Second-Generation Bioethanol: Advancement of Ethanologenic …
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The concentration and type of phenolic compounds are highly dependent on the
raw material since lignin content and chemical structure differ among the different lignocellulosic materials. The hydrolytic conditions during pretreatment are also
very important for the functionality of the degradation products, that is, the phenolic aldehydes have been shown to be favored at oxidative acidic conditions (Klinke
et al. 2002). After soda pulping wheat straw, phenols r-cumaric and ferulic acids
are produced by the hydrolysis of esterified hemicellulose and lignin. Alkaline wet
oxidation of wheat straw also produces cinnamic acid derivates. Furthermore, owing
to oxidative cleavage of the conjugated double bonds, 4-hydroxybenzoic acid and
vanillic acid are formed (Klinke et al. 2002). Some other more abundant phenolic
compounds are 4-hydroxybenzaldehyde, vanillin, synringaldehyde, syringic acid,
and cathecol. These compounds are toxic because they affect the integrity of biological membranes (Almeida et al. 2007). In general, it is accepted that there is a high
amount of degradation products derived from lignin that remain unidentified.
As have been mentioned, severe conditions during pretreatment lead to the generation of some toxic compounds that could affect the subsequent hydrolysis and
fermentation steps. Some studies have also reported that small amounts of acetic,
levulinic, or formic acid could increase glucose consumption rates and ethanol yields
because low concentration of acids stimulated the production of ATP (Almeida et al.
2007; Keating et al. 2006). Production of bioethanol usually will undergo whether
submerged fermentation or solid-state fermentation (SSF). Submerged fermentation
usually implemented in the industrial scale because of the familiarity of the process
but this process required a lot of water. Whereas, solid-state fermentation did not gain
an interest because of the difficulty in process scale up. However, solid-state fermentation mimic the natural environment that microorganism inhibits which offers more
possibilities (Jain et al. 2013). Currently, ethanol fermentation is carried out mainly
by submerged fed-batch processes with cell recycle, and a small part is produced
through multistage continuous fermentation with cell recycle (Mussatto et al. 2010).
7 Submerge Fermentation
In order to produce bioethanol in a cost-effective way, it is important to have a lowcost substrate for fermentation. Commonly submerged fermentation of agricultural
wastes such as corn, banana, and potato peels, molasses, and waste food grain was
utilized to produce bioethanol. As the agricultural waste is in solid form, in order to
carry out submerged fermentation, a solution containing carbon source especially
glucose needs to be obtained. Hydrolysate containing different composition of
saccharides could be done using acid hydrolysis method or enzyme hydrolysis
method. This additional step usually referred to pretreatment process. Pretreatment
of the waste such as hydrolysis can be carried out by HCl hydrolysis. Higher
yield was observed when 10% substrate concentration, pH 5.5 and particle size of
0.157 mm were used. Saccharomyces species was normally used for fermentation.
A 10% inoculum was used and temperatures of 30–35 °C wee applied. A range
75
The concentration and type of phenolic compounds are highly dependent on the
raw material since lignin content and chemical structure differ among the different lignocellulosic materials. The hydrolytic conditions during pretreatment are also
very important for the functionality of the degradation products, that is, the phenolic aldehydes have been shown to be favored at oxidative acidic conditions (Klinke
et al. 2002). After soda pulping wheat straw, phenols r-cumaric and ferulic acids
are produced by the hydrolysis of esterified hemicellulose and lignin. Alkaline wet
oxidation of wheat straw also produces cinnamic acid derivates. Furthermore, owing
to oxidative cleavage of the conjugated double bonds, 4-hydroxybenzoic acid and
vanillic acid are formed (Klinke et al. 2002). Some other more abundant phenolic
compounds are 4-hydroxybenzaldehyde, vanillin, synringaldehyde, syringic acid,
and cathecol. These compounds are toxic because they affect the integrity of biological membranes (Almeida et al. 2007). In general, it is accepted that there is a high
amount of degradation products derived from lignin that remain unidentified.
As have been mentioned, severe conditions during pretreatment lead to the generation of some toxic compounds that could affect the subsequent hydrolysis and
fermentation steps. Some studies have also reported that small amounts of acetic,
levulinic, or formic acid could increase glucose consumption rates and ethanol yields
because low concentration of acids stimulated the production of ATP (Almeida et al.
2007; Keating et al. 2006). Production of bioethanol usually will undergo whether
submerged fermentation or solid-state fermentation (SSF). Submerged fermentation
usually implemented in the industrial scale because of the familiarity of the process
but this process required a lot of water. Whereas, solid-state fermentation did not gain
an interest because of the difficulty in process scale up. However, solid-state fermentation mimic the natural environment that microorganism inhibits which offers more
possibilities (Jain et al. 2013). Currently, ethanol fermentation is carried out mainly
by submerged fed-batch processes with cell recycle, and a small part is produced
through multistage continuous fermentation with cell recycle (Mussatto et al. 2010).
7 Submerge Fermentation
In order to produce bioethanol in a cost-effective way, it is important to have a lowcost substrate for fermentation. Commonly submerged fermentation of agricultural
wastes such as corn, banana, and potato peels, molasses, and waste food grain was
utilized to produce bioethanol. As the agricultural waste is in solid form, in order to
carry out submerged fermentation, a solution containing carbon source especially
glucose needs to be obtained. Hydrolysate containing different composition of
saccharides could be done using acid hydrolysis method or enzyme hydrolysis
method. This additional step usually referred to pretreatment process. Pretreatment
of the waste such as hydrolysis can be carried out by HCl hydrolysis. Higher
yield was observed when 10% substrate concentration, pH 5.5 and particle size of
0.157 mm were used. Saccharomyces species was normally used for fermentation.
A 10% inoculum was used and temperatures of 30–35 °C wee applied. A range