Scale-up of Bioprocesses
499
Fig. 11.8 Shear stress as a function of shear rate for different fluids.
A medium containing unicellular microorganisms is normally Newtonian, and the viscosity is close
to that of pure water (except for the case of very high biomass concentrations). If the
microorganisms produce extracellular polysaccharides, however, there is a significant effect on the
rheology of the medium. The change in viscosity can be ascribed to the formation of the polymers,
and the effect by the cells themselves is negligible. The rheology of these media can therefore
normally be described with a rheological model derived from a pure polymer solution (often the
power law model). In fermentations with filamentous microorganisms, the medium gradually
becomes very viscous. The reason for this non-Newtonian behavior is the formation of a mycelial
network. The rheological properties depend on whether free mycelia or agglomerates of hyphal
elements, in the form of pellets, are present, but the power law model can be used to describe the
rheology of both morphological forms (Pedersen
et ah,
1993). Normally, both the degree of shear
thinning and the viscosity are higher in a medium containing a mycelium than in a medium where
pellets are formed (at the same biomass concentration), i.e., the power law index is smaller and the
consistency index is higher for media with mycelia than for media containing pellets. Other models,
e.g., the Casson model, have also been used to describe the rheology of fermentation media
containing filamentous microorganisms, but for reasonably high shear rates in a bioreactor
(y>
20
s'1) it is not possible to distinguish between the power law model and the Casson model (Roels
et
ah,
1974).
In Fig. 11.9, the power law parameters are shown as functions of the biomass concentration in
fermentations with
P. chrysogenum
(pellet morphology). It is observed that the power law index is
approximately constant n « 0.45, whereas the consistency index increases with the biomass
concentration.
previous page 521 Bioreaction Engineering Principles, Second Edition  read online next page 523 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off