ABSTRACT
Xylose-fermenting recombinant bacteria and recombinant and wild-type yeasts are being examined for use in large-scale hemicellulose conversion processes for the production of fuel ethanol. This chapter provides a summary of representative performance data for xylose fermentations carried out using synthetic media. It summarizes performance levels achieved on detoxified hydrolyzates for the two microorganisms for which the most performance data are available, recombinant Escherichia coli bacteria and xylose-fermenting Pichia stipitis yeast. The major metabolic pathways for xylose fermentation are believed to be similar in bacteria, yeasts, and fungi, despite important differences in transport, regulation, cofactor requirements, and pyruvate fermentation pathways. A variety of factors influence xylose fermentation performance. For wild-type xylose-fermenting yeasts, aeration is one of the dominant factors influencing performance. Secondary factors affecting the performance of yeasts are medium composition, pH, and temperature. To achieve high-yield ethanol production, media must be formulated to optimize the levels of vitamins and trace minerals, and the type of nitrogen source.
