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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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
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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 Read more!