EXPERIMENTAL INVESTIGATION AND MODELING OF OSCILLATORY BEHAVIOR IN THE CONTINUOUS-CULTURE OF ZYMOMONAS-MOBILIS

The mechanism causing oscillation in continuous ethanol fermentation b y Zymomonas mobilis under certain operating conditions has been examin ed. A new term, ''dynamic specific growth rate,'' which considers inhi bitory 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 fer mentation of Zymomonas mobilis. Forced oscillation fermentation experi ments, in which exogenous ethanol was added at a controlled rate to ge nerate oscillatory behavior, were performed in order to obtain estimat es for the model parameters and to validate the proposed model. In add ition, data from a literature example of a sustained oscillation were analyzed by means of the model, and excellent agreement between the mo del simulation and experimental results was obtained. The lag in the c ells' 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 o f Zymomonas mobilis under certain operating conditions. (C) 1997 John Wiley & Sons, Inc.