Rheology of filamentous fermentations

The performance of a bioreactor containing a filamentous fermentation broth is greatly influenced by the rheological properties of the broth. These properties are determined mainly by the concentration of biomass, its growth rate and morphology. Included in the morphology are such factors as the geometry of hyphae (length, diameter, branching frequency), hyphal flexibility and hyphal-hyphal interactions, which can all be affected by the operational design of the reactor. Thus, correlations describing viscosity as a function of biomass only are of limited value. A better understanding of the relations between morphology and rheology may be achieved by a combination of rheological and morphological studies. Rheological properties are normally determined using off-line measurements in-spite of associated problems with sample treatment influencing the results. Equipment for dynamic, on-line, measurement of morphology and rheology is available, but little used in filamentous fermentations. Controlling the rheological properties of mycelial fermentations may be difficult because of the great number of factors influencing mycelial development and/or hyphal-hyphal interactions.Polymer solutions are often used to simulate flow behaviour of filamentous fermentations and scale-up and mass transfer considerations are based on these studies. Although much information has been gained this way, the predictions developed do not include the effect of an active biomass on the mass transfer and flow properties of the culture. It is important to carry out studies on the non-homogeneous fermentation fluids, and develop correlations based on these studies.