Interaction of Mechanical Stresses and Productivity of Biological Agglomerates in Stirred Fermenters

This research study aims at achieving a deeper understanding of the influence of fluid dynamic stress on the fungus’ morphology in lab-scale aerated stirred tank bioreactors, and at identifying low-stress stirring conditions beneficial for the growth of stress-sensitive microorganisms. The effect of stirring on the morphology of various strains of the filamentous fungus Aspergillus niger was on focus. Various conventional, commercial, and newly designed impellers were examined. The investigations were expanded significantly by using model particle systems. Focus was given on the rheology of the cultivation broth and the stress that particles experience in non-Newtonian flow conditions. The additional stress on particles due to aeration was also investigated. Different parameters, such as the maximum energy dissipation rate εmax or the circulation frequency tc-1 of the particles in the reactor, were evaluated to interpret the impellers’ stress-intensity and to set a base for a successful scale-up. To determine such flow properties, experimental and numerical fluid dynamic analysis was carried out. In this chapter, the main findings of the research conducted within SPP 1934 are presented. The reader can access further details and extended analysis in the group’s publications given as references in this chapter.