238
Chapter 7
Specify model complexity
i
Set up kinetics
4
«
I
Set up mass balances
|
Redefine model complexity
Estimate parameters
i
Simulate fermentation process
Figure 7.1 Different steps in quantitative description of fermentation processes.
Biological processes are
per se
extremely complex, and in Chapter 2 we illustrated that cell
growth and metabolite formation is the result of a very large number of cellular reactions and
events such as gene expression, translation of mRNA into functional proteins, further processing
of proteins into functional enzymes or structural proteins, and long sequences of biochemical
reactions leading to building blocks needed for synthesis of cellular components. It is quite clear
that a complete description of all these reactions and events cannot be attempted in a
mathematical model - at least with our current knowledge of the underlying molecular events
and with the currently available computer power. When in a fermentation process the cell
population is characterized by a stochastic distribution of properties such as cell activity and
cellular composition further complexity is added to the model. In setting up fermentation models
lumping of cellular reactions and events is therefore always done, but the level of detail
considered in the model,
i.e.
the degree of lumping, depends on the aim of the modeling.
Fermentation models can roughly be divided into four groups depending on the level of detail in
the model (Fig. 7.2). The simplest description is the so-called
unstructured models
where the
biomass is described by a single variable (often the total biomass concentration) and where no
segregation in the cell population is considered,
i.e.
the cell population is assumed to be
completely homogeneous. These models can be extended to a
segregated
population model,
where the individual cells in the population are described by a single variable,
e.g.
the cell mass
or cell age, but often it is also relevant to add further structure to the model when segregation in
the cell population is considered. In the so-called
structured models
the biomass is described
with more than one variable,
i.e.
structure in the biomass is considered. Structure may be
anything from diving cell mass into a few compartments to a microscopic view of the cell as
made up of individual enzymes and macromolecular pools.
previous page 261 Bioreaction Engineering Principles, Second Edition  read online next page 263 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off