Page 104 - Biofuels Refining and Performance

Bioethanol: Market and Production Processes 87

to temperature, and substrate concentration. Furthermore, P. stipitis is
able to ferment a wide variety of sugars to ethanol and has no vitamin
requirement for xylose fermentation [2].
Olsson and Hahn-Hägerdal [20] have presented a list of bacteria, yeasts,
and filamentous fungi that produce ethanol from xylose. Certain species
of the yeasts Candida, Pichia, Kluyveromyces, Schizosaccharomyces,
and Pachysolen are among the naturally occurring organisms. Jeffries and
Kurtzman [53] have reviewed the strain selection, taxonomy, and genetics
of xylose-fermenting yeasts.
Utilization of cellobiose is important in ethanol production from lig-
nocellulosic materials by SSF. However, a few ethanol-producing
microorganisms are cellobiose-utilizing organisms. The requirement for
addition of -glucosidase has been eliminated by cellobiose utilization
during fermentation, since presentation of cellobiose reduces the activity
of cellulase. Cellobiose utilization eliminates the need for one class of
cellulase enzymes [2]. Brettanomyces custersii is one of the yeasts iden-
tified as a promising glucose- and cellobiose-fermenting microorganism
for SSF of cellulose for ethanol production [54].
High temperature tolerance could be a good characterization for
ethanol production, since it simplifies fermentation cooling. On the other
hand, one of the problems associated with SSF is the different optimum
temperatures for saccharification and fermentation. Many attempts
have been made to find thermotolerant yeasts for SSF. Szczodrak and
Targonski [55] tested 58 yeast strains belonging to 12 different genera
and capable of growing and fermenting sugars at temperatures of
40–46 C. They selected several strains belonging to the genera
Saccharomyces, Kluyveromyces, and Fabospora, in view of their capacity
to ferment glucose, galactose, and mannose at 40 C, 43 C, and 46 C,
respectively. Kluyveromyces marxianus has been found to be a suitable
strain for SSF [56].

3.11.2 Bacteria
A great number of bacteria are able to produce ethanol, although many
of them generate multiple end products in addition to ethanol.
Zymomonas mobilis is an unusual Gram-negative bacterium that has
several appealing properties as a fermenting microorganism for ethanol
production. It has a homoethanol fermentation pathway and tolerates
up to 120 g/L ethanol. Its ethanol yield is comparable with S. cerevisiae,
while it has much higher specific ethanol productivity (2.5 ) than the
yeast. However, the tolerance of Z. mobilis to ethanol is lower than that
of S. cerevisiae, since some strains of S. cerevisiae can produce ethanol
to give concentrations as high as 18% of the fermentation broth. The tol-
erance of Z. mobilis to inhibitors and low pH is also low. Similarly,