Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2009
Publication Date: 8/1/2009
Citation: Dien, B.S., Sarath, G., Pedersen, J.F., Sattler, S.E., Chen, H., Funnell-Harris, D.L., Nichols, N.N., Cotta, M.A. 2009. Improved Sugar Conversion and Ethanol Yield for Forage Sorghum (Sorghum bicolor L. Moench) Lines with Reduced Lignin Contents. BioEnergy Research. 2(3):153-164. Interpretive Summary: In this research, we discovered that more ethanol could be made from specially bred sorghum plants than from the same amount of regular plants. While these special plants are not ready for farmers to grow, the seed companies can use this information to breed better sorghum plants. This is important because biomass sorghum has great potential as a crop for making cellulosic ethanol; it produces a lot of biomass and grows on farmland that you would not grow corn on. However, sorghum is harder to convert to fuel alcohol than corn and does not make as much ethanol. This research is expected to help manufacturers of cellulosic ethanol because the higher yield of the specially bred sorghum plants opens a door to greater profits and less financial risk.
Technical Abstract: Lignin is known to impede conversion of lignocellulose into ethanol. In this study, forage sorghum plants carrying brown midrib (bmr) mutations, which reduce lignin contents, were evaluated as bioenergy feedstocks. The near isogenic lines evaluated were: wild-type, bmr-6, bmr-12, and bmr-6 bmr-12 double mutant. The bmr-6 and bmr-12 mutations were equally efficient at reducing lignin contents (by 13 and 15% respectively) and the effects were additive (27%) for the double-mutant. Reducing lignin content was highly beneficial for improving biomass conversion yields. Sorghum biomass samples were pretreated with dilute-acid and recovered solids washed and hydrolyzed with cellulase to liberate glucose. Glucose yields for the sorghum biomass were improved by 27%, 23%, and 34% for bmr-6, bmr-12, and the double mutant respectively compared to wild-type. Sorghum biomass was also pretreated with dilute-acid followed by co-treatment with cellulases and Saccharomyces cerevisiae for simultaneous saccharification and fermentation (SSF) into ethanol. Conversion of cellulose to ethanol for dilute-acid pretreated sorghum biomass was improved by 22%, 21%, and 43% for bmr-6, bmr-12, and the double mutant compared to wild-type, respectively. Electron microscopy of dilute-acid treated samples showed an increased number of lignin globules in double mutant tissues as compared to the wild-type, suggesting the lignin had become more pliable. The mutations were also effective for improving ethanol yields when the (de-grained) sorghum was pretreated with dilute alkali instead of dilute acid. Following pretreatment with dilute ammonium hydroxide and SSF, ethanol conversion efficiency was 72% for the double-mutant and 59% for the wild-type.