with the high
solid loading; it causes high sugar concentration in fermentation broth which can
cause high osmotic pressure on cell leading to the cell lysis and loss of cell viability,
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B. Kumar et al.
Theoretical ethanol yield from conversion of 1 kg of glucose is around 0.51 kg, and
the remaining 0.49 kg is CO 2 . However, the actual yield is comparatively low as
microbes require glucose to meet their nutrient need (Demirbas 2005). Saccharomyces cerevisiae is the most widely used organism apart from some bacteria, i.e.,
Zymomonas mobilis and recombinant E. coli. The properties of ethanologenic
microbes are discussed later under different headings.
7.3.3.1 Solid-State Fermentation for Bioethanol Production
During solid-state fermentation, the substrate is fermented in its natural state and
does not require additional water or extraction of juice from crops, and thus it is
economically and technologically easy (Zabed et al. 2017). The solid-state fermentation has several other advantages such as low sterilization cost, easier aeration
(large surface area), and lower contamination risks, which make the process an ideal
choice for bioethanol generation (Yu et al. 2008). The limitations associated with the
solid-state fermentations are poor heat transfers, design of reactors and its operation
on large scale and difficulty in agitation of substrate (Li et al. 2013). The major
advantage of this process, i.e., low water requirement, may sometimes act as a
disadvantage which limits the microbial growth under low humidity. In order to
overcome these limitations, several technological advancements such as design of
special rotary drum bioreactors can help in proper agitation and mass heat transfer
and result in enhanced production of bioethanol under short fermentation time (Han
et al. 2010; Wang et al. 2010).
7.3.3.2 Very High Gravity Fermentation (VHG) for Bioethanol
Production
There is a need of fermentation process with high initial substrate concentration to
enhance the concentration of ethanol in fermentation broth that can result in
decreases in overall energy consumption during distillation process (Alkasrawi
et al. 2002). Therefore, normal fermentation has fixed initial solid concentration of
20–25% for starch and 10–15% for lignocellulosic biomass and thus have low
ethanol yield. Advancement of gravity fermentation that is “very high gravity”
(VHG) fermentation is a promising technology that can overcome several limitations
of the normal fermentation process. It can lead to several benefits in the form of
savings associated with water and energy requirements through high initial solid
concentration of approximately 270 g/L or more solids (Bayrock and Ingledew
2001; Puligundla et al. 2011). Several studies demonstrated that VHG can be
exploited for enhanced and prolonged growth of Saccharomyces cerevisiae under
fermentation-induced oxidative stress or low oxygen level (Burphan et al. 2018).
Other advantages associated with the VHG are high fermentation efficiency, better
utilization of fermenter space, low water requirement, low energy/power consumption, low distillation cost, reduced contamination risk, and high ethanol yield (Lim
et al. 2013). Apart from benefits, there are certain limitations associated with the high
solid loading; it causes high sugar concentration in fermentation broth which can
cause high osmotic pressure on cell leading to the cell lysis and loss of cell viability,
172
B. Kumar et al.