Interaction of mechanical stresses and productivity of biological agglomerates in stirred fermenters

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This project aims at investigating the mechanical stress acting on filamentous microorganisms in stirred fermenters. In the first funding period, the necessary methods to characterise mechanical stress were established. Based on the obtained results and developed methods, the goals set for the 2nd FP focus on three main topics:1. From cultivation broth to model solution and backIn the 1st FP, based on cultivation broths of Aspergillus niger, model systems were developed and used to quantify the particle stress in stirred tanks. The model systems will be further used. In the 2nd FP, to approach better to real cultivation broth while still ensuring good reproducibility, fragmentation experiments will be performed based on diluted broths. This experimental procedure can ensure that a larger number of broths with similar characteristics is produced. Moreover, it allows a controlled design of the properties of the system since the rheology of the broth can be set at a desired level, while the morphology of the bio-agglomerates can be easier characterised in the diluted broth. In the frame of several working packages, cultivations will be performed at the department and in cooperation with other groups of the SPP to broad the data base with results from investigations with other microorganisms, allowing a generalization of the conclusions.2. Fluid dynamic characterization of stirred tanks in diverse configurationsIn the 1st FP, during fundamental investigations of conventional and new stirrer types further influencing factors of specific importance for lab and larger scale fermenters were identified. Small changes in the configuration of the stirred system (e.g., baffle position) can influence the fluid dynamics in the tank. Such effects will be investigated with the methods established in the 1st FP. Conventional stirrer types will be optimized and the development of the novel stirrer geometries will be expanded to new radial flow stirrer types. Multiple stirrer configurations will also be part of the study.3. Analysis of the stress history of single bio-agglomerates in the stirred tankBesides the macroscopic investigation of global properties in the system, the numerical work in the 1st FP showed that local phenomena, as well as the investigation of single particle tracks within the reactor gives a deep insight and significant potential to interpret the impact of particle stress. The characterization of the stress experienced by a particle along its track and its residence time in a region of specific shear rates will be further developed in the 2nd FP. In a joined work with other groups, the results will be compared to different biological systems and apparatuses and further investigated down to molecular level.As outcome of the investigations a base for the choice of stirred tank configurations and stirrer types will be built, to allow a better defined and controlled operation of biotechnological processes in comparison to the current ones.