ENRICHED OXYGEN FERMENTATION

Oxygen is one of the fundamental requirements in any aerobic fermentations. The oxygen used by the microorganisms used in the fermentation process comes from two main sources:

1 Oxygen incorporated in the organic substrate which are used directly in the metabolism of biosynthesis of new biomass
2 Oxygen from the air which is pumped through the fermentation broth and used by the microorganisms in their respiration activities as their terminal electron acceptors

In aerobic fermentation although we see the mandatory requirements for oxygen by the microorganisms yet it exhibits the characteristics of Dr Jekyll and Mr Hyde of metabolism. It is needed but if exceeded it can be toxic and can kill the aerobic microorganisms

Oxygen too face other limitations in the sense of:
1 Its limited partial composition in air
2 Its limited solubility in water
3 Other parameters such as solutes, and temperature which affect its solubility
4 That microorganisms can only use oxygen not directly from air but only in the form of dissolved oxygen

In most fermentation process using fermentors attempts to increase the supply of oxygen to the broth and microorganisms have always been in the form of:

1 Increasing stirrer speed or mixing
2 Increasing volume of flow
3 Increasing the air pressure

Using the above three parametric controls have unavoidably resulted in building very large and costly fermentors

One of the alternative methods of increasing oxygen supply lately is by the increase in partial pressure of oxygen or enriched oxygen in the fermentation process. There are reports of increasing the efficiency of the fermentation process using this method but at the same time negative reports are cited of increasing microbial toxicity and poisoning by enriched oxygen.
There is also increase in hazards using enriched oxygen that may cause accidental combustions

PROBLEM OF MICROBIAL OXYGEN TOXICITY
-----------------------------------------------------

Most accept that oxygen are toxic to anaerobic microorganisms. Exposure to oxygen could kill obligate anaerobic microorganisms. Anaerobes could not stand the toxic effect of oxygen because their physiology are not equipped with enzymes that can make the toxic oxygen harmless. Aerobic microorganisms on the other hand have these enzymes to neutralize the toxic oxygen molecules. But what most people do not know is that oxygen can be toxic to aerobic microorganisms depending on the situation of operation.

It has often been regarded that increasing the amount of oxygen to the microbes during aerobic fermentation is a good option to improve the yield of the fermentation. However, under certain situation of exposure time and concentration or pressure of oxygen the process might be detrimental. Try reading articles by JG Morris and JWT Wimpenny for better understanding of the oxygen physiology of aerobic and anaerobic microbes!

The microbes are killed by the oxygen because they are not adapted to the operating conditions of oxygenation. The only alternative is to adapt microorganisms to the high oxygen exposure before using them as inocula for the fermentation process