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Bioethanol: Market and Production Processes 83

3.8 Conversion of Simple Sugars
to Ethanol
Conversion of simple hexose sugars, such as glucose and mannose, in
fermentation into ethanol can take place anaerobically as follows:

Microorganisms
C H O (Hexoses) ⎯⎯⎯⎯⎯→ 2C H OH (ethanol) 2CO 2
12
5
2
6
6
If the entire sugar is converted into ethanol according to the above reac-
tion, the yield of ethanol will be 0.51 g/g of the consumed sugars, mean-
ing that from 1.0 g of glucose, 0.51 g of ethanol can be produced. This
is the theoretical yield of ethanol from hexoses. However, the ethanol
yield obtained in fermentation does not usually exceed 90–95% of the
theoretical yield, since part of the carbon source in sugars is converted
to biomass of the microorganisms and other by-products such as glycerol
and acetic acid [9, 31].
A similar reaction for anaerobic conversion of pentoses, such as xylose
to ethanol, might be considered. Xylose is generally converted first to
xylulose by a one-step reaction catalyzed by xylose isomerase (XI) in
many bacteria, or by a two-step reaction through xylitol in yeasts and
fungi. It can then be converted to ethanol anaerobically through a pentose
phosphate pathway (PPP) and glycolysis. The general reaction can be
written as
Microorganisms
3C H O (Pentoses) ⎯⎯⎯⎯⎯→ 5C H OH (Ethanol) 5CO 2
5
10
5
5
2
In this case, we can expect a theoretical ethanol yield of 0.51 g/g from
xylose, as we had from glucose. However, the redox imbalance and slow
rate of ATP formation are two major factors that make anaerobic ethanol
production from xylose very difficult [32, 33]. A few anaerobic ethanol-
producing strains have been developed from xylose in research labora-
tories, but no strain is so far available for industrial-scale processes.
Attempts have been made to overcome this problem of xylose assimila-
tion by cometabolization or working with microaerobic conditions, where
oxygen is available at low concentrations. A number of microorganisms
can produce ethanol aerobically from xylose, where the practical yield
of ethanol from xylose and other pentoses is usually lower than its the-
oretical yield. The challenges in ethanol production from xylose have
been reviewed by van Maris et al. [34].

3.9 Biochemical Basis of Ethanol
Production from Hexoses
Asimplified central metabolic pathway for ethanol production in yeast and
bacteria under anaerobic conditions is presented in Fig. 3.7 [15, 35–37].