(200a) Thermotolerant Bioprocessing of Lignocellulosic Biomass for Cost-Efficient Bioethanol Production

Among the bottlenecks in the development of a cost-effective process for cellulosic ethanol production are the slow rates of enzyme hydrolysis of lignocellulose and the relatively high enzyme production costs. From this perspective, the search for and the discovery of novel enzymes with unique properties and enhanced capabilities for cellulose degradation may lead to significant improvements in the bioethanol process. Due to improved substrate solubility and mass transfer rates, thermostable cellulases provide greater stability and reaction rates in the enzymatic hydrolysis of cellulose. This presentation summarises research on lignocellulose hydrolysis and fermentation to bioethanol utilizing a thermophilic lignocellulose-degrading microorganism, Geobacillus sp. R7, isolated from the deep subsurface of the Homestake gold mine in Lead (South Dakota, USA), now known as the National Science Foundation Deep Underground Science and Engineering Laboratory (NSF DUSEL). Geobacillus sp. R7 produced extracellular cellulase when grown at 60?aC on cellulose as a sole carbon source. The crude cellulase was most active at 80oC and exhibited a remarkable thermostability: it retained 50% of its initial activity at 70oC after incubation for 7 days. The thermostable enzyme was used in a 2-stage simultaneous saccharification and fermentation (SSF) of corn stover and native prairie grasses to bioethanol. Initially, biomass with varying solids loadings was enzymatically hydrolyzed at 70oC for a period of 36 to 72 h. Thereafter, the temperature was reduced to 30oC and biomass was fermented to bioethanol using a Saccharomyces cerevisiae strain. At solids loadings of 15 and 20%, the biomass was liquefied after 36 h of enzymatic hydrolysis thereby reducing the overall time of SSF. The biomass conversion to fermentable sugars using the thermostable R7 cellulase and commercial enzymes was compared and discussed. Furthermore, under limited oxygen conditions, Geobacillus sp. R7 was shown to ferment biomass to ethanol in a single step. This warrants further investigations for the establishment of a cost-effective consolidated bioprocessing (CBP) for bioethanol production.