Amongst the many microorganisms used for bioethanol production, Saccharomyces sp. remains the prime species. Currently, both alcoholic beverages and ethanol fuels are produced through fermentation performed by Saccharomyces sp. The species are resistant to high temperatures and provide a high ethanol tolerance level, allow­ing fermentation to continue at ethanol concentrations of 16-17% (v/v) [8]. Bacteria, particularly Zymomonas mobilis and Escherichia coli, have been successfully used to ferment biomass for bioethanol production [34]. However, bacteria are less robust than yeast and their growth requires a narrow pH range (6.0-8.0), thus less prefer­able to be used in the fermentation process. Table 6 shows some wild-type of micro­organisms commonly used for industrial ethanol production. Most of the listed microorganisms fail to ferment xylose even though it is one of the sugars obtained from the hydrolysis process. In order to overcome the hurdle, genetically-modified microorganisms have been cultivated and tested to ferment xylose [25, 65]. To date, the successful use of genetically-modified strains for fermentation has been reported only at a laboratory scale.

The fermentation of simple sugars into bioethanol involves a glycolytic pathway which occurs in two major stages. The first stage is the conversion of the various sugar molecules to a common intermediate, glucose-6-phosphate. The second phase is the metabolism of each molecule of glucose-6-phosphate to yield two molecules of pyruvate [38]. The products from the glycolysis steps are further metabolized to complete the breakdown of glucose. Under anaerobic conditions, the pyruvate is further reduced to ethanol with a simultaneous release of CO2 as a by-product.