Low-waste technology for the production of bioethanol from sorghum grain: Comparison of Zymomonas mobilis and Saccharomyces cerevisiae in fermentation with stillage reusing

Zymomonas mobilis and Saccharomyces cerevisiae have been used for simultaneous saccharification and fermentation of sorghum grain with the reuse of the stillage. This study investigated the effects of replacing half the water by a stillage liquid fraction on ethanol yield and volatile compound synthesis. Studies have shown that Z. mobilis is more efficient in ethanol production (55.96–61.48 g/L) than S. cerevisiae (55.26–58.38 g/L). Additionally, Z. mobilis had the advantage of a shorter fermentation time (48 h) and lower fermentation temperature (35 °C) than distillery yeast. No relationship was observed between successive stillage reusing and ethanol content. Reusing the liquid fraction of the stillage did not increase the amount of volatile compounds identified in the raw distillates. Significant differences were observed in the volatile compounds profile of bacterial and yeast raw distillates. Acetaldehyde was the dominant compound in bacterial raw distillates, whereas 3-methylbutan-1-ol was the dominant compound in yeast raw distillates. The characteristic of Z. mobilis was the production of fewer (4.31–6.03 g/L spirit 100%) volatile compounds compared to yeast (4.04–7.44 g/L spirit 100%). The fermentation process reusing the liquid part of the stillage made it possible to reduce process water consumption and waste generation in bioethanol production from sorghum grain. In addition, Z. mobilis made ethanol production from sorghum grain grown in Eastern Europe more beneficial.