64
H. A. Tajarudin et al.
rigorous and are therefore often regarded unfeasible (Hendriks and Zeeman 2009).
However, in combination with other pretreatment methods they can be useful.
Once pretreatment was done, the important component of the cellulose still
remains in polymeric form, and depending on the pretreatment method, some of
the hemicelluloses may also remain in polymeric or oligomeric form. To breakdown
the reminder of the sugars, a set of enzymes, mainly cellulases, are needed. For a long
time, enzymatic hydrolysis was regarded as the primary bottleneck in the production
of bioethanol from lingocellulose (Lynd et al. 2008). This was mainly due to the
slow action of the cellulase mixtures and the need for large amounts of expensive
enzymes.
For the fermentation part, there are many potential candidates for bioethanol
production such Saccharomyces cerevisiae, Scheffersomyces stipitis, Kluyveromyces
marxianus, Dekkera bruxellensis which are commonly used microorganism in
sugar- and starch-based bioethanol production today (De Souza Liberal et al. 2007).
This will be elaborated later in the fermentation of ethanologenic microorganism’s
subsection.
2 Biomass Resource for Second-Generation Bioethanol
Biomass is a transformed solar energy from photosynthesis, hence stored chemical
energy available for satisfying renewable energy demand. The biomass processes of
sucrose or starch containing crops such as sugarcane, sugar beet, maize, and wheat
are relatively easy and simple, considering the accessibility of matured bioconversion
technology (Lin and Tanaka 2006). However, the increase in bioethanol production
from food crops will affect global agricultural commodity prices and food security.
This is especially true for most of the feedstocks except sugarcane scenario in Brazil
where Brazilian biofuel program promotes agricultural development (Koizumi 2015;
Sims et al. 2010). Overall, albeit with conflicts from different studies, many agreed
that first-generation bioethanol has several limitations in term of land competition
for growing crops and residence (Mohr and Raman 2013; Naik et al. 2010).
The conversion of biomass into second-generation bioethanol is different from the
first generation due to its variation of biochemical composition. The first-generation
biomass conversion is easier due to the presence of readily fermented soluble sugars
(mono and disaccharides) and hydrolysed starch prior to fermentation. In contrast,
second-generation bioethanol faces more difficulties in the preparation of soluble
sugars for the fermentation process. This is due to the presence of recalcitrant
molecules with complex linkages, which in turn provides robustness toward enzymatic and chemical degradation. Nevertheless, the concept of second-generation
bioethanol is promising due to the abundance of lignocellulosic biomass and its
accessibility without the interference of additional land for crops cultivation. The
worldwide production of lignocellulosic biomass is around 200 Gt per year, where
roughly 20 Gt is available for biofuel production (Limayem and Ricke 2012).
H. A. Tajarudin et al.
rigorous and are therefore often regarded unfeasible (Hendriks and Zeeman 2009).
However, in combination with other pretreatment methods they can be useful.
Once pretreatment was done, the important component of the cellulose still
remains in polymeric form, and depending on the pretreatment method, some of
the hemicelluloses may also remain in polymeric or oligomeric form. To breakdown
the reminder of the sugars, a set of enzymes, mainly cellulases, are needed. For a long
time, enzymatic hydrolysis was regarded as the primary bottleneck in the production
of bioethanol from lingocellulose (Lynd et al. 2008). This was mainly due to the
slow action of the cellulase mixtures and the need for large amounts of expensive
enzymes.
For the fermentation part, there are many potential candidates for bioethanol
production such Saccharomyces cerevisiae, Scheffersomyces stipitis, Kluyveromyces
marxianus, Dekkera bruxellensis which are commonly used microorganism in
sugar- and starch-based bioethanol production today (De Souza Liberal et al. 2007).
This will be elaborated later in the fermentation of ethanologenic microorganism’s
subsection.
2 Biomass Resource for Second-Generation Bioethanol
Biomass is a transformed solar energy from photosynthesis, hence stored chemical
energy available for satisfying renewable energy demand. The biomass processes of
sucrose or starch containing crops such as sugarcane, sugar beet, maize, and wheat
are relatively easy and simple, considering the accessibility of matured bioconversion
technology (Lin and Tanaka 2006). However, the increase in bioethanol production
from food crops will affect global agricultural commodity prices and food security.
This is especially true for most of the feedstocks except sugarcane scenario in Brazil
where Brazilian biofuel program promotes agricultural development (Koizumi 2015;
Sims et al. 2010). Overall, albeit with conflicts from different studies, many agreed
that first-generation bioethanol has several limitations in term of land competition
for growing crops and residence (Mohr and Raman 2013; Naik et al. 2010).
The conversion of biomass into second-generation bioethanol is different from the
first generation due to its variation of biochemical composition. The first-generation
biomass conversion is easier due to the presence of readily fermented soluble sugars
(mono and disaccharides) and hydrolysed starch prior to fermentation. In contrast,
second-generation bioethanol faces more difficulties in the preparation of soluble
sugars for the fermentation process. This is due to the presence of recalcitrant
molecules with complex linkages, which in turn provides robustness toward enzymatic and chemical degradation. Nevertheless, the concept of second-generation
bioethanol is promising due to the abundance of lignocellulosic biomass and its
accessibility without the interference of additional land for crops cultivation. The
worldwide production of lignocellulosic biomass is around 200 Gt per year, where
roughly 20 Gt is available for biofuel production (Limayem and Ricke 2012).