Fig. 16.1. How kinetics determined on an absolute basis can be converted to a relative basis, in order to allow comparison between the model predictions and experimental results. This is necessary since samples removed from a bioreactor are processed to give biomass contents on a relative basis. Note that, even though growth has finished by the end of the fermentation in absolute terms, the relative biomass concentration continues to rise through the conversion of substrate into CO2 due to maintenance metabolism as the growth decelerates at the end of the process. In addition, the water activity of the substrate bed may start at the optimum but may then decrease during the fermentation due to the evaporation of water from the bed. Further, these two variables can be influenced significantly by the manner in which the bioreactor is operated, and bioreactor models that describe the effects of these two variables on growth can be used to explore strategies of bioreactor operation that attempt to minimize the deviation of these variables from the optimum values for growth and product formation.
In kinetic models, the effect of these varying environmental variables on growth is taken into account by expressing the parameters in the kinetic equation as functions of the local conditions. Table 16.1 indicates, for each of the kinetic equations, which of the parameters might be expressed as functions of the environmental variables.
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