Sunday, May 3, 2009
2-16
Fed-batch schemes and yeast adaptation for improving ethanol production at high substrate loading
Elia Tomás-Pejó1, José M. Oliva1, Mercedes Ballesteros1, and Lisbeth Olsson2. (1) Renewable Energies Department, CIEMAT, Research Centre for Energy, Environment and Technology, Avda. Complutense, 22, Madrid, Spain, (2) Chemical and Biological Engineering. Industrial Biotechnology, Chalmers University of Technology, Kemivägen 10, Göteborg, Sweden
Renewable lignocellulosic biomass is expected to be the major future bioenergy feedstock. In this context, the simultaneous saccharification and fermentation process (SSF) has become one of the most successful methods for ethanol production since early 90’s. Nowadays, fed-batch SSF process is appearing as a promissing alternative allowing conversion of higher substrate loading.
One of the limitations when using lignocellulosic materials at high substrate loading is the presence of inhibitors, including weak acids, phenolic compounds and furan derivates produced during biomass pretreatment which affect yeast fermentation. Thus, the employment of more tolerant yeast is required for the development of large-scale ethanol production. Since not only glucose is contained in the pretreated material an efficient pentose fermentation is also required for making the process profitable.
In the present study, directed evolution of the xylose fermenting yeast Saccharomyces cerevisiae F12 was performed by sequential transfer of cultures to diluted prehydrolysate media with increasing concentration of inhibitors. The evolved strain was tested in prehydrolysates from steam-pretreated wheat straw obtaining in all cases higher ethanol concentration and faster sugar consumption than the parental strain. In the best case, an increase of 34% in the ethanol concentration was observed together with 45% increase in the xylose uptake. Differences were also found in the levels of glycerol, acetate and xylitol which reflected better growth rates and cell viability when using the adapted strain.
Hydrolysates that were found to be unfermentable with the parental strain could be fermented with the evolved strain both in batch and fed-batch SSF.
One of the limitations when using lignocellulosic materials at high substrate loading is the presence of inhibitors, including weak acids, phenolic compounds and furan derivates produced during biomass pretreatment which affect yeast fermentation. Thus, the employment of more tolerant yeast is required for the development of large-scale ethanol production. Since not only glucose is contained in the pretreated material an efficient pentose fermentation is also required for making the process profitable.
In the present study, directed evolution of the xylose fermenting yeast Saccharomyces cerevisiae F12 was performed by sequential transfer of cultures to diluted prehydrolysate media with increasing concentration of inhibitors. The evolved strain was tested in prehydrolysates from steam-pretreated wheat straw obtaining in all cases higher ethanol concentration and faster sugar consumption than the parental strain. In the best case, an increase of 34% in the ethanol concentration was observed together with 45% increase in the xylose uptake. Differences were also found in the levels of glycerol, acetate and xylitol which reflected better growth rates and cell viability when using the adapted strain.
Hydrolysates that were found to be unfermentable with the parental strain could be fermented with the evolved strain both in batch and fed-batch SSF.