[Show abstract][Hide abstract] ABSTRACT: The incidence of oscillatory behavior in the continuous culture of Zymomonasmobilis has been examined using a combination of experimental investigations and a predictive model. The tendency to oscillatory behavior was assessed by perturbing the feed substrate concentration and dilution rate in a continuous 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 capabilities of a model previously proposed by our research group were confirmed over 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 associated with historical ethanol concentration change rate. Parameters and relationships estimated for the model were used to identify key characteristics leading to oscillatory behavior. In particular, differences in the sensitivities of ethanol production rate versus specific growth rate to ethanol concentration and its rate of change dictate whether sustained oscillations 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 formation characteristics.
[Show abstract][Hide abstract] ABSTRACT: The mechanism causing oscillation in continuous ethanol fermentation by Zymomonas mobilis under certain operating conditions has been examined. A new term, "dynamic specific growth rate," which considers inhibitory culture conditions in the recent past affecting subsequent cell behavior, is proposed in this article. Based on this concept, a model was formulated to simulate the oscillatory behavior in continuous fermentation of Zymomonas mobilis. Forced oscillation fermentation experiments, in which exogenous ethanol was added at a controlled rate to generate oscillatory behavior, were performed in order to obtain estimates for the model parameters and to validate the proposed model. In addition, data from a literature example of a sustained oscillation were analyzed by means of the model, and excellent agreement between the model simulation and experimental results was obtained. The lag in the cells' response to a changing environment, i.e., ethanol concentration change rate experienced by the cells, was shown to be the major factor contributing to the oscillatory behavior in continuous fermentation of Zymomonas mobilis under certain operating conditions.
Biotechnology and Bioengineering 10/1997; 56(1):99-105. · 4.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The inhibition effects onZymomonas mobilis of ethanol concentration history (time-integrated exposure to ethanol) and ethanol concentration change rate have been investigated. It was found that the ethanol concentration history had no significant effect on the fermentative capability ofZ. mobilis, while the ethanol concentration change rate had a quite intense inhibitory effect.