Page 103 - Biofuels Refining and Performance

86 Chapter Three

3.11 Microorganisms Related
to Ethanol Fermentation
The criteria for an ideal ethanol-producing microorganism are to have
(a) high growth and fermentation rate, (b) high ethanol yield, (c) high
ethanol and glucose tolerance, (d) osmotolerance, (e) low optimum fer-
mentation pH, (f) high optimum temperature, (g) general hardiness
under physiological stress, and (h) tolerance to potential inhibitors pres-
ent in the substrate [31, 47]. Ethanol and sugar tolerance allows the con-
version of concentrated feeds to concentrated products, reducing energy
requirements for distillation and stillage handling. Osmotolerance
allows handling of relatively dirty raw materials with their high salt con-
tent. Low-pH fermentation combats contamination by competing organ-
isms. High temperature tolerance simplifies fermentation cooling.
General hardiness allows microorganisms to survive stress such as that
of handling (e.g., centrifugation) [47]. The microorganisms should also
tolerate the inhibitors present in the medium.

3.11.1 Yeasts
Historically, yeasts have been the most commonly used microorganisms
for ethanol production. Yeast strains are generally chosen among S. cere-
visiae, S. ellypsoideuse, S. fragilis, S. carlsbergensis, Schizosaccharomyces
pombe, Torula cremoris, and Candida pseudotropicalis. Yeast species
which can produce ethanol as the main fermentation product are
reviewed, e.g., by Lin and Tanaka [8].
Among the ethanol-producing yeasts, the “industrial working horse”
S. cerevisiae is by far the most well-known and most widely used yeast
in industry and research for ethanol fermentation. This yeast can grow
both on simple hexose sugars, such as glucose, and on the disaccharide
sucrose. S. cerevisiae is also generally recognized to be safe as a food
additive for human consumption and is therefore ideal for producing
alcoholic beverages and for leavening bread. However, it cannot fer-
ment pentoses such as xylose and arabinose to ethanol [14, 31]. There
have been several research efforts to genetically modify S. cerevisiae to
be able to consume xylose [33, 48–50]. Several attempts have been made
to clone and express various bacterial genes, which is necessary for fer-
mentation of xylose in S. cerevisiae [51, 52]. It resulted in great success,
but probably not enough yet to efficiently ferment xylose with high yield
and productivity [32].
Alternatively, xylose is converted to ethanol by some other naturally
occurring recombinant. Among the wild-type xylose-fermenting yeast
strains for ethanol production, Pichia stipitis and C. shehatae have
reportedly shown promising results for industrial applications in terms of
complete sugar utilization, minimal by-product formation, low sensitivity