There exists a significant number of species of yeasts and bacteria having the ability to synthesize ethanol. Main ethanol-producing microorganisms that are currently used in the industry or have potential utilization in the future are shown in Table 6.2. Among the main criteria in choosing a microorganism producing

Ethanol Tolerance Remarks References 150 g/L Lip to 95% of theor. yield Claassen et al. (1999) High osmotic tolerance Bullock (2002); Goyes and Bolanos (2005) 80-90% of theor. yield Kadar et al (2004) 30-45 g/L 94% of theor. yield on xylose; Olsson and Hahn-Hagerdal produces moderate amounts of xylitol (1996); Jeffries and Jin (2000) 35-47 g/L 92% of theor. yield on xylose Olsson and Hahn-Hagerdal (1996); Jeffries and Jin (2000) 37.5-75 g/L 47-55% of theor. yield on Olsson and Hahn-Hagerdal xylose; produces large amounts of xylitol (1996); Jeffries and Jin (2000) 100 g/L Up to 97% of theor. yield Claassen et al. (1999) 10-30 g/L Produces acetic acid Claassen et al. (1999); South et al. (1993) <30 g/L 41 % of theor. yield on xylose Olsson and Hahn-Hagerdal (1996)

ethanol, the ability to assimilate a wide range of substrates should be emphasized. Unfortunately, alcoholic fermentation presents an end-product inhibition, so one of the most desired features of the potential microorganisms is a high ethanol tol­erance. Similarly, the cultivation conditions allow defining some desirable features of the microorganisms to be used. For instance, elevated temperatures allow the reduction of cooling costs as well as the acceleration of the metabolic processes (although this can imply an increase in the inhibitory effect of ethanol). Increased temperatures can be advantageous when the microorganisms are cultivated along with hydrolytic enzymes in simultaneous hydrolysis and fermentation processes of either starch or cellulose since these enzymes have, in general, optimum tem­peratures of enzymatic activity higher than fermentation temperature. Finally, one of the most important selection criteria is the achieved ethanol yield on substrate because the economy of the overall process directly depends on this parameter.