Dekkera bruxellensis – a Competitive Yeast for Ethanol Production from Conventional and Non-conventional Substrates

Abstract: In the ethanol industry and research community, Saccharomyces cerevisiae is regarded as the most competitive fermentation yeast and therefore industrial yeast populations are rarely studied. This thesis investigates yeast and lactic acid bacteria (LAB) populations in a Swedish ethanol plant. Interestingly, the yeast population was dominated by Dekkera bruxellensis and not by the inoculated S. cerevisiae. High numbers of lactic acid bacteria (LAB) were also found, with Lactobacillus vini dominating. Since there was no indication of a reduction in productivity, we regarded D. bruxellensis together with L. vini as a production consortium. In test fermentations, the industrial D. bruxellensis strains had similar or higher ethanol yield than two industrial S. cerevisiae strains. Glycerol yield was lower and biomass yield higher, indicating more energy-efficient metabolism of D. bruxellensis. To test the ability of our isolates to produce ethanol from lignocellulosic substrate, we cultured D. bruxellensis in aspen sawdust hydrolysate. D. bruxellensis was slightly more sensitive to the hydrolysate than S. cerevisiae in batch culture, but was able to adapt to the inhibitors, achieving an ethanol production comparable to S. cerevisiae. Five years after the first isolation, the ethanol plant made substantial process changes. Renewed investigations showed that D. bruxellensis and L. vini were still the dominant microbial population. In continuous competition experiments, D. bruxellensis was only able to outcompete S. cerevisiae during glucose limitation. This suggests that in glucose-limited conditions, D. bruxellensis is more competitive due to its more energy-efficient metabolism, probably together with a higher affinity for glucose uptake in such conditions. Although D. bruxellensis is indeed facultatively anaerobic, it has a higher nutritional demand under anaerobic conditions than S. cerevisiae. This is probably due to a redox imbalance caused by low glycerol production. The results show that D. bruxellensis is a competitive yeast owing to its energy-efficient metabolism in glucose-limited conditions. It can act as production yeast for ethanol production from both first- and second-generation substrates under conditions of optimal aeration and nutrient supply.