Mathematische Analyse eines biotechnologischen Prozesses am Beispiel der Fermentation von Streptococcus thermophilus als Starterkultur (English shop)

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The industrial process for cultivating Streptococcus thermophilus as a starter culture for the dairy industry is still characterised by traditional process procedures, heuristic optimisation methods and manual process control. In order to apply modern and innovative methods of process development and control, a mathematical model of this system is required. Analysis of the cultivation process of Streptococcus thermophilus shows that its dynamics are determined by the three system states cell count, concentration of the substrate lactose and concentration of the product lactate. The process model therefore consists of differential equations describing these three system states. The most important quality characteristic of a starter culture is its acidification activity. By mathematically describing this activity as a function of the lactate production rate, it is possible to integrate this property into the mathematical model. Parameter identification is carried out by means of a hybrid of a genetic algorithm and the downhill simplex method. This hybrid is able to solve the parameter estimation problem robustly and automatically despite nonlinearities of the model, measurement noise and parameter correlations. The model structure can be transferred to processes with modified process parameters, to further strains of the bacterium Streptococcus thermophilus and to fed-batch experiments. The parameter estimation method delivers reliable results without the algorithm having to be readapted to the problem. In combination with the parameter estimation method, the model thus enables the application of modern methods of control engineering and process optimisation to the production of Streptococcus thermophilus as a starter culture.