Modeling of the mixed culture and periodic control for PHB production

The unstructured mathematical model was developed in the present investigation for the mixed culture, where the metabolites produced by one microorganism is assimilated by the other microorganism. For this, we specifically employed such model system in which sugars such as glucose were converted to lactate by Lactobacillus delbrueckii and the lactate was converted in turn to poly-beta-hydroxybutyrate (PHB) by Ralstonia eutropha in one fermentor. Several batch and fed-batch culture experiments were conducted using each microorganism at different dissolved oxygen (DO) concentrations. Those experimental data were then fitted to the mathematical model, which can describe the dynamics of a mixed culture. Some of the model parameters were expressed as functions of DO concentrations, and some of the other model parameters were tuned based on the mixed culture experiments. The model developed describes the effects of such concentrations of glucose, lactate, DO, and NH3 on the dynamic behavior of such concentrations as both microorganisms, glucose, lactate, and PHB. Optimal operating condition was then investigated using the model developed, It was found that the periodic change in DO concentration improved such performance as PHB yield, and it was verified by experiments. The optimal NH3 concentration profile was also obtained for the efficient PHB production by the application of the maximum principle, (C) 2002 Elsevier Science B.V. All rights reserved.