Sunday, April 29, 2007

Performance evaluation of selected ethanologens in dilute-acid pretreated corn stover hydrolysate

Kent W. Evans and Daniel J. Schell. National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Blvd., MS 3511, Golden, CO 80401

The developing biorefinery industry will need robust microorganisms to be able to effectively ferment biomass-derived sugars to ethanol or other products. Therefore, the objective of this study was to compare the ability of seven of these microorganisms to ferment sugars in a dilute acid pretreated corn stover hydrolysate. The microorganisms were tested using various concentrations of hydrolysate liquor (40%, 65%, and 85% [v/v] of the original hydrolyzate concentration) that had been neutralized or conditioned by overliming. Corn stover hydrolysate was produced in a pilot scale reactor at a 30% total solids concentration with solids subsequently removed by pressing. In some cases, the liquor was spiked with additional glucose to represent glucose that would be produced by enzymatic cellulose hydrolysis of the pretreated solids were the solids still present.

 All strains performed better in conditioned hydrolysate than in hydrolysate that was only neutralized. Although for yeast fermentations, the performance differences were minor. The Saccharomyces cerevisiae yeast strains D5A, a Broin commercial yeast, and a yeast obtained from ATCC (#26602) performed well in a 85% neutralized hydrolysate converting glucose to ethanol at yields of 67%, 68%, and 71%, respectively. A recombinant Zymomonas mobilis 8b achieved ethanol yields in neutralized and conditioned hydrolysate of 68% and 74% in 40% hydrolysate, respectively, but only 17% and 41% in 85% hydrolysate, respectively. High ethanol concentrations (85 – 90 g/L) were produced by the Broin yeast and ATCC #26606 in glucose supplemented hydrolysate achieving ethanol yields of 70.4 % and 73.8%, respectively.