Sunday, April 29, 2007

Novel approach for conversion of xylose to ethanol

Kripa Rao1, Silpa Chelikani2, Patricia Relue2, and Sasidhar Varanasi1. (1) Department of Chemical & Environmental Engineering, The University of Toledo, 3062 Nitschke Hall, 2801 West Bancroft Street, Toledo, OH 43606, (2) Department of Bioengineering, The University of Toledo, 3062 Nitschke Hall, 2801 West Bancroft Street, Toledo, OH 43606

Of the sugars recovered from lignocellulose, D-glucose can be readily converted into ethanol by bakers or brewers yeasts (Saccharomyces Cerevisiae). However xylose obtained by the hydrolysis of hemicellulosic portions is not fermentable by the same species of yeasts. Alternatively xylose can be converted into its ketose isomer, xylulose by the enzyme xylose isomerase and this isomer can then be converted into ethanol. The economics of this two-step approach can be significantly improved by conducting the isomerization reaction of xylose and fermentation of the resulting xylulose to ethanol in a single vessel.  This approach has the additional advantage that it would enable one to overcome the unfavorable equilibrium of the isomeraization reaction, by continuous removal of the xylulose via the fermentation process. A major obstacle; however, to this approach is that the pH at which xylose isomerase displays its optimal activity (pH of 7-8.0) is much different from the pH at which the fermentation is carried out (pH of 4.0-5.0). Herein, we propose a novel scheme which allows the isomerization and fermentation to take place simultaneously in a single vessel. This technique would involve the immobilization of xylose isomerase in a porous polymer pellet with an additional layer of urease enzyme immobilized on it. These bilayered pellets are dispersed in a fermentation broth which contains prespecified amounts of urea in addition to the other necessary ingradients.  It will be shown how this system will achieve the objective and the resulting kinetics of the combined isomerization-fermentation process will presented.