Introduction

Quantitative description of cellular processes is an indispensable tool in the design of fermentation processes. Thus, the two most important quantitative design parameters, yield and productivity, are quantitative measures that specify how the cells convert the substrates to the product. The yield specifies the amount of product obtained from the substrate (or raw material) and the productivity specifies the rate of product formation. These two design parameters can easily be derived from experimental data, e.g. from measurement of the substrate consumption and the product formation. However, what is more difficult is to predict how they change with the operating conditions, e.g. if the medium composition changes or the temperature changes. In order to do this it is necessary to set up a mathematical model (see Box 6.1), which may be anything from a simple empirical correlation that specifies the product formation rate as a function of the medium composition, to a complex model that accounts for all the major cellular reactions involved in the conversion of the substrates to the product. Independent of the model structure, the process of defining a quantitative description of a fermentation process involves a number of steps (Fig. 6.1).

A key aspect in setting up a model is to specify the model's complexity. This depends much on the aim of the work, i.e. what the model is going to be used for, as discussed in Section 6.2.1.