In traditional ethanol fermentations S. cerevisiae has been used to produce this biofuel from either sugarcane or corn. These substrates contain glucose or sucrose (a disaccharide of glucose and fructose) and do not contain pentose sugars. Although a natural xylose-ferment­ing yeast, Pichia stipitis produces ethanol with comparatively good productivity (Hahn- Hagerdal et al. 2006- from pentose sugars; it is inhibited when biomass hydrolysates are employed. For this and other reasons, an ideal ethanol-producing culture should possess the following characteristics: should not be inhibited by inhibitors produced during biomass pretreatment as well as its own metabolic byproducts such as acetic and lactic acids; should be able to utilize hexose and pentose sugars; and should be able to produce and tolerate a high concentration of ethanol. Production of byproducts such as acetic acid reduces ethanol yield and arrests cell growth and the fermentation process. A large number of studies are

being focused toward this direction with the aim of developing suitable microbial strains; however, only limited success has been achieved to date. The recombinant strains of yeast and bacteria that have been developed include Escherichia coli (KO11 and FBR5; Ingram et al. 1987) , Klebsiella oxytoca, Zymomonas mobilis, and S. cerevisiae (424A [LNF-ST], TMB3006, TMB3400). So far only recombinant S. cerevisiae strains have been able to produce ethanol from xylose contained in non-detoxified hydrolysates. Some of these S. cerevisiae strains have been used in fed-batch systems in combination with extremely inhibi­tory softwood hydrolysates (Hahn-Hagerdal et al. 2006). Details of some of the fermentation parameters that resulted from the use of these xylose-utilizing strains have been listed in Table 2.1.