Environmental Engineering Reference
In-Depth Information
taBle 8.8
Products obtained by Fermentation
theoretical yield
(g product/
g substrate)
Product
Biochemical reaction
reference
Butanol
0.41
Nelson and Cox 2000; Liu
et al. 2009
C
6
H
12
O
6
→ C
4
H
10
O + 2CO
2
+ H
2
O
Acetone,
hydrogen
C
6
H
12
O
6
+ H
2
O → C
3
H
6
O + 3CO
2
+ 4H
2
0.32 for acetone
0.04 for hydrogen
Nelson and Cox 2000; Liu
et al. 2009
Ethanol
0.51
Nelson and Cox 2000; Liu
et al. 2009
C
6
H
12
O
6
→ 2C
2
H
6
O + 2CO
2
Butyric acid
C
6
H
12
O
6
→ C
4
H
8
O
2
+ 2CO
2
+ 2H
2
0.49
Antonopoulou et al. 2008;
Liu et al. 2009
Acetic acid
0.66
Nelson and Cox 2000; Liu
et al. 2009;
Antonopoulou et al.
2008
C
6
H
12
O
6
+ 2H
2
O → 2CH
3
COOH + 2CO
2
+ 4H
2
Gasoline-like
hydrocarbons
Renewable sugars are metabolized into fatty acids,
which are subsequently converted into hydrocarbons
Not available
LS9, Inc.
the carbohydrates mentioned before are metabolized as depicted in the previous section after some
modifications, entering in the glycolysis cycle in different stages. Mannose, galactose, and fructose
are phosphorylated and isomerized to fructose-6-phosphate, which is then fed to glycolysis.
8.4.4.3 Fermentation of Five-carbon sugars
Five-carbon carbohydrates such as xylose and arabinose are transferred into the cells of microorganisms
and converted into xylulose, a five carbon sugar. Xylulose is then fed into the pentose phosphate pathway,
where six units of xylulose are converted into five units of glucose, or its isomer fructose (McMillan
1993; Ostergaard et al. 2000; Jeffries and Jin 2004; Dmutruk et al. 2008), which are then fed to the
glycolysis pathway. The intermediaries of this process are sedoheptulose, a seven-carbon carbohydrate,
erythrose, a four-carbon carbohydrate, and glyceraldehyde. The six-carbon sugars are then fed to the
glycolysis process. Overall the process of fermenting six units of xylose to ethanol, by means of the
pentose phosphate pathway followed by glycolysis, is summarized by the following reaction with a
theoretical yield of xylose to ethanol of 0.51 g of ethanol per g of xylose (McMillan 1993):
6[ Xylo s e] → 10[Ethanol] + 10CO
2
+ 10H
2
O
(8.5)
8.4.4.4 Fermentation of Five- and six-carbon sugars to ethanol and other Biofuels
Ethanol production from lignocellulosic materials deals with the fermentation of xylose (derived
from hemicelluloses) and glucose (mostly derived from cellulose). The fermentation of these sugars
could be achieved separately or simultaneously. When the fermentation of xylose and glucose
occurs in different vessels, it is possible to select a specific microorganism to ferment xylose and a
specific microorganism to ferment glucose. When the fermentation is simultaneous, it is possible to
achieve the fermentation of both carbohydrates by the use of a mixture of microorganisms or a single
specially engineered microorganism (DOE 2006). Also, during the selection of the microorganisms,
it is important to consider the technology used to make available the carbohydrates because of
the possibility of generation of some inhibitors of fermentation. When lignocellulosic material is
subjected to dilute acid hydrolysis at high temperature, furfural and 5-hydroxymethylfurfural are
formed.
Saccharomyces cerevisiae
is inhibited by these compounds (Liu 2006). Table 8.9 shows
information related to ethanol production from lignocellulosic materials.
