(230d) Model Systems for Mixed Particulate Fermentation Broth

Model systems for mixed particulate fermentation
broth

C Bliatsiou, L
Böhm, M Kraume

Chair of Chemical and Process
Engineering, Technische Universität Berlin, Germany

FH6-1, Fraunhoferstrasse
33-36, 10587 Berlin, Germany. Tel: +493031472791, email:
lutz.boehm@tu-berlin.de

The project which is part of the Priority
Programme “SPP1934 DiSPBiotech”
is concerned with the experimental and numerical characterization of local
mechanical stresses occurring in gassed stirred fermenters and their effect on
biological agglomerates. The findings thus obtained are practically tested on
the fermentation of Aspergillus niger. Furthermore,
the results are used for the identification and development of low-shear
stirrers.

To create controlled and repeatable
conditions in experiments, suitable model systems which represent the
biological broth are of high interest. The material properties such as density
and rheology but also characteristics of the agglomerates depend on numerous parameters
such as biological behavior, the mechanical and operational process settings as
well as time. Even for such well known processes as the fermentation of A.niger, material property data can rarely be found.  It is known that, depending on the
conditions, the broth can have Newtonian (e.g., for pellet growth; Fig.1a) or
non-Newtonian characteristics (incl. shear thinning, shear thickening,
viscoelastic, e.g., for mycelia growth; Fig.1b) or even changing rheology over
time due to increasing biomass concentration in the batch process. The same is
valid for the characteristics of the agglomerates with rather large round
particles in case of pellets down to smaller, in parts branched, filamentous
shapes. This broad variety of conditions in real fermentation broth cannot all
be covered by one model system. The requirements that the model solution should
fulfill are challenging. It should be able to mimic all the material properties
of the real system while covering the requirements of the applied measurement
techniques as well. To name one example, e.g., for optical measurements, the
continuous phase must be transparent.

Here, material and particle information is
collected from literature and generated by own work and together with project
partners within the frame of the Priority Programme.
This information will be used to find proper model solutions. For a Newtonian
rheology of the continuous phase, aqueous solutions with different additives
can be used. Furthermore, for high viscous conditions, silicone oil or glycerol
are valid options. To generate a non-Newtonian behavior carboxyl cellulose,
xanthan gum or polyacrylamid solutions show a
shear-thinning and, in parts, additionally viscoelastic behavior while still
being transparent.

Covering the task of finding a model system
for the dispersed phase as found in real systems with a comparable morphology
and morphological behavior during the application of mechanical stress is an even
the more complicated task. Suitable options are, e.g., a second immiscible liquid
phase (Fig.1a) or blue-clay flocs (Fig.1b). The latter one, especially, is
highly promising. Still it must be kept in mind that the dispersed phase can
influence the rheology of the whole mixture, as well. To find a guideline for
choosing the best model solution representing a particulate fermentation broth,
these options must be investigated with according techniques for the
determination of material properties, such as rheometers. Here, the accuracy of
the measurements is of high importance especially for liquid-solid-mixtures as
the solids can settle which changes the properties of the sample over time. As
mentioned above, in the frame of this project, the morphology and morphological
behavior during the application of mechanical stress, as found in stirred tanks,
is of highest interest. This will be investigated in the stirred tank by using
an endoscope measurement techniques together with an automated image analysis
delivering the particle properties.

Figure 1. Images of a) pellet-like growth
of A.niger (upper left) and an according potential model
system (liquid-liquid dispersion) and b) of branched filamentous agglomerates of
the fungi (upper left) with an according image of blue-clay flocs in a stirred tank

Acknowledgments

Financial
support by the Deutsche Forschungsgesellschaft (DFG)
in the frame of the Priority Programme SPP1934 DiSPBiotech is gratefully acknowledged. Special
acknowledgements go to the working group of Prof. Krull
from Technische Universität Braunschweig for the
support in the fermentation and imaging of A.niger.