PROBLEMS OF HIGH CELL DENSITY FERMENTATION

The microorganisms are important components in any microbial fermentation. The microorganisms in the fermentation process are either the agents of change which convert the substrate to valuable fermentation products or they themselves are the products sought in the fermentation process.
In either situation the objective of the fermentation process it selves is to obtain the highest number of microbes per unit volume. This trend is termed as achieving high cell density fermentation.
If we can achieve high cell density it is akin to saying that we have higher number of cells to carry out the transformation of substrate to products. More cells mean more “factory workers” to carry out the production. Thus more products will hopefully be produced over the period of fermentation process.
It should be noted although increasing the number of cells can increase the amount of fermentation products formed, that does not mean that the real efficiency of the fermentation process has increased in terms of fermentation yield by the unit cell. It is only the numbers of cells has increased leading to more conversions.
Only new strains or improved strains of producing microorganisms will increase the fermentation yield.
Logically, it is a good option to increase the fermentation product formation by increasing the number of microorganisms that will transform the fermentation substrates to fermentation products. However, this is not easily achieved in reality as there are many physical and physiological constraints that will try to prevent this objective.
The microbial cell is unique in its own right. During the process of fermentation not only it transforms the fermentation substrates to fermentation products, it will in the process of metabolism extract energy and carbon from the substrate to produce new biomass and undergo microbial growth.
In a way this would be ideal in achieving the high cell density fermentation, but such growth is not infinite and will ultimately slow down as in the typical sigmoid growth curve of the batch culture fermentation
Even before achieving this stationary phase the cells will it selves undergoes major changes in its metabolism or physiology will be of negative impact to the fermentation process and product formation
So how can we achieve the optimal high cell density fermentation? How shall we operate so that the fermentation of high cell density will be at optimum fermentation product formation in terms of economic and efficient control?
The whole idea of obtaining high cell density fermentation is ideal when:
1 You can have high density or number of microbial cells in the bioreactor
2 The high density protocol will not negatively affect the fermentation process or the products in terms of the quantity, quality and stability of products
3 That the fermentation process will be able to be carried out efficiently and economically in terms of substrate usage and efficient mass transfers
There are various strategies which could be applied in trying to obtain high density cell fermentation not only through feeding mode but also bioreactor configurations and physiological manipulations of the microbial populations





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