Production of glucose–fructose oxidoreductase and ethanol by Zymomonas mobilis ATCC 29191 in medium containing corn steep liquor as a source of vitamins

Different concentrations of corn steep liquor (CSL) were tested in the cultivation of Zymomonas mobilis. Cell growth, ethanol production, and the formation of glucose–fructose oxidoreductase (GFOR) and glucono-δ-lactonase (GL), the enzymes responsible for the bio-production of gluconic acid and sorbitol, were examined. The cell yields using 25 g CSL l–1 and 40 g CSL l–1 (YX/S≈0.031 g g–1) were close to that obtained with 5 g yeast extract (YE) l–1. With 5 g CSL l–1 and 15 g CSL l–1, the nutritional limitation led to smaller YX/S. Using 100 g CSL l–1 produced an inhibitory effect on cell growth. Similar ethanol yields (92–95%) were calculated for each concentration of CSL and also for YE medium. The highest specific GFOR/GL activities (13.2–13.5 U g–1 dry cell) were reached with 25 g CSL l–1 and 40 g CSL l–1, values comparable to that achieved with 5 g YE l–1. The results confirm that CSL is an effective and cheap supplement for Z. mobilis medium, increasing the economic potential of a large-scale bio-production of sorbitol and gluconic acid by untreated Z. mobilis cells. The economic feasibility of the process is discussed.