Tuesday, May 3, 2011
Alkaline hydrogen peroxide (AHP) pretreatment is capable of very high carbohydrate digestibilities in grasses and results in the minimal generation of compounds toxic to fermentation. For this work, AHP-pretreated switchgrass (Panicum virgatum, cv. Cave-in-Rock) hydrolyzate was tested for ethanol fermentation by using 2 xylose-fermenting yeasts, wild-type Pichia stipitis CBS 6054 and Saccharomyces cerevisiae GLBRC Y35 which expressed a heterologous xylose fermentation pathway (xylose reductase, xylitol dehydrogenase, and xylulokinase). Without detoxification or Na+ removal, both yeasts were capable of growth and glucose and fermentation on hydrolyzates, however xylose utilization was particularly slow. Fermentation inhibitors in the hydrolyzate include inorganics (sodium from NaOH and sulfate used for pH adjustment) and solubilized organics (aromatics, acetate, and oxidation products such as aliphatic carboxylic acids). Removal of aromatics in the hydrolyzate by detoxification with activated carbon significantly improve the rate of growth, glucose, and xylose fermentation indicating aromatics were the primary contributor to inhibition. Growth tests with increasing sodium phosphate buffers indicated that P. stipitis rates of growth and fermentation decrease strongly at [Na+] > 0.5 M and were completely halted at [Na+] > 1.0 M which was still much higher than the salt concentrations in these hydrolyzates. Process integration challenges for AHP pretreatment at higher solids loadings which are necessary for generating suitably high ethanol titers include increasing concentrations of these soluble fermentation inhibitors. High-solids AHP pretreatment and enzymatic hydrolysis is capable of yielding more than 100 g/L of fermentable sugars with [Na+] < 0.5 M which is promising.