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Bioethanol: Market and Production Processes 85

reduce NADP . In Saccharomyces cerevisiae, 6–8% of glucose passes
through the PPP under anaerobic conditions [8, 15].
The TCA cycle functions to convert pyruvic and lactic acids and
ethanol aerobically to the end products CO and H O. It is also a common
2
2
channel for the ultimate oxidation of fatty acids and the carbon skele-
tons of many amino acids. In cells containing the additional aerobic
pathways, the NADH that forms during glycolysis results in ATP gen-
eration in the TCA cycle [8].
Ethanol production from hexoses is redox-neutral, i.e., no net forma-
tion of NADH or NADPH occurs. However, biosynthesis of the cells
results in net formation of NADH and consumption of NADPH. The

PPP is mainly used to reduce NADP to NADPH. Oxidation of surplus
NADH under anaerobic conditions in S. cerevisiae is carried out through
the glycerol pathway. Furthermore, there are other by-products—mainly
carboxylic acids: acetic acid, pyruvic acid, and succinic acid—that add
to the surplus NADH. Consequently, glycerol is also formed to com-
pensate the NADH formation coupled with these carboxylic acids. Thus,
formation of glycerol is coupled with biomass and carboxylic acid for-
mation in anaerobic growth of S. cerevisiae [15, 39].
We should keep in mind that growth of the cells and increasing their
biomass is the ultimate goal of the cells. They produce ethanol under
anaerobic conditions in order to provide energy through catabolic reac-
tions. Glycerol is formed to keep the redox balance of the cells, and car-
boxylic acids may leak from the cells to the medium. Therefore, the
ethanol-producing microorganisms produce ethanol as the major product
under anaerobic conditions, while biomass, glycerol, and some carboxylic
acids are the by-products.

3.10 Chemical Basis of Ethanol Production
from Pentoses
In general, yeast and filamentous fungi metabolize xylose through a two-
step reaction before they enter the central metabolism (glycolysis)
through the PPP. The first step is conversion of xylose to xylitol using
xylose reductase (XR), and the second step is conversion of xylitol to
xylulose using another enzyme, xylitol dehydrogenase (XDH) [40–42].
Wild strains of S. cerevisiae possess the enzymes XR and XDH, but
their activities are too low to allow growth on xylose. Although S. cere-
visiae cannot utilize xylose, it can utilize its isomer, xylulose. Thus, if
S. cerevisiae is to be used for xylose fermentation, it requires a genetic
modification to encode XR/XDH or XI [40, 43].
Bacteria have a slightly different metabolic pathway for xylose uti-
lization. They convert xylose to xylulose in one reaction using XI [10,
44–46].