6-08: Simultaneous saccharification and fermentation of xylan with Kluyveromyces marxianus IMB2

A great amount of today’s bioethanol production is based on corn starch. Since this substrate stands in conflict with the food industry, researchers are studying alternatives like cellulosic biomass, namely switchgrass. Switchgrass grows naturally in the US and has great potential for utilization. Switchgrass has ~20% hemicellulose with xylan as the main constituent. Since traditional yeast like Saccharomyces cerevisiae are unable to utilize xylose, the monomer of xylan, alternative yeast need to be found. Kluyveromyces marxianus IMB1-5 strains have shown the ability to utilize xylose at 40-50°C, which has the advantage to work in areas with higher average temperatures and in applications like simultaneous saccharification and fermentation (SSF). SSF includes enzymatic hydrolysis and fermentation in the same reactor. Since enzymes have higher temperature optima, usually 50 to 70°C, a yeast strain with a higher temperature optimum is preferred to accommodate both temperature optima. In this work different commercially available enzyme mixtures were evaluated in terms of xylan degradation. The enzyme mixture that produced the highest xylose concentration was used together with K. marxianus IMB2 in SSF with different enzyme concentrations and temperatures. Highest ethanol production was achieved at 40°C but with 3µl/ml enzyme concentration. However, this was only 17% of the theoretical maximum value. The results imply that under these conditions, Kluyveromyces marxianus IMB2 is not suitable for ethanol production based on xylose.