Simultaneous saccharification and fermentation of furfural residues by mixed cultures of lactic acid bacteria and yeast to produce lactic acid and ethanol

In a cellulosic ethanol production system, yeasts cannot be reused, and it is difficult to avoid the formation of lactic acid. A novel mixed fermentation system based on water-rinsed furfural residue was designed to produce lactic acid and ethanol simultaneously by yeast and lactic acid bacteria. The fermentation broth can be used for the production of lactic acid, ethanol, or ethyl lactate, which is a very suitable alternative green solvent. Simultaneous saccharification and fermentation by mixed cultures of lactic acid bacteria and yeast (MSSF) in different conditions were carried out. Acid/alcohol molar ratios (molar ratio of lactic acid to ethanol) were investigated to determine the effects of temperature, substrate concentration, and mass rate of yeast to lactic acid bacterial cells on MSSF. Cellulose conversion rate was also calculated to evaluate the effectiveness of MSSF. The cellulose conversion rate of MSSF was higher than those of ethanol simultaneous saccharification and fermentation and lactic acid simultaneous saccharification and fermentation. The cellulose conversion rate increased with increasing substrate concentration, while the acid/alcohol molar ratio decreased with increasing substrate concentration. These results indicate that yeast cells could provide nutrients for lactic acid bacteria. MSSF at 38 °C is also apt to obtain a low acid/alcohol molar ratio. A 1:1 lactic acid/alcohol molar ratio will be obtained at a fluctuating temperature (38 °C between 0 and 54 h, 42 °C after 54 h) and a substrate concentration of 9%, while keeping a high cellulose conversion rate and final lactic acid concentration. A probable bottleneck for MSSF and its potential solution are also proposed in this paper.