In general, yeast and filamentous fungi metabolize xylose through a two — step reaction before they enter the central metabolism (glycolysis) through the PPP. The first step is conversion of xylose to xylitol using xylose reductase (XR), and the second step is conversion of xylitol to xylulose using another enzyme, xylitol dehydrogenase (XDH) [40-42].

Wild strains of S. cerevisiae possess the enzymes XR and XDH, but their activities are too low to allow growth on xylose. Although S. cere­visiae cannot utilize xylose, it can utilize its isomer, xylulose. Thus, if

S. cerevisiae is to be used for xylose fermentation, it requires a genetic modification to encode XR/XDH or XI [40, 43].

Bacteria have a slightly different metabolic pathway for xylose uti­lization. They convert xylose to xylulose in one reaction using XI [10, 44-46].

3.7 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 fer­mentation pH, (f) high optimum temperature, (g) general hardiness under physiological stress, and (h) tolerance to potential inhibitors pres­ent in the substrate [31, 47]. Ethanol and sugar tolerance allows the con­version 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 con­tent. Low-pH fermentation combats contamination by competing organ­isms. 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.