Oscillatory Behavior Control in Continuous Fermentation Processes

The continuous fermentation processes involving yeast Saccharomyces cerevisiae or bacterium Zymomonas mobilis often exhibit oscillatory behavior. Because, the oscillatory mode of operation may lead to higher or lower average ethanol concentrations, hence, there is a natural need to control this behavior. The idea presented in this paper is based on the use of a mixture of two substrates that are continuously fed into the reactor chamber and the continuous fermentation process is described by an unstructured mathematical model with a product inhibition on cell growth. The relative contribution of both substrates will be treated as a new control variable. Moreover, it is assumed that the microorganisms exhibit diauxic growth and that the occurrence of the oscillatory behavior is related to a time delay in the response of cells to changes in the environment. From the bifurcation analysis of the system, it is shown that an appropriate ratio of both substrates to the mixture allows for induction and elimination of the oscillatory behavior.