Fermentation of steam-pretreated wheat straw using recombinant strains of Saccharomyces cerevisiae

The production cost of ethanol from lignocellulosic materials is higher compared with production from starch or sugar. There are several alternatives to reduce the production costs. An obvious way is to increase the ethanol concentration in the fermentation broth, resulting in less energy requirement in the distillation step. However, to be able to increase the ethanol concentration, higher solids loading during simultaneous saccharification and fermentation (SSF) can be used. This alternative may result in mixing problems associated with the increased solids loading. Another alternative is to co-ferment hexose and pentose sugars using a pentose and hexose fermenting organism.

In this study different recombinant strains of Saccharomyces cerevisiae were used for co-fermentation of the pretreatment liquid after steam pretreatment of acetic-acid-impregnated wheat straw. First, focus was on fermentation of the xylose present in the pretreatment liquid. Then, also the cellulose-rich part of the pretreated material was added using SSF, either in batch or fed-batch mode. The impact of different variables was investigated to optimize and better understand the fermentation step. In particular, addition of nutritional compounds, e.g. yeast extract, high amount of monomer sugars, addition of a small amount of pretreated solids and temperature were investigated.  The results from the on-going study will be presented.