also within the steam explosion treatment and is found to reduce the temperature and
the retention time. Another advantage is that complete hydrolysis of hemicellulose
can be achieved (Mood et al. 2013).
1.4.3.2 Hydrolysis
During the hydrolysis, polysaccharides are broken down to simple sugars. Two
examples of hydrolysis methods of cellulose into glucose are (Lynd et al. 2002):
1. Concentrated acid (H 2 SO 4 30–70%, 40
C, a few hours to achieve >90% glucose
yields)
2. Enzymatic hydrolysis (cellulase mixture, 50
C several days to reach 75–95%
glucose yields)
The current trend is to use enzymatic hydrolysis to avoid costly recovery and
wastewater treatment requirements, resulting from the use of acid hydrolysis. Enzymatic hydrolysis is attractive because it produces better yields than acid-catalyzed
hydrolysis and enzyme producers have recently reduced their cost using biotechnology (Ruane et al. 2010). The conversion of cellulose and hemicellulose is catalyzed
by cellulase and hemicellulase enzymes, respectively.
1.4.3.3 Fermentation
The ability to use the hemicellulose component in biomass feedstock is critical for
any bio-ethanol plant. Saccharomyces cerevisiae and Zymomonas mobilis, the
commonly employed organisms in alcohol fermentation, are not able to ferment
hemicellulose-derived pentose (C5) sugars. There are organisms that can ferment C5
sugars (e.g., Pichia stipitis, Pachysolen tannophilus, Candida shehatae), but their
efficiencies are low. They also need microaerophilic conditions. This implies that for
more than 20 years research activities have focused on the development of improved
microorganisms for the fermentation of pentose sugars (Hahn-hägerdal et al. 2007).
Besides this, currently there are not known natural organisms that have the ability to
convert both these C6 and C5 sugars at high yields. While pentose fermentation has
been achieved on ideal substrates, (i.e., laboratory preparations of sugars designed to
imitate a perfectly pretreated feedstock), significant work remains to apply this to
real lignocellulosic feedstocks (Sims et al. 2008). Lignocellulosic biomass conversion into bioethanol flow diagram is shown in Fig. 1.8.
A typical process for making cellulosic ethanol starts with pretreatment and
separation of the insolubles. The insoluble fraction is then hydrolyzed with cellulase
and glycosidases to release glucose, which is fermented to produce ethanol. The
residual insoluble material, mostly lignin, is burned to generate energy (Ruane et al.
2010). If the fermentation process is performed after the hydrolysis, this is called
separate hydrolysis and fermentation (SHF). The fermentation process produces
wastewaters which can be used to recover a nutrient-rich microbial cell mass (Kurian
et al. 2013). Pentose fermentation, when it is carried out, is accomplished in an
independent unit. The advantage of SHF is the ability to carry out each step under
16
P. Bartocci et al.
the retention time. Another advantage is that complete hydrolysis of hemicellulose
can be achieved (Mood et al. 2013).
1.4.3.2 Hydrolysis
During the hydrolysis, polysaccharides are broken down to simple sugars. Two
examples of hydrolysis methods of cellulose into glucose are (Lynd et al. 2002):
1. Concentrated acid (H 2 SO 4 30–70%, 40
C, a few hours to achieve >90% glucose
yields)
2. Enzymatic hydrolysis (cellulase mixture, 50
C several days to reach 75–95%
glucose yields)
The current trend is to use enzymatic hydrolysis to avoid costly recovery and
wastewater treatment requirements, resulting from the use of acid hydrolysis. Enzymatic hydrolysis is attractive because it produces better yields than acid-catalyzed
hydrolysis and enzyme producers have recently reduced their cost using biotechnology (Ruane et al. 2010). The conversion of cellulose and hemicellulose is catalyzed
by cellulase and hemicellulase enzymes, respectively.
1.4.3.3 Fermentation
The ability to use the hemicellulose component in biomass feedstock is critical for
any bio-ethanol plant. Saccharomyces cerevisiae and Zymomonas mobilis, the
commonly employed organisms in alcohol fermentation, are not able to ferment
hemicellulose-derived pentose (C5) sugars. There are organisms that can ferment C5
sugars (e.g., Pichia stipitis, Pachysolen tannophilus, Candida shehatae), but their
efficiencies are low. They also need microaerophilic conditions. This implies that for
more than 20 years research activities have focused on the development of improved
microorganisms for the fermentation of pentose sugars (Hahn-hägerdal et al. 2007).
Besides this, currently there are not known natural organisms that have the ability to
convert both these C6 and C5 sugars at high yields. While pentose fermentation has
been achieved on ideal substrates, (i.e., laboratory preparations of sugars designed to
imitate a perfectly pretreated feedstock), significant work remains to apply this to
real lignocellulosic feedstocks (Sims et al. 2008). Lignocellulosic biomass conversion into bioethanol flow diagram is shown in Fig. 1.8.
A typical process for making cellulosic ethanol starts with pretreatment and
separation of the insolubles. The insoluble fraction is then hydrolyzed with cellulase
and glycosidases to release glucose, which is fermented to produce ethanol. The
residual insoluble material, mostly lignin, is burned to generate energy (Ruane et al.
2010). If the fermentation process is performed after the hydrolysis, this is called
separate hydrolysis and fermentation (SHF). The fermentation process produces
wastewaters which can be used to recover a nutrient-rich microbial cell mass (Kurian
et al. 2013). Pentose fermentation, when it is carried out, is accomplished in an
independent unit. The advantage of SHF is the ability to carry out each step under
16
P. Bartocci et al.