Theory and applications of unstructured growth models: Kinetic and energetic aspects

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Abstract
Microbial kinetics and energetics are discussed in connection with the formulation of unstructured growth models. The development of microbial energetics and the use of macroscopic methods in the study of microbial growth are briefly evaluated. The general approach to the modelling of microbial growth has been critically discussed and a strategy for the formulation of unstructured models is presented. A simple unstructured model based on Monod kinetics and the linear relation for substrate consumption is evaluated with reference to extensive experimental and simulation data obtained in batch, fed-batch, and continuous cultivation modes. Choice for a kinetic expression is discussed and has been shown not to be critical in most situations. It is shown that during growth in batch mode, the behavior of the system is rigidly fixed by the kinetic parameter: the maximum specific growth rate. The energetic parameters have minimal influence. In continuous cultivation the behavior is fixed by the energetic parameters: the maximum yield and the coefficient of maintenance. Implications of these observations have also been discussed. The linear relation for substrate consumption is tested with continuous culture data. It is shown that significant deviations at low growth rates cannot be fully accounted by the loss of viability. The situations where unstructured models will be adequate or not for system description, are evaluated and checked experimentally. Influence of an environmental factor, the temperature, on the unstructured model parameters is also quantitatively described. It is concluded that the art of unstructured model building has already reached its maturity and that now much effort should be channelled into the development and verification of structured models.
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