OXYGEN CONTROL AS FEED CONTROL IN FERMENTATION

I have always found the study of oxygen very interesting and complex, both at the level of microbial metabolism as well at the level of fermentors. The two levels of aeration and oxygen activity are connected by involving the same cultures. The differences in response to oxygen are more at the concentration of microorganisms involved and the scale of the effect.

In most fermentation studies, the supply of oxygen is looked upon more as a requisite of the process where oxygen is the terminal electron acceptors. Aeration has never been really exploited in the controlling of the fermentation process. In most cases just supply it continuously. Very rarely oxygen is seen from the point of view of controlling and manipulating the activity of the microorganisms. Maybe this is attributed to the poor understanding of the physiology of oxygen upon the metabolism of the microbes.

A lot of researches has been done on the effect of oxygen on the metabolism of the microbes. While in a way oxygen seems or is required by obligate aerobes and may even be lethal to obligate anaerobes, the effect is not clear cut.

In aerobic microorganisms .although oxygen is not lethal but in reality the aerobic microorganisms have a shield of enzymes such as catalase and superoxide dismutase which detoxify the oxygen. In the case of strict anaerobes the absence of such enzymes makes oxygen lethal

The point is simply oxygen is too reactive for comfort!

Even though aerobic microorganisms need oxygen due to the presence of the TCA cycle in their metabolism, there are part of the metabolic pathway which feeds to the TCA cycle as being anaerobic or fermentative. The success of these microorganisms depend on the smooth equilibrium which occur between the two types of pathways. Sudden changes to the smooth coupling between the aerobic and anaerobic part of the metabolic pathway could throw the physiology of these microorganisms out of gear

Many studies have shown that oscillatory or transient aeration of cultures could result in sudden changes in the metabolism leading to higher metabolic rate or even diversion of metabolism and formation of fermentation products. The bad news is that such transient aeration often result in the quick death of the microorganisms

The most crucial point in all these studies is the time lag after oxygen is manipulated and the ability of the microorganisms to respond. Quick alternations between the different phases of aeration tend to upset the physiology of the microorganisms

The point is studies need to be done if oxygen can be used to manipulate fermentation positively!

The most common manipulation of oxygen in industrial fermentation are:

1 Varying the supply of oxygen by controlling the stirrer speed
2 Varying the supply of oxygen with the demand of microbial growth in fermentors
3 Using pure oxygen or oxygen under increased pressure at certain stages of the fermentation process
4 Increasing the mass transfer rate of oxygen by using different size bubbles





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