Simultaneous Saccharification and Fermentation with substrate, enzyme and yeast feed facilitate bioethanol production at high solids loadings

Simultaneous saccharification and fermentation (SSF) is an interesting process option for lignocellulosic bioethanol production. The economical viability and commercialization of cellulose-to-ethanol demands the process to work under high-solid loadings to result in high sugar yield and final ethanol titer in S. cerevisiae based SSF process. In a conventional batch SSF process practical limitations to high-solid loadings include, poor mixing and accessibility of enzymes to substrates and high inhibitors concentration that reduces the yeast viability and metabolism. In order to overcome these limitations, we propose an improved SSF process configuration involving feeding of substrate, enzyme and yeast. It is possible to achieve maximum dilution effect with substrate, enzyme and yeast feed thereby overcoming the mixing issues associated with a batch SSF at high-solid loadings. The feed of freshly cultivated yeast throughout the fermentation process ensures active metabolic state of yeast. In addition, the substrate feed ensures low inhibitors concentration at any given time point increasing the survival ability of yeast in contrast to a batch SSF. The enzyme feed ensures slow release of glucose providing an opportunity to xylose consuming yeast strain to co-consume xylose together with glucose. With a feed of enzyme, cells and substrate, a SSF process with 20% WIS of spruce biomass yielded 40 g/l of ethanol compared to the conventional batch SSF that yielded only 13 g/l of ethanol with severe yeast and enzyme inhibition. This novel process was able to work even at 25% WIS of spruce biomass without any difficulties in mixing and therefore reducing the total power consumption due to stirring. These features make this novel configuration of SSF an extremely viable commercial approach to lignocellulosic bioethanol production at high solids loadings