Effect of hydrothermolysis on ethanol yield from Alamo switchgrass using a thermotolerant yeast

Switchgrass is a perennial grass that has potential as a feedstock for ethanol production. Using switchgrass for ethanol production would reduce dependence on food crops, such as corn, that are currently used for fuel ethanol. Hot compressed liquid water was used to treat Alamo switchgrass in a method called hydrothermolysis to disrupt lignin, dissolve hemicellulose, and increase accessibility of cellulose to hydrolysis enzymes. Hydrothermolysis was selected instead of other common methods to minimize formation of inhibitors, chemical use, and corrosion of process equipment. Three temperatures (190, 200, and 210°C) and hold times (10, 15, and 20 min) were used to pretreat Alamo switchgrass at 10% solids to prepare it for SSF (Simultaneous saccharification and fermentation). Prehydrolyzate from switchgrass treated at 190°C for 10 min had the greatest xylan recovery in the hydrolyzate. From all treatment conditions, less than 0.65 g/L glucose were released into the prehydrolyzate, indicating most glucose was retained as cellulose in the solid substrate. HMF (5-hydroxymethylfurfural) and furfural formation in the prehydrolyzate were found to be less than 1 g/L for all treatments. The highest theoretical yield of ethanol (82%, 18.6 g/L) was produced from switchgrass pretreated at 200°C and 10 min using SSF at 45°C with thermotolerant yeast Kluyveromyces marxianus IMB 4 and 15 FPU cellulase/g glucan loading. The glucan loading for SSF was 40 g/L.