Epsilon Archive for Student Projects

Abstract

Bioethanol is one alternative energy fuel that can be produced from different types of biomass. Second generation bioethanol comes from fermentation of lignocellulosic material and can be seen as a more environmental friendly alternative as e.g. forest residues and agricultural by-products are used. To be able to use lignocellulosic biomass in ethanol production, it has to be subjected to different treatments first. In this study, aspen sawdust was used as lignocellulosic material. After pretreatment with steam explosion and hydrolysis, using enzymes, a hydrolysate was obtained containing dissolved sugars, e.g. glucose. Fermentation of glucose into ethanol was done using two different yeasts to compare the differences: the traditional baker’s yeast Saccharomyces cerevisiae J672 and the yeast Dekkera bruxellensis CBS 11269, most known as a spoilage yeast but recently found to be the producing organism of ethanol in an ethanol production plant. In the study, the addition of (NH4)2SO4 at 2 g/L did not affect the growth negatively. Yeast extract was observed to be an essential media component for growth of D. bruxellensis CBS 11269 and improved the growth for S. cerevisiae J672. Due to the high amounts of toxic compounds acting inhibitory to the yeast, the hydrolysate produced needed to be diluted in order to achieve yeast growth. Unadapted S. cerevisiae J672 grew in 1:5 and 1:10 diluted hydrolysate while D. bruxellensis CBS 11269 only grew in the 1:10 dilution. Adapted yeast cells, of both yeasts, in a continuous experiment with recirculation, grew well in both 1:10 and 1:5 diluted hydrolysate resulting in an ethanol yield between 0.21-0.26 g ethanol/g glucose. No clear difference between the yeasts was seen. When applying the same procedure in fermenters, the same dilution rate as in the smaller scale experiment was not achieved properly and neither did the dilution rate become the same for both yeasts. No comparison between the yeasts could therefore be made for the fermenter part. The ethanol yield for S. cerevisiae J672 was between 0.35-0.36 g ethanol/g glucose for the media compositions that occurred during the experiments and for D. bruxellensis CBS 11269 it varied between 0.31-0.37 g ethanol/g glucose.