Bifurcation analysis and control of yeast cultures in continuous bioreactors

Continuous cultures of ordinary baker's yeast can exhibit sustained oscillations under normal bioreactor operating conditions. The bifurcations leading to these periodic solutions are investigated using an unstructured, segregated model in which the population balance equation (PBE) for the cell mass distribution is coupled to the mass balance of the rate limiting substrate. The PBE model is able to reproduce experimentally observed bifurcation behavior that includes the coexistence of stable steady-state and periodic solutions over a large range of dilution rates. The possibility of modifying the bifurcation behavior by nonlinear feedback is explored via simulation. Controller design is based on a low dimensional moment representation of the PBE model. A simple feedback linearizing controller is shown to effectively attenuate undesirable oscillations that adversely affect bioreactor stability and productivity.