72
H. A. Tajarudin et al.
5 Fermentation of Ethanologenic Microorganisms
The cellulose and hemicellulose fraction of rice straw can be converted to ethanol by
either simultaneous saccharification and fermentation (SSF) or separate enzymatic
hydrolysis and fermentation (SHF) processes. SSF is more favored because of its
low potential costs (Wyman 1994). It results in higher yield of ethanol compared
to SHF by minimizing product inhibition. One of the drawbacks of this process is
the difference in optimum temperature of the hydrolyzing enzymes and fermenting microorganisms. Most of the reports state that the optimum temperature for
enzymatic hydrolysis is at 40–50 °C, while the microorganisms with good ethanol
productivity and yield do not usually tolerate this high temperature. This problem
can be avoided by applying thermo-tolerant microorganisms such as Kluyveromyces
marxianus, andida lusitaniae, and Zymomonas mobilis or mixed culture of some
microorganisms like Brettanomyces clausenii and Saccharomyces cerevisiae (Golias
et al. 2002; Spindler et al. 1988). Table 2 shows the different substrate, method, and
microorganism involved in previous ethanol production research
Cellulose processing cannot commence until the improvement of (i) the relatively slow kinetics of breaking down pure cellulose into sugars, (ii) the low yields
of sugars from other plant polysaccharides, and (iii) the removal of lignin, a relatively intractable polymer of phenylpropanoid subunits. It is clear that technological
advances must be realized to make biofuels sustainable and cost-effective.
6 Toxic Compounds Generated from Pretreatment
The attribute and concentration of toxic compounds generated from various
pretreatment depend on biomass source, pretreatment condition and the use of
catalyst. Three types of toxic compounds were known, viz., furans, phenolic compounds, and carboxylic acids. Among furan derivatives, 2-furaldehyde (furfural)
and 5-hydroxymethylfurfural (HMF) constitute the main degradation compounds
generated from pentoses and hexoses degradation, respectively. Pretreatments which
employ acids as hydrolytic agents and utilize high temperature and time to the
reaction will produce furfural and HMF at higher levels (Wyman 2007). Most of
the fermenting microorganisms are able to reduce furans to their corresponding
less toxic alcohols. HMF is reduced to 2,5-bis-hydroxymethylfuran and furfural to
furfuryl alcohol, and both could be also oxidized to formic acid under anaerobic
conditions (Taherzadeh et al. 1999). If furans are present at high concentration,
they exert an inhibitory effect interfering with glycolytic enzymes and synthesis
of macromolecules provoking an enlarge of the lag phase and reducing the ethanol
productivity (Almeida et al. 2007; Klinke et al. 2004). These effects depend on
furan concentration but are highly related with the yeast strain.
Main carboxylic acids generated during pretreatment are acetic acid, produced
from the acetyl groups in hemicelluloses, and formic acid, derived from furfural and
H. A. Tajarudin et al.
5 Fermentation of Ethanologenic Microorganisms
The cellulose and hemicellulose fraction of rice straw can be converted to ethanol by
either simultaneous saccharification and fermentation (SSF) or separate enzymatic
hydrolysis and fermentation (SHF) processes. SSF is more favored because of its
low potential costs (Wyman 1994). It results in higher yield of ethanol compared
to SHF by minimizing product inhibition. One of the drawbacks of this process is
the difference in optimum temperature of the hydrolyzing enzymes and fermenting microorganisms. Most of the reports state that the optimum temperature for
enzymatic hydrolysis is at 40–50 °C, while the microorganisms with good ethanol
productivity and yield do not usually tolerate this high temperature. This problem
can be avoided by applying thermo-tolerant microorganisms such as Kluyveromyces
marxianus, andida lusitaniae, and Zymomonas mobilis or mixed culture of some
microorganisms like Brettanomyces clausenii and Saccharomyces cerevisiae (Golias
et al. 2002; Spindler et al. 1988). Table 2 shows the different substrate, method, and
microorganism involved in previous ethanol production research
Cellulose processing cannot commence until the improvement of (i) the relatively slow kinetics of breaking down pure cellulose into sugars, (ii) the low yields
of sugars from other plant polysaccharides, and (iii) the removal of lignin, a relatively intractable polymer of phenylpropanoid subunits. It is clear that technological
advances must be realized to make biofuels sustainable and cost-effective.
6 Toxic Compounds Generated from Pretreatment
The attribute and concentration of toxic compounds generated from various
pretreatment depend on biomass source, pretreatment condition and the use of
catalyst. Three types of toxic compounds were known, viz., furans, phenolic compounds, and carboxylic acids. Among furan derivatives, 2-furaldehyde (furfural)
and 5-hydroxymethylfurfural (HMF) constitute the main degradation compounds
generated from pentoses and hexoses degradation, respectively. Pretreatments which
employ acids as hydrolytic agents and utilize high temperature and time to the
reaction will produce furfural and HMF at higher levels (Wyman 2007). Most of
the fermenting microorganisms are able to reduce furans to their corresponding
less toxic alcohols. HMF is reduced to 2,5-bis-hydroxymethylfuran and furfural to
furfuryl alcohol, and both could be also oxidized to formic acid under anaerobic
conditions (Taherzadeh et al. 1999). If furans are present at high concentration,
they exert an inhibitory effect interfering with glycolytic enzymes and synthesis
of macromolecules provoking an enlarge of the lag phase and reducing the ethanol
productivity (Almeida et al. 2007; Klinke et al. 2004). These effects depend on
furan concentration but are highly related with the yeast strain.
Main carboxylic acids generated during pretreatment are acetic acid, produced
from the acetyl groups in hemicelluloses, and formic acid, derived from furfural and