Dilute-acid pretreatment of lignocellulosic biomass hydrolyzes hemicellulose to monomeric and oligomeric sugars and improves enzymatic hydrolysis of cellulose to glucose. We have previously demonstrated that fermenting the hemicellulosic-sugar-rich stream and the cellulosic-rich stream of pretreated corn stover separately improves ethanol yields compared to a process in which streams are combined. Separating hemicellulosic sugars from the cellulose-rich stream prior to enzymatic hydrolysis improves cellulose digestibility by removing sugars and other inhibitors to enzymatic hydrolysis. However, the separate process is more expensive because of the additional cost for solid-liquid separation equipment.
This study expands on our previous research into the separate hemicellulose/cellulose process configuration. Three strains of a xylose-glucose co-fermenting Zymomonas mobilis bacteria were compared for their ability to convert sugars in the hemicellulosic-rich stream to ethanol. The cellulosic stream was subjected to a simultaneous saccharification and fermentation process or separate enzymatic hydrolysis and fermentation at insoluble solids loading as high as 21%. For this process, we also compared the performance of two glucose fermenting yeasts Saccharomyces pastorianus and Saccharomyces cerevisiae D5A and Z. mobilis ZW 705 at 12% insoluble solids. Sugar conversion and ethanol yields were measured in both streams as a function of the total solids loading of the pretreated corn stover The results demonstrate how solids loading, liquor dilution, the microorganism, and enzyme loading all affect conversion yields.