We consider the optimization of fed-batch fermentation processes involving one limiting substrate for biomass growth and product synthesis, with respect to the volumetric feed rate of this substrate. Until now classification of these processes and also the determination of the corresponding optima feed rate strategy, has been based only on the qualitative behavior of the specific rates for growth and production as functions of substrate concentration. In this paper we illustrate that the optimal control sequence largely depends on other model characteristics as well, such as the value of the product degradation constant. A detailed optimization study for all possible combinations of the specific rates for growth and production reveals that the typical biphasic behavior of processes with growth/production decoupling disappears if product degradation is not modeled. In this sense numerical optimization can prove very useful in solving the model structure discrimination problem.
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