PROBLEM OF MYCELIAL SUBMERGED FERMENTATION

Get a large beaker of water. Fill it various strands of fine cotton strings of varying lengths. Then slowly switch on your stirrer. Observe what will happen?
You will observe that as the water starts flowing and mixing, the strings will start getting entangled not only with other strings to form pellets, but also with the spinning shaft and impeller.
This is what will happen too during submerged mycelia fermentation involving fungal or streptomycetes with hyphal structure. Strands of hypha will behave like the strings in forming pellets or getting entangled with the impeller or shaft. The hydrodynamic characteristics of the broth during mixing will cause collision between the various hypha resulting in the formation of pellets.
Of course the use of the string is just a simple model to explain what happens in the broth turbulence. In the real situation involving the mycelia the effect of mixing upon submerged fermentation is far more complex
A lot of submerged fermentation is involved in antibiotic fermentation. Despite the decades of experiences in mycelia antibiotic fermentation, there is relatively poor understanding of the behavior and physiology of mycelia in fermentation which resulted in poor control of the fermention process.The behavior and growth forms of the mycelia in the fermentor affects various mass transfer processes and even the microorganism itself.
The fermentation of the antibiotics initially requires the build up of large amount of biomass in the trophophase before secondary metabolism could be initiated to form antibiotics at the idiophase. Large amount of mycelia will be generated in the log phase prior to the onset of the stationary phase.
Intense aeration and mixing at this stage will result in collisions of the hyphae and mycelia forming fungal mats, pellets and other microbial aggregates. High shear forces are generated at the impeller tips, blades, and fluid turbulence and even at the bubbles paths.
On one hand, such shearing forces will damage the cells or hyphal and thus affecting metabolism and antibiotic formation. While on the other hand such hydrodynamics will form microbial aggregates which will affect the mass transfers across the aggregate and even the viscosity of the broth
The fungal pellets in antibiotic fermentation are interesting in their own way. The morphological and biochemical characteristics of the fungal pellets change as the fermentation progresses
However we can divide the fungal pellets into three classes:
1 Open pellets
2 Semi dense pellets
3 Dense pellets
The open pellets are not dense showing diffuse growth of hyphae,fused together and autolysed while the dense pellets are darker inner mass with thicker hyphae and healthy apical tips. Fungal pellets are larger compared to streptomycete which are smaller and simpler.
Correct shape and form of pellets indicate fermentation going as expected. This is useful diagnostic tool to indicate the onset of secondary metabolite formation during the antibiotic formation




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