Kinetic modeling-based optimization of multi-feed simultaneous saccharification and co-fermentation of wheat straw for ethanol production
Conference contribution, 2015
Fed-batch simultaneous saccharification and co-fermentation (SSCF) enables production of lignocellulosic ethanol with high content of water insoluble solids (WIS), and therefore high cellulose loadings (the major sugar source in lignocellulose). The viscosity of the SSCF broth and the mass/heat transfer efficiency, depend on the feeding frequency of solid substrates and the hydrolytic activities of the added cellulases. An ideal feeding scheme should avoid over-feeding which leads to mixing problems, while feeding as much substrates as possible to shorten the process time and increase the final ethanol titer.
A previously developed kinetic model  was modified to predict the performance of cellulases on steam pre-treated wheat straw, and to decide when and how much WIS to feed in the next feeding event. With this approach, mixing problems could be completely avoided up to 22.2% WIS in lab scale stirred tank reactors, and ethanol concentrations reached 56 g/L within 72 hours of SSCF. The process was tested at demonstration scale in 10 m3 reactors, and a similar fermentation performance as that in lab scale was observed.
Further feeding of solid substrate (>20% WIS) did not lead to increases in the ethanol concentration, while a substantial loss of yeast viability (colony forming unit) were observed in SSCF medium at high WIS contents. This was likely due to toxic compounds retained in the pre-treated lignocellulose. We are currently investigating different xylose fermenting Saccharomyces cerevisiae strains in the SSCF process to increase the ethanol titer further.
 Wang et al. Bioresour. Technol., 2014