MATERIALS FOR FERMENTORS

As previously discussed, fermentations can be carried out in any vessel or containers. The only limitation of such vessels is that the fermentation process will not be efficiently carried out as those carried out in specially designed and built bioreactors.

When we build or buy fermentors it is important to know the roles and the choice of materials that go into a fermentor. This is especially so when we carry out fermentation process that are fastidious, require high sanitary requirements and can withstand all the demanding requirements of the intended fermentation process.

A typical fermentor today is made up of various materials which will confer certain advantages or disadvantages to the fermentor and to the fermentation process. The strength and function of a fermentor depends on its weakest link or the weakest material!

GLASS
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Glass to be used in building a fermentor have certain beautiful as well as poor characteristics.

It's positive attributes are that the glass material is inert and do not react with the components of the fermentation broth. Glass make ideal materials for us to observe directly the fermentation process that is occurring in the fermentor.

The bad thing about glass is that glass easily breaks. We cannot build large industrial fermentors using glass alone! Of course there are tempered glass but still they are not suitable to build large fermentors. In most large fermentors, glass are only used as observation windows or ports. Only small fermentors can be build using glass body.Glasses that are used in building fermentors are usually the strong type of glass which are not easily broken. These glasses are called boro-silicate glass

Glass as a material show poor heat conductivity which becomes a problem during mass transfer of heat between the contents of the fermentor and the surrounding environment



STAINLESS STEEL
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Presently stainless steel are very popular materials to build the fermentor. It has a number of positive attributes that is ideal to considered as the main material to build fermentors. The only bad thing about using stainless steel in building fermentors is that its costly!

Stainless steel are strong, robust and durable. It is inert and do not react with chemicals or contents of the fermentors. It does not rust and lasts a lifetime. It shows good heat conductivity which is ideal in fermentation operation

Stainless steel can be easily can easily be produced in sheets to build very large fermentors by welding the various plates

RUBBER and PLASTICS
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Rubber and plastics are polymers which show characteristics like compressibility, deformation and inert properties. They make good seals between two pressed surfaces to ensure tight fit and maintain the integrity aseptic conditions and prevent leakage.

Rubber and plastics such as tygon tubings are excellent for for various supplies of air and chemicals to the fermentors.

All these rubber and plastic stoppers can withstand continuous and frequent sterilizations

They are also good insulators for various electrodes and sensors

GREASE and LUBRICANTS
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Special kind of grease or lubricant are often used as part of the fermentor operation. Grease are used not only to seal tight the flanges but also as part of the mechanical seal encompassing the stirrer shaft. The grease are usually viscous and can withstand vacuum pressure


COMPOSITE FERMENTORS
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Most fermentors are made up of using various materials as stated above. All these different materials show different physical and chemical properties such as exhibiting different coefficient of expansions under different temperature during heating and cooling of the fermentors. Considerations must be taken in adjusting the requirements to ensure that the fermentor will function properly

Care must be taken to see the ' expected lifetime' of each component material especially those components made of rubber and plastics. They need to be examined periodically for wear outs and defect and replaced to ensure that the fermentation will be successful


ARE WE OVER DESIGNING OUR FERMENTORS?
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Specialized factory built fermentors are expensive and represent a huge capital investment! Can we still have fermentors that are cheaper using cheaper materials? There is no law saying that we cannot have cheaper fermentors

It all depends on the type of fermentation process we are trying to run and the lifetime expected of the fermentors.

Although the use of stainless steel is popular but at the same time it is very expensive. We use stainless steel as fermentor components more from the point of sanitary requirements especially if we are fermenting pharmaceuticals or food. If we are fermenting other non sanitary demanding fermentors then we can go for other materials options

The good thing about stainless steel fermentors is that we can always adapt the fermentors for other types of fermentations should we decide to change.

It is sad most imagine only the continuous stirred tank reactor as THE FERMENTOR. Many fermentations can be runned using other variations of fermentor such as fibre glass and high density plastics.( Here, you need the prior advice of a fermentation consultant first!)