Comparison of SHF and SSF processes from steam-exploded wheat straw for ethanol production by xylose-fermenting and robust glucose-fermenting Saccharomyces cerevisiae strains

In this study, bioethanol production from steam-exploded wheat straw using different process configurations was evaluated using two Saccharomyces cerevisiae strains, F12 and Red Star. The strain F12 has been engineerically modified to allow xylose consumption as cereal straw contain considerable amounts of pentoses. Red Star is a robust hexose-fermenting strain used for industrial fuel ethanol fermentations and it was used for comparative purposes. The highest ethanol concentration, 23.7 g/L, was reached using the whole slurry (10%, w/v) and the recombinant strain (F12) in an SSF process, it showed an ethanol yield on consumed sugars of 0.43 g/g and a volumetric ethanol productivity of 0.7 g/L h for the first 3 h. Ethanol concentrations obtained in SSF processes were in all cases higher than those from SHF at the same conditions. Furthermore, using the whole slurry, final ethanol concentration was improved in all tests due to the increase of potential fermentable sugars in the fermentation broth. Inhibitory compounds present in the pretreated wheat straw caused a significantly negative effect on the fermentation rate. However, it was found that the inhibitors furfural and HMF were completely metabolized by the yeast during SSF by metabolic redox reactions. An often encountered problem during xylose fermentation is considerable xylitol production that occurs due to metabolic redox imbalance. However, in our work this redox imbalance was counteracted by the detoxification reactions and no xylitol was produced.