Fermentation media frequently contain starch as a carbon source which may render the medium non-Newtonian and relatively viscous. However, as the organism grows it will degrade the starch and thus modify the rheology of the medium and reduce its viscosity. Such a situation was described by Tuffile and Pinho (1970) in their study of the growth of Streptomyces aureofaciens on a starch-containing medium. Before inoculation, the medium displayed Bingham plastic characteristics with a well-defined yield stress and an apparent viscosity of approximately 18 pseudopoise; after 22 hours the organism's activity had decreased the medium viscosity to a few pseudopoise and modified its behaviour to that of a Newtonian liquid; from 22 hours onwards the apparent viscosity of the broth gradually increased, due to the development of the mycelium, up to a maximum of approximately 90 pseudopoise and the rheology of the broth became increasingly pseudoplastic in nature. Thus, this example suggests that the rheological prob-
esented t|le medium are minor compared
''•Tthose presented by a high mycelial biomass, espe-
r*i when it is considered that the total oxygen de-
1 nd is relatively low in the early stages of a fermenta-
1,1,1 however, it is worth remembering that in non-
'""•otis unicellular fermentations the highest power iw will occur when the medium is sterilized in situ hen the vessel is not being aerated and this will
•pcnond with the time when a starch-based medium concsp
nerkman-Dik et al. (1992) observed that the medium composition could affect the rheological properties of mycelium suspensions by affecting the interactions between the hyphae.
11II-; fiPT'IvCT OF MICROBIAL BIOMASS ON K,a i i i Agitator design for non-Newtonian fermentations
The biomass concentration and its morphological loim in a fermentation has been shown to have a profound effect on oxygen transfer. Most bacterial and yeast fermentations tend to give rise to relatively non-u^'ous Newtonian broths in which conditions of turbulent flow may be achieved. Such fermentations present relatively few oxygen-transfer problems. However, the highly viscous non-Newtonian broths of fungal and streptomycete fermentations present major difficulties in oxygen provision, the productivities of many such fermentations being limited by oxygen availability. Banks (1977) stressed the difference in the pattern of oxygen uptake between unicellular and mycelial fermentations as illustrated in Fig. 9.16. In both unicellular and mycelial fermentations the pattern of total oxygen uptake is very similar during the exponential growth phase, up to the point of oxygen limitation. However, during oxygen limitation, when arithmetic growth occurs, the oxygen uptake rate remains constant in a unicellular system whereas it decreases in a mycelial one. Banks claimed that the only possible explanation for such a decrease is the increasing viscosity of the culture caused by the increasing mycelial concentration.
Several groups of workers have demonstrated the detrimental effect of the presence of mycelium on oxygen transfer. Figure 9.17 represents some of the data of Deindoerfer and Gaden (1955) illustrating the effect of Penicillium chrysogenum mycelium on KLa. Buckland et al. (1988), using different agitator systems, reported that the KLa decreased approximately in proportion with the square root of the broth viscosity, i.e:
KLa a 1/^viscosity.
Steel and Maxon (1966) investigated the problem of oxygen provision to mycelial clumps in the Streptomyces
Oxygen uptake rate of the culture and dissolved oxygen concentration
~>—Oxygen — limitation
N i V / X / N / \ / \ / \ /
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