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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