Warren, Simon John Curtis; (1994) The relationship between the morphology and rheology of mycelial fermentations. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The rheology and morphology of culture broths are believed to influence the production of many secondary metabolites during fermentation. Consequently these topics have been the subject of intense research interest in the past and several researchers have postulated that morphology of filamentous microorganisms determines the rheology of the fermentation broth. However, no unequivocal evidence exists to support this hypothesis and the question of the factors that influence the rheology of the broth still remains open. In this thesis an investigation was made to study the factors determining the rheology of three industrial strains of Actinomycete specifically Streptomyces rimosus, Actinomadura roseorufa and Streptomyces erythraeus (recently re-classified as Saccharopolyspora erythraea). The rheological data were collected using two different viscometers; a Brookfield 2HAT DVII and a Contraves Rheomat 115. All rheological data collected with the Brookfield were in good agreement with the data from the Contraves and were analysed using Mitschka's technique (113). These measurements revealed that all culture broths studied were highly viscous, pseudoplastic and could be characterised using a power law relationship. Both the flow behaviour index (0.2 < n < 1) and the consistency coefficient (0.001 < K < 15 Pa.sn) were monitored and were found to vary throughout the course of the fermentation. A sophisticated image analysis technique has recently been developed for morphological studies of filamentous microorganisms at the Department of Chemical and Biochemical Engineering at UCL (and the department of Chemical Engineering at Birmingham University). In this investigation a Magiscan image analyser was used for the rapid collection and accurate analysis of morphological parameters, namely total hyphal length, main hyphal length, branch length, number of tips and the length of the hyphal growth unit. These parameters were collected because they have been shown to be reasonably sensitive to changes in the conditions of the culture. The results of these measurements indicated that the morphology of the microorganisms changed during the whole of the fermentation period. However no observable correlation could be detected between these measurements and the rheology of the culture. The variation in the rheology of the culture broth was found to depend on the biomass concentration and on the pH of the culture. The culture rheology changed rapidly and dramatically from an initial Newtonian behaviour (n = 1) to a highly pseudoplastic behaviour (n ≈ 0.3) within forty hours of the fermentation inoculation, after which the flow behaviour index of the culture remained practically unchanged for the remainder of the fermentation. Additionally the consistency coefficient (K) was found to depend on biomass concentration during the initial growth phase for all three Actinomycete cultures. The data for S. erythraeus indicated that the consistency coefficient (K) increased with increasing biomass concentration and then dropped as biomass concentration decreased after the initial growth phase. These observations, however, could not be confirmed for S. rimosus and A. roseorufa beyond the initial growth phase because of experimental difficulties associated with measurements of biomass concentration during this phase. Theories on suspension, flocculation and precipitation of particles (e.g protein particles) and limited measurements of zeta potential of microorganisms suggest that interparticle forces can under certain circumstances cause changes in the rheology of the mixture and pH is shown to influence these interaction forces. A limited number of experiments were conducted during which the pH of the culture was changed to establish whether this concept could be extended to fermentation broths. The results indicated that the rheology of the culture was strongly influenced by its pH indicating that charges around the microorganisms influence the rheology.

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