(126c) Xylose to Ethanol – Why is it still a challenge?

It was early recognized that sugars with five carbon atoms, pentose (C5) sugars,  had to be utilized efficiently when renewable lignocellulose biomass is converted to replace fossil transportation fuels. Lignocellulose biomass includes agriculture and forest residues as well as dedicated energy crops. Typical examples are corn stover, sugar cane bagasse, wheat straw, poplar, pine and switchgrass. In these raw materials the C5 content ranges from 7% DM in pine to 22% DM in switchgrass. The most investigated substitute for fossil transportation fuel is ethanol. Biomass conversion to ethanol is a very old and well established process based on sugar and starch, which contain only hexose (C6) sugars. The preferred microorganism in these processes is the yeast Saccharomyces cerevisiae because of its superior tolerance to osmostic pressure and to ethanol. However, this organism does not naturally ferment C5 sugars to ethanol.

C5 fermenting S. cerevisiae strains have been generated by applying genetic engineering first in haploid laboratory strains and later in diploid and polyploid industrial strains. Many C5 strains have then been further adapted to lignocellulose materials such as wheat straw and corn stover. An alternative approach is to select S. cerevisiae host strains based on their suitability for a particular lignocellulose raw material and then to introduce the xylose fermenting pathway. The latter rational engineering approch will be discussed in relation to the fermentation of different lignocellulose raw materials.