Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 205

3 Fuels and Chemicals from Hemicellulose Sugars
3.1 Ethanol
Ethanol is the most commonly employed liquid biofuel, either as a fuel or as a
gasoline enhancer. The demand for fuel ethanol is expected to increase greatly as a
safer alternative to methyl tertiary butyl ether (MTBE), the most common additive
to gasoline used to provide cleaner combustion [ 8 ]. Ethanol as a fuel additive can
be utilized to elevate the oxygen content of gasoline, allowing a better oxidation of
hydrocarbons and reducing tailpipe emissions of carbon dioxide and unburned
hydrocarbons into the atmosphere [ 27 ].
The production of fuel ethanol from hemicellulosic hydrolysates is essential for
the economic success of lignocellulosic ethanol [ 14 ]. Low-cost feedstocks for
production of ethanol can be found from various waste and under-utilized ligno-
cellulosic agricultural residues. These lignocellulosic feedstocks containing
hemicellulose generate a mixture of sugars when pretreated alone or in combi-
nation with enzymatic hydrolysis, and the utilization of hemicellulosic sugars,
especially xylose, is essential for efficient and cost-effective conversion of ligno-
cellulosic materials to ethanol. The microbial conversion of hexose into ethanol is
well established, while the ability of ethanol fermentation using pentose by these
microorganisms is somewhat less so.
The yeasts Pachysolen tannophilus, Candida shehatae, Pichia stiptis, and
Kluveromyces marxianus have been studied extensively for their capability to
ferment xylose to ethanol [ 28 ]. Other yeasts, such as recombinant Saccharomyces
cerevisiae, were also investigated for their xylose-fermenting ability [ 29 , 30 ].
However, commercial utilization of these yeasts for ethanol production from
xylose is restricted by their slow fermentation rate, low ethanol tolerance, difficulty
in controlling the optimal oxygen supply rate, and sensitivity to inhibitors
generated during pretreatment and hydrolysis of lignocellulosic substrates [ 31 , 32 ].
Bacterial species, including Clostridium thermohydrosulfuricum, Clostridium
thermocellum, Clostridium thermosaccharolyticum, recombinant Escherichia coli,
recombinant Zymomonas mobilis, recombinant Klebsiella planticola, and several
pentose-utilizing fungal species like Fusarium oxysporum, Neurospora crassa,
Mucor sp., and Chalara parvispora, have been extensively reviewed for their
capability to convert xylose to ethanol [ 33 ]. Several representatives of these
microorganisms have different natural characteristics that can be regarded as either
advantages or disadvantages in processes of ethanol production from hemicellu-
losic sugars (Table 4 ).
In China, different microorganisms, including yeasts, bacteria and fungi, have
been used and optimized for ethanol fermentation, using both pure xylose and
hemicellulose-derived sugar mixtures [ 34 - 40 ]. Zhao and Xia [ 40 ] detoxified the
acid hydrolyzate of corn stover (the most favored lignocellulosic resource for
large-scale ethanol production) by rotoevaporation and lime neutralization, and