(116bf) Screening of Xylose-Fermenting Yeast Strains for Production of Cellulosic Ethanol

Obtaining ethanol from sugar cane bagasse is regarded as a possible source of this alternative fuel for two reasons: it does not hinder food accessibility and, reduces solid waste generation from sugar cane processing. Cellulose, hemicellulose and lignin are the main components of bagasse; cellulose can be hydrolyzed into glucose yet hemicellulose is hydrolyzed into glucose and other pentoses, among which xylose is abundant. Unlike glucose, xylose can only be fermented by a limited number of yeast strains. During this project, the main objective was to determine the kinetics and ethanol yields of xylose-fermenting yeast strains Pichia stipitis NRRL Y-11545, Pachysolen tannophilus NRRL Y-12885 and Candida shehatae NRRL Y-12858 by employing the unstructured and non-segregated Monod model in order to screen for the most efficient xylose fermenting strain. Batch fermentations for each strain were conducted at a pH range between 4.5 and 5.5 and, temperature and agitation speed of 32°C and 115 rpm, respectively. Batch culture data and ethanol production were determined by means of periodical optical density measurements, dry biomass weighing and HPLC analysis for xylose and ethanol. The maximum specific growth rate, μ_max, overall ethanol yields, Y_P/S and Y_P/X, and overall biomass yield, Y _X/S were obtained for each strain with the purpose of developing a mathematical model to describe the xylose fermentation kinetics. For Pichia stipitis and Candida shehatae, μ_max values were 0.036 h^-1 and 0.0586 h^-1, respectively. Based on data analysis, Pichia stipitis offers the highest ethanol yield from xylose fermentation. Ultimately, the most efficient xylose fermenting strain will be employed along with Saccharomyces cerevisiae in a subsequent study on co-fermentation of glucose and xylose to implement an efficient cellulosic ethanol process design.