Controlled feeding of cellulases improves conversion of xylose in simultaneous saccharification and co-fermentation for bioethanol production

Simultaneous saccharification and fermentation (SSF) is an interesting option for ethanol production from lignocellulosic materials. To meet desired overall yields during ethanol production from lignocellulosic materials, it is important to use both hexoses and pentoses. This can be achieved by co-fermentation of sugars in SSF, so called SSCF (simultaneous saccharification and co-fermentation), using genetically modified yeast strains. However, high concentration of glucose in the pretreated material makes xylose utilization challenging due to competitive inhibition of sugar transport. The present work demonstrates a new approach for controlling the glucose release rate from the enzymatic hydrolysis by controlling the addition of enzymes in SSCF using spruce as the raw material. Enzyme kinetics and yeast sugar uptake rates for a recombinant xylose utilizing strain of Saccharomyces cerevisiae, TMB3400, were determined in a real hydrolyzate medium. A simplified model for glucose release and uptake was created, to be used as a tool for control of the glucose concentration in a SSCF process. With help of this model, an SSCF process with efficient co-utilization of glucose and xylose was successfully designed. The results showed that the total xylose uptake could be increased from 40% to as much as 80% by controlling the enzyme feed.