CONTINUOUS ETHANOL-PRODUCTION FROM SUGAR-BEET MOLASSES USING AN OSMOTOLERANT MUTANT STRAIN OF ZYMOMONAS-MOBILIS

A laboratory process was established for ethanol production by fermentation of sugar beet molasses with the bacterium Zymomonas mobilis. Sucrose in the molasses was hydrolyzed enzymatically to prevent levan formation. A continuous system was adopted to reduce sorbitol formation and a two-stage fermentor was used to enhance sugar conversion and the final ethanol concentration. This two-stage fermentor operated stably for as long as 18 d. An ethanol concentration of 59.9 g/l was obtained at 97% sugar conversion and at high ethanol yield (0.48 g/g, 94% of theoretical). The volumetric ethanol productivity (3.0 g/l.h) was superior to that of batch fermentation but inferior to that of a single-stage continuous system with the same medium. However, the ethanol concentration was increased to a level acceptable for economical recovery. The process proposed in this paper is the first of successful fermentation of sugar beet molasses in the continuous mode using the bacterium Z. mobilis.