Simultaneous Saccharification and Fermentation Performance of S. cerevisiae strains on spruce and bagasse slurries

Simultaneous Saccharification and Fermentation (SSF) is considered to be the most preferred option for bioethanol production from lignocellulosic raw materials. It is a process where enzymatic hydrolysis of water insoluble solids (WIS) in a slurry and fermentation of hydrolyzed sugars takes place simultaneously. It is possible to achieve high final ethanol concentrations and a high overall ethanol yield by having high WIS content in a SSF process. A wide variety of lignocellulosic materials such as softwood, hardwood and grasses are available as options for raw materials and their compositions show huge diversity. Subsequently, the fermentation performance of yeast show large variations in these varied raw materials. The present work demonstrates the performance of two strains of S. cerevisiae (parental strain and RKU903) in a SSF process using spruce and bagasse slurries at 7.5% and 11% WIS content. Enzyme mixture from Novozymes (NS-22074) at 5 FPU/g WIS is used for saccharification with an initial cell concentration of 3 g dry weight/l. The process is maintained at pH 5.0 and 35 \ub0C for a period of 96h. The analysis of SSF show that both strains perform well at 7.5% WIS spruce and no major differences observed between the strains in terms of consumption of sugars and ethanol production. However, at 11% WIS spruce both strains were not able to completely convert HMF and resulted in lower ethanol yields compared to SSF at 7.5% WIS spruce. On the other hand, at 7.5% WIS bagasse the parental strain displayed poor consumption of sugars despite low levels of HMF and furfural. This indicates the presence of other inhibitors which might limit the utilization of sugars by yeast cells. Analyses also show that considerable amounts of residual xylose were present at the end of SSF. The results from SSF experiments also suggests that for a complete characterization of yeast strains SSF at higher WIS contents in varied raw materials are needed to be performed