Submitted to: Biomass and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2014
Publication Date: 4/26/2014
Publication URL: http://handle.nal.usda.gov/10113/60323
Citation: Nichols, N.N., Hector, R.E., Saha, B.C., Frazer, S.E., Kennedy, G.J. 2014. Biological abatement of inhibitors in rice hull hydrolyzate and fermentation to ethanol using conventional and engineered microbes. Biomass and Bioenergy. 67:79-88.
Interpretive Summary: This research discovered that a microbe found in soil may be used to improve production of ethanol from rice hulls, a by-product equal to 20% of harvested rice by weight. Rice hulls are harvested with the grain, but the hulls are poorly suited as animal feed and have low value. Rice hulls do have sugars that could be fermented to ethanol for fuel. However, the sugars obtained from rice hulls also contain compounds that are detrimental to fermentation. Use of a specially selected microbe to remove the inhibitory compounds yields “cleaner,” more useable sugars that can be efficiently converted to ethanol by fermentation. This research will benefit producers of renewable fuels and chemicals.
Technical Abstract: Microbial inhibitors arise from lignin, hemicellulose, and degraded sugar during pretreatment of lignocellulosic biomass. The fungus Coniochaeta ligniaria NRRL30616 has native ability to metabolize a number of these compounds, including furan and aromatic aldehydes known to act as inhibitors toward relevant fermenting microbes. In this study, C. ligniaria was used to metabolize and remove inhibitory compounds from pretreated rice hulls, which comprise a readily available agricultural residue rich in glucose (0.32-0.33 g glucan/g hulls) and xylose (0.15-0.19 g xylan/g hulls). Samples were dilute-acid pretreated and subjected to bioabatement of inhibitors by C. ligniaria. The bioabated rice hull hemicellulose hydrolysates were then utilized for ethanol fermentations. In bioabated liquors, glucose was converted to 0.58% (w/v) ethanol by Saccharomyces cerevisiae D5a at 100% of theoretical yield, while in fermentations of unabated hydrolysates, little glucose was consumed and 0.11% (w/v) ethanol was produced. In fermentations using ethanologens engineered for conversion of pentoses, bioabatement of hydrolysates similarly resulted in increased ethanol production. Fermentation of xylose and arabinose by ethanologenic Escherichia coli yielded 2.25% and 0.05% (w/v) ethanol from bioabated and unabated samples, respectively. Metabolism of xylose by Saccharomyces cerevisiae YRH400 increased in bioabated samples, as reflected in decreased fermentation lag times. However, in S. cerevisiae YRH400, xylose metabolism was strongly affected by pH and acetate concentration.