Xylose fermentation: Analysis, modelling, and design
Abstract
Ethanolic fermentation is a means of utilizing xylose-rich industrial wastes, but an optimized bioprocess is lacking. Pachysolen tannophilus NRRL Y-7124 was the first yeast discovered capable of significant ethanol production from xylose and has served as a model for studies of other yeasts mediating this conversion. However, a comparative evaluation of strains led us to focus on Pichia stipitis NRRL Y-7124 as the yeast with highest potential for application. Given 150 g/L xylose in complex medium, strain Y-7124 functioned optimally at 25-26$\sp\circ$C and pH 4-7 to accumulate 56 g/L ethanol with negligible xylitol production. Dissolved oxygen concentration was critical to cell growth; and in order to measure it accurately, a colorimetric assay was developed to allow calibration of electrodes based on oxygen solubility in media of varying composition. Specific growth rate was a Monod function of limiting substrate concentration (oxygen and/or xylose). Both specific ethanol productivity and oxygen uptake rate were growth-associated, but only the former was maintenance-associated. Both growth and fermentation were inhibited by high xylose and ethanol concentrations. Carbon and cofactor balances supported modelling xylose metabolism as a combination of four processes: assimilation, pentose phosphate oxidation, respiration, and ethanolic fermentation. A mathematical model describing the stoichiometry and kinetics was constructed, and its predictive capacity was confirmed by comparing simulated and experimental batch cultures. Consideration of example processes indicated that this model constitutes an important tool for designing the optimum bioprocess for utilizing xylose-rich wastes.
Degree
Ph.D.
Advisors
Okos, Purdue University.
Subject Area
Agricultural engineering
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