Submitted to: Journal of Industrial Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2004
Publication Date: 6/8/2004
Citation: Keating, J.D., Robinson, J., Cotta, M.A., Saddler, J.N., Mansfield, S.D. 2004. An ethanologenic yeast exhibiting unusual metabolism in the fermentation of lignocellulosic hexose sugars. Journal of Industrial Microbiology and Biotechnology. 31:235-244. Interpretive Summary: Biomass resources, like agricultural by-products and residues, wood, and municipal solid waste, have the potential to serve as substrates for production of fuel ethanol in much greater quantities than is currently produced from corn. However, these sources are composed of complex mixtures of sugars which are inefficiently fermented to alcohol. In addition, preparation of these lignocellulose rich materials for use is accompanied by the generation of fermentation inhibitors which further contributes to inefficient fermentation. In the current study, two strains of yeast were evaluated for their ability to ferment the sugars in wood hydrolysates to ethanol. Both strains grew well, displaying little inhibition by toxic compounds likely to have been present in the hydrolysates. Growth of both was accompanied by rapid fermentation of the sugars present in the wood hydrolysates, but one strain was superior in the use of galactose which it could use more rapidly than the other sugars measured (glucose and mannose). Interestingly, cultures combining both strains out performed either of the strains when grown separately. This information will be used to direct further development of the galactose fermenting strain for industrial application and the potential for use of combinations of yeasts in the bioconversion of biomass to ethanol.
Technical Abstract: Three lignocellulosic substrate mixtures (liquid fraction of acid-catalyzed steam-exploded softwood, softwood spent sulfite liquor, and hardwood spent sulfite liquor) were separately fermented by a spent sulfite liquor-adapted strain of Saccharomyces cerevisiae (Tembec T1) and a natural galactose-assimilating isolate of S. cerevisiae (Y-1528) to compare fermentative efficacy. The performance of strain Y-1528 exceeded that of Tembec T1 on all 3 substrate mixtures, with complete hexose sugar consumption occurring in approximately 10-18 h for Y-1528, versus greater than 24 to greater than 48 h for T1. Furthermore, Y-1528 consumed galactose prior to glucose and mannose, in contrast to Tembec T1, which exhibited conventional catabolite repression. Ethanol yields were comparable regardless of substrate utilized. Strains T1 and Y-1528 were also combined in mixed culture to determine the effects of integrating their distinct metabolic capabilities during synthetic hexose sugar and spent sulfite liquor fermentations. Sugar consumption in the synthetic mixture was accelerated with complete exhaustion of hexose sugars occurring in just over 6 h. Galactose was consumed first, followed by glucose and mannose. Ethanol yields were slightly reduced relative to pure cultures of Y-1528, but normal growth kinetics were not impeded. Sugar consumption in the spent sulfite liquors was also accelerated, with complete utilization of softwood- and hardwood-derived hexose sugars occurring in 6 and 8 h, respectively. Catabolite repression was absent in both spent sulfite liquor fermentations.