THE QUESTION OF SIZE AGAIN.....PART TWO

It does appear that the discussion about the size of fermentors is not as simple as it may seem. In fact it is still a very complex issue and the point of contention among many fermentation technologists. Even till today, arguments on the pros and cons of choosing the right size of fermentors is still not settled.
There are generally two schools of thoughts on these matters. The first group who are advocates of going for large size fermentors or the scale up party, and those that goes for small and miniaturized fermentors or the scale down fermentation technologists
In reality both groups have their pros and cons. Every group has their advantages and disadvantages. What is right depends on the situation or the nature of the fermentation problems.
Size of fermentors was not an issue in the early days of fermentation. The simple rule the size of the vessel dictates the volume to be fermented. But with the advent of industrial microbiology where economics dictates everything, size and other parameters as efficiency, energy input suddenly becomes critical.
We have a golden rule in economics called the economics of scale. Where increasing volume produced will result in lowering the cost price of production per unit product. This often explains why fermentation industries have huge fermentors especially those involved in high volume low value products.
This rule could not be similarly applied to low volume high value fermentation products where other factors such as limitations in down stream processing is the constraining factor and purity of product is stringent
Lately in the last few years there have been a trend towards miniaturization of bioreactors or fermentors. This involves the use of fermentors of less than 10 ml or using of microtitreplates
The use of these very small bioreactors offer the main advantage of using small volume of media and allowing multi variate experiments to be easily carried out simulataneously or in parallel configuration. This is almost akin to the advantages of using solid media on petri dishes during primary and secondary screening.
The problem in using these miniaturized bioreactors differ fro the use of petri dishes in that it uses liquid media and tries to mimic what really happened in a liquid fermentation process.
This is not easy as the key issues in any liquid fermentation is attempts to get homogenous mixing, mass transfers and monitoring of the various fermentation process parameters.
The behavior of fluid mixing in miniature fermentors differs greatly from those larger fermentors where mixings can be carried out effectively by various mixing techniques from stirring to even shaking the conical flasks. In microbioreactors due to the small size the mixing of the liquid broth is hard to achieve especially due to the physical interaction between the liquid and the walls of the bioreactors. The phenomenon of surface tension and capillary effect will be significant.
Any new techniques to measure or detect efficacy of mixing in microbioreactors do have to depend in parallel development in techniques such as computational fluid dynamics.
The use of micro fermentor will generate its own set of unique problems not faced significantly when using large fermentors. Small volume of liquid broth will have higher surface area to volume ratio which will affect processes such as evaporation, surface tension. This if not controlled or taken care off will introduce errors in data to be used especially during scale up exercises .It doesn’t matter even if you have come up with miniaturized sensors the problem of mass transfers will be severely affected
Due to poor mixing any samples obtained would be questionable to its representative function. Wrong sample means wrong data despite the use of the most sophisticated microanalysers.
In my own personal view the use of microfermentors are still in the research stage and are of very limited applications in fermentation technology as of now 




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WHICH IS BETTER; ONE BIG FERMENTOR OR MANY SMALL FERMENTORS?

When intending to acquire acquire fermentors, with our limited financial constraint, we are often faced with the choice of either buying one unit of a large fermentor or buying a few smaller size fermentors. If you are already clear with your purpose and objective of what type of studies or scale of fermentation you want to carry out, then it is not a problem at all. The headache confronting the buyer who have to made the choice are those that are often with no fermentation experience or just a novice who is starting to learn the rudiments of fermentation using the fermentor the first time.

Sad to say it is not really wise to listen to the sales people trying to sell and dump their fermentors to you. It is better in any instance before buying a big or small fermentors to ask those people or colleagues who have been working with fermentors over a number of years. Buying fermentors is a capital investment, and in many cases the buyers end up buying fermentors that are over designed or under desogned for his purpose. In most cases through the author's personal experience visiting a number of fermentation laboratories that ultimately the fermentors bought are just expensive white elephants sitting in one corner of the laboratory collecting dusts.

Of course, buying a very large fermentor is a prestige for the person. even though many might not know the person who bought the fermentor might be under using the fermentor!

THE OBJECTIVE OF BUYING THE FERMENTOR
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Questions that you will find useful in making the choice between big and small fermentors are:

1 What sort of fermentation studies are you carrying out? What are the parameters and operating regimes you are studying?

2 What is the level or scale of studies are you involving? Is it some basic fermentation studies or are you looking for scale up works? Is the scale up exercise you are looking for significant in magnitude to require a pilot scale fermentor , for example.

3 What is the volume of fermentation broth that you are really looking? Are you interested in producing large volume of broth for test samplings or for complete analyses?


Any fermentation studies involving bioreactors must pass through various stages of study. In the earlier phase of studies where crude data are needed, or not so detailed monitoring of the fermentation process are required, even table top fermentors may be avoided. Sometimes shake flasks and petri dishes studies suffices at that level

But at the level of using sensors and monitoring the fermentation using liquid broth fermentors are needed. Normal table top or teaching and research fermentors should suffice. A fermentor capacity around 3 litres or less working volume would suffice. There is no need for larger fermentors at this stage of the study. Investing in larger fermentors would not yield the level of data which is not yielded by the smaller fermentors. In fact you waste more fermentation broth and poorer mass transfers.......and of course more costly!

In buying small fermentors at this stage you can get a number of small fermentors for the price of one big fermentor without significantly affecting your data or results. With such a high number of fermentors you can carry out more fermentation runs at any one time or do variability studoes such as different mixing rate in one go! Your fermentation results too would be statistically more significant!

Fermentors being mechanical and electrical equipment can and do suffer from breakdowns. With so many fermentors at disposal there is no problem of interchanging parts or even salvaging parts that can be used in the other fermentors. This will guarantee your fermentation lab will be operating for decades

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Using small fermentors will only need the normal microbiology laboratory environment. There is no need to have special large air compressors or high pressure water pumps or even special electrical supply. All these additions will incur additional costs. Is your laboratory up to it? Read more!