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Aeration and agitation in fermentation technology

Dr.Jitender Kumar

Department of Biotechnology

HMV,Jalandhar

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INTRODUCTION

The majority of fermentation processes are aerobic and therefore, require the provision

of oxygen.

If the stoichiometry of respiration is considered, then the oxidation of glucose may be represented as

C6H12O6 + 6O2 = 6H2O + 6CO2

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Industrial Fermentation Process

The oxygen demand of an industrial fermentation process is normally satisfied by aerating and agitating the fermentation broth.
However, the productivity of many fermentations is limited by oxygen availability and, therefore, it is important to consider the factors which affect a fermenter’s efficiency in supplying microbial cells with oxygen.

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OXYGEN REQUIREMENTS OF INDUSTRIAL FERMENTATIONS

192 g of oxygen are required for the complete oxidation of 180 g of glucose.
However, both components must be in solution before they are available to a microorganism and oxygen is approximately 6000 times less soluble in water than is glucose (a fermentation medium saturated with oxygen contains approximately 7.6 mg dm–3 of oxygen at 30°C). Thus, it is not possible to provide a microbial culture with all the oxygen it will need for the complete oxidation of the glucose.

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OXYGEN REQUIREMENTS OF INDUSTRIAL FERMENTATIONS

Although a consideration of the stoichiometry of respiration gives an appreciation of the problem of oxygen supply, it gives no indication of an organism’s true oxygen demand as it does not take into account the carbon that is converted into biomass and products.
A number of workers have considered the overall stoichiometry of the conversion of oxygen, a source of carbon, and a source of nitrogen into biomass and have used such relationships to predict the oxygen demand of a fermentation.

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Fermentation Economics

Most fermentation equations only include biomass production and do not consider product formation
Product and biomass both are important for fermentation process

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Metabolism of the culture

To calculate that the production of 1 g penicillin consumes 2.2 g of oxygen however, it is inadequate to base the provision of oxygen for a fermentation simply on an estimation of overall demand, because the metabolism of the culture is affected by the concentration of dissolved oxygen in the broth.

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Effect of dissolved oxygen concentration

The effect of dissolved oxygen concentration on the specific oxygen uptake rate (millimoles of oxygen consumed per gram dry weight of cells per hour) has been shown to be of the Michaelis–Menten type equations.

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Specific oxygen uptake rate

It has been observed that the specific oxygen uptake rate increases with increase in the dissolved oxygen concentration up to a certain point (referred to as the critical dissolved oxygen concentration,) above which no further increase in oxygen uptake rate occurs.

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Biomass production

Thus, maximum biomass production may be achieved by satisfying the organism’s maximum specific oxygen demand by maintaining the dissolved oxygen concentration greater than the critical level.

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Feed back inhibition

If the dissolved oxygen concentration were to fall below the critical level then the metabolism of the cells may be metabolically disturbed.
However, it must be remembered that it is frequently the objective of the fermentation technologist to produce a product of the microorganism rather than the organism itself and that metabolic disturbance of the cell by oxygen starvation may be advantageous to the formation of certain products

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Effect on product

Equally, provision of a dissolved oxygen concentration far greater than the critical level may have no influence on biomass production, but may stimulate product formation. Thus, the aeration conditions necessary for the optimum production of a product may be different from those favoring biomass production.

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Amino acid biosynthesis

Investigations of amino acid biosynthesis by Brevibacterium flavum provide an excellent example of the effects of the dissolved oxygen concentration on the production of a range of closely related metabolites

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The industry’s chosen strain

Brevibacterium flavum is effectively the same organism as Corynebacterium glutamicum, the industry’s chosen strain for producing amino acids and other metabolites.

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Critical dissolved oxygen concentration

Workers demonstrated the critical dissolved oxygen concentration for B. flavum to be 0.01 mg/dm3 and considered the extent of oxygen supply to the culture in terms of the degree of “oxygen satisfaction,” that is the respiratory rate of the culture expressed as a fraction of the maximum respiratory rate

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Production of a range of amino acids

Thus, a value of oxygen satisfaction below unity implied that the dissolved oxygen concentration was below the critical level.
The effect of the degree of oxygen satisfaction on the production of a range of amino acids is fixed and it may be seen that the production of members of the glutamate (glutamine, proline, and arginine) and aspartate (lysine, threonine, and isoleucine) families of amino acids was affected detrimentally by levels of oxygen satisfaction below 1.0, whereas optimum production of phenylalanine, valine, and leucine occurred at oxygen satisfaction levels of 0.55, 0.60, and 0.85, respectively.

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References

Stanbury, P.F., Whitaker, A. and Hall, S.J. (2001), Principles of Fermentation Technology 2nd ed., Pergamon Press, Oxford.
Young, M.Y. (2000), Comprehensive Biotechnology (Vol. 1-4), Pergamon Press, Oxford.
Young, M.Y. (1996), Environmental Biotechnology, Principles & Applications, Kluwer
S.J. Pirt (1985), Principles of microbes and cell cultivations. Blackwell Scientific Publication, London.

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