The incidence of oscillatory behavior in the continuous fermentation of Zymomonas mobilis

The incidence of oscillatory behavior in the continuous culture of Zymomona s mobilis has been examined using a combination of experimental investigati ons and a predictive model. The tendency to oscillatory behavior was assess ed by perturbing the feed substrate concentration and dilution rate in a co ntinuous fermentation starting from a number of distinct initial conditions . The entire range of qualitative dynamic behavior was observed: overdamped , underdamped, and sustained oscillatory responses. The predictive capabili ties of a model previously proposed by our research group were confirmed ov er this range of operation. A key component of this model is the inclusion of a dynamic specific growth rate term which accounts for the inhibition as sociated with historical ethanol concentration change rate. Parameters and relationships estimated for the model were used to identify key characteris tics leading to oscillatory behavior. In particular, differences in the sen sitivities of ethanol production rate versus specific growth rate to ethano l concentration and its rate of change dictate whether sustained oscillatio ns will occur. Experimental evidence indicates that a change in morphology is associated with oscillatory behavior. The change in morphology to a more filamentous form may explain the change in specific growth and product for mation characteristics.