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 fer­ment 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 fer­mentation 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 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-Hagerdal [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 iden­tified 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 40-46oC. They selected several strains belonging to the genera Saccharomyces, Kluyveromyces, and Fabospora, in view of their capacity to ferment glucose, galactose, and mannose at 40oC, 43oC, and 46oC, respectively. Kluyveromyces marxianus has been found to be a suitable strain for SSF [56].