STUDIES ON THE SIMULTANEOUS ENZYMATIC ISOMERIZATION AND YEAST FERMENTATION OF D-XYLOSE
Abstract
The addition of the enzyme D-xylose isomerase (glucose isomerase) to fermentation medium facilitates the conversion of D-xylose into ethanol by a two step process in which D-xylose is first isomerized to D-xylulose, which is in turn converted to ethanol by yeast. Detailed kinetic studies were conducted independently for the isomerization reaction and for the fermentation of D-xylulose by the yeast Schizosaccharomyces pombe. The kinetics of the isomerization reaction has been examined using a differential batch reactor at different temperatures and pH values. The kinetic parameters of the reversible isomerization reaction were evaluated at the optimal pH value, 7.5, and 40(DEGREES)C, and at pH 6.0 and 30(DEGREES)C. The kinetics of the utilization of D-xylulose by the yeast Schizosaccharomyces pombe has been examined in presence of D-xylose and ethanol under anaerobic batch conditions. The effects of the concentration of each of the two sugars upon the production of ethanol were also examined. Based upon fermentations done with D-xylulose, and mixtures of D-xylose and D-xylulose, it was observed that D-xylose has a significant effect upon D-xylulose utilization. The pattern of inhibition was observed to be too complex to be attributed to pure competitive inhibition and was found to be correlated to the ratio of D-xylose to D-xylulose concentrations. Strong end-product inhibition was also observed. The pattern of non-competitive ethanol inhibition on specific ethanol production rate was found to be linear with respect to ethanol concentration. These relationships for sugar utilization and ethanol production have provided the basis for a kinetic model for alcoholic fermentation of mixtures of D-xylose and D-xylulose. A combined model has been formulated that predicts the performance of the batch conversion of D-xylose to ethanol by the simultaneous isomerization/fermentation process. A reasonable agreement was found between experimental data and values calculated from the model, which also simulates the effects of varying the isomerase, yeast, and initial pentose sugar concentration on fermentation rate, ethanol yield and ethanol productivity values.
Degree
Ph.D.
Subject Area
Chemical engineering
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