conditioning prior to its addition to the bioreactor for fermentation in mineral-defined aqueous fermentation medium
(Devarapalli and Atiyeh 2015; Phillips et al. 2017).
Fermentation-strategies development has taken place over
the years with respect to the degree of consolidation of
various steps involved in bioethanol production.
A schematic diagram of the same is shown in Fig. 3.
Separate hydrolysis and fermentation (SHF): Enzyme
production (if present), hydrolysis, and fermentation (hexose
and pentose fermentation) all take place in different bioreactors with different operating conditions and, thus,
increasing the processing time for each. Since pretreatment
and hydrolysis of the biomass are carried out separately,
enzyme inhibition due to the fermentable-components
accumulation in the hydrolysate prior to hydrolysis may
take place (Margeot et al. 2009).
Simultaneous saccharification and fermentation (SSF):
The process modification adopted in SSF was to avoid
enzyme inhibition as faced in SHF. That is why hydrolysis
and hexose fermentation take place in a single bioreactor and
the sugar monomers are, therefore, concomitantly hydrolyzed along with their formation. SSF thus helps in efficiency enhancement of the enzymatic process. The problems
faced in SSF are with respect to the inability of the fermenting organisms to utilize both hexoses and pentoses and
different requirements of the operating conditions for the
enzymatic and hydrolysis processes (Devarapalli and Atiyeh
2015; Lin and Tanaka 2006). The operating cost of the
overall process with respect to the energy investment is
lesser due to the process intensification, i.e. the merging of
two intermediate stages (hydrolysis and fermentation) into
one. Temperature monitoring and control is an important
factor pertaining to SSF due to the difference between
temperature required for enzyme activity for hydrolysis (45–
60 °C) and ethanol fermentation (30-35°C). Moreover, the
heat released during the fermentation reaction also contributes to the temperature increase in the system (Mejía-Barajas et al. 2018). A trade-off between the inefficiencies of
activities of enzyme(s) for hydrolysis and of organism(s) for
fermentation would therefore seem inevitable for the best
possible choice of temperature in such a scenario, for
example, Liu et al. (2014) had chosen 39°C as the best
possible temperature.
Simultaneous saccharification and co-fermentation
(SSCF): To overcome the issues faced in SSF, fermentation
of both sugars (hexoses and pentoses) is carried out in the
same fermentor by bioengineered fermenting organisms with
the ability to utilize different classes of sugars simultaneously (Devarapalli and Atiyeh 2015; Öhgren et al. 2006).
This allows for maintaining a steady rate of glucose release
ensuring a high xylose to glucose ratio. This facilitates
higher rates of xylose utilization (Mejía-Barajas et al. 2018).
Consolidated bioprocessing (CBP): This is a biorefinery
where all the bioprocessing steps of enzyme production, and
hexose and pentose fermentations are integrated into a single
operation in a single fermentor at the same time. The
selection of the fermenting organism or of a microbial
consortium, with the capability of participating in all the
enzymatic as well as fermentation reactions, is very crucial.
Moreover, the existence of compatibility between
co-cultures, if used, is necessary for increasing the overall
process effectiveness (Ibrahim et al. 2018).
Fig. 3 A schematic diagram of
ethanol production from
lignocellulosic-based biomass
(Illustration adapted from
Devarapalli and Atiyeh (2015)
with permission from Biofuel
Research Team)
Bioconversion of Food Waste into Ethanol: A Review
53
(Devarapalli and Atiyeh 2015; Phillips et al. 2017).
Fermentation-strategies development has taken place over
the years with respect to the degree of consolidation of
various steps involved in bioethanol production.
A schematic diagram of the same is shown in Fig. 3.
Separate hydrolysis and fermentation (SHF): Enzyme
production (if present), hydrolysis, and fermentation (hexose
and pentose fermentation) all take place in different bioreactors with different operating conditions and, thus,
increasing the processing time for each. Since pretreatment
and hydrolysis of the biomass are carried out separately,
enzyme inhibition due to the fermentable-components
accumulation in the hydrolysate prior to hydrolysis may
take place (Margeot et al. 2009).
Simultaneous saccharification and fermentation (SSF):
The process modification adopted in SSF was to avoid
enzyme inhibition as faced in SHF. That is why hydrolysis
and hexose fermentation take place in a single bioreactor and
the sugar monomers are, therefore, concomitantly hydrolyzed along with their formation. SSF thus helps in efficiency enhancement of the enzymatic process. The problems
faced in SSF are with respect to the inability of the fermenting organisms to utilize both hexoses and pentoses and
different requirements of the operating conditions for the
enzymatic and hydrolysis processes (Devarapalli and Atiyeh
2015; Lin and Tanaka 2006). The operating cost of the
overall process with respect to the energy investment is
lesser due to the process intensification, i.e. the merging of
two intermediate stages (hydrolysis and fermentation) into
one. Temperature monitoring and control is an important
factor pertaining to SSF due to the difference between
temperature required for enzyme activity for hydrolysis (45–
60 °C) and ethanol fermentation (30-35°C). Moreover, the
heat released during the fermentation reaction also contributes to the temperature increase in the system (Mejía-Barajas et al. 2018). A trade-off between the inefficiencies of
activities of enzyme(s) for hydrolysis and of organism(s) for
fermentation would therefore seem inevitable for the best
possible choice of temperature in such a scenario, for
example, Liu et al. (2014) had chosen 39°C as the best
possible temperature.
Simultaneous saccharification and co-fermentation
(SSCF): To overcome the issues faced in SSF, fermentation
of both sugars (hexoses and pentoses) is carried out in the
same fermentor by bioengineered fermenting organisms with
the ability to utilize different classes of sugars simultaneously (Devarapalli and Atiyeh 2015; Öhgren et al. 2006).
This allows for maintaining a steady rate of glucose release
ensuring a high xylose to glucose ratio. This facilitates
higher rates of xylose utilization (Mejía-Barajas et al. 2018).
Consolidated bioprocessing (CBP): This is a biorefinery
where all the bioprocessing steps of enzyme production, and
hexose and pentose fermentations are integrated into a single
operation in a single fermentor at the same time. The
selection of the fermenting organism or of a microbial
consortium, with the capability of participating in all the
enzymatic as well as fermentation reactions, is very crucial.
Moreover, the existence of compatibility between
co-cultures, if used, is necessary for increasing the overall
process effectiveness (Ibrahim et al. 2018).
Fig. 3 A schematic diagram of
ethanol production from
lignocellulosic-based biomass
(Illustration adapted from
Devarapalli and Atiyeh (2015)
with permission from Biofuel
Research Team)
Bioconversion of Food Waste into Ethanol: A Review
53