Cultivation of Zymomonas mobilis 113 S at different mixing regimes and their influence on the levan formation

The Zymomonas mobilis 113 S strain was cultivated in a bioreactor with a wo rking volume of 1.41 at different stirring regimes in a 15% initial sucrose medium. The levan obtained in the fermentation process was analyzed by gel filtration. Because the sucrose/biomass ratio in the fermentation broth de creased to below 300 g/g, the insufficient concentration of sucrose might h ave decreased the concentration of levan. Besides the growth characteristic s of the population, the mixing intensity and flow structure were also foun d to influence the molecular mass of levan. At 600 rpm, the microorganisms produced levan with a molecular mass lower than at 300 rpm. The stirring of a fermentation broth with levan without cells at 300 rpm and 900 rpm showe d changes in the molecular mass approximately at 20 kD and 5 kD. The size o f eddies in the fermenter was supposed to determine the size of a levan mol ecule. Because the size of the eddy may be compared with that of a levan mo lecule, it explains the decline in the molecular mass of levan.