Saturday, July 30, 2011


This is one of the most common and most abused terms used in fermenter when dealing with the speed of rotation of stirrer in fermentation studies. The speed of rotation of the stirrer is commonly expressed as rounds per min (RPM). The power of stirring is often associated directly with the speed of RPM. Higher RPM means more power to stir

There is a simple correlation generally accepted that the higher the RPM, the higher will be the agitation or mixing of the fermentation broth and hopefully a better mass transfer is achieved between the microorganisms and the environment.
One of the common beliefs is that with higher mixing or RPM we should be able to achieve higher mass transfer of oxygen leading to more efficient fermentation process.

This belief has its limitation as there are many factors that affect the mass transfers of oxygen besides RPM other than just obtaining the optimum RPM
What is most surprising is that the use of RPM seems to be carried out with out the proper understanding of the various processes that occur during the fermentation in the bioreactor.

In most textbooks there seems to be a complete trust in one particular set value of RPM without thinking the use of various other options to enhanced the fermentation process and minimizing the negative impact of excessive or under use of RPM
I have seen almost majority of books suggesting the range of RPM around 200 to 250 RPM. Simple visual observations can show how fast or how damaging such high speed of stirring can be on the fermentation process.

We all know that in the case of mass transfer of oxygen to the fermentation broth will be influenced by the volume of air introduced into the fermentor and stirring among other factors. A choice of high RPM may be even more damaging to filamentous microorganisms. Thus great care must be taken in choosing the proper RPM and not to trust blindly by the 200 to 250 RPM often recommended

Mixing of the fermentation broth is not a simple process. Each researcher must find their very own optimum values in terms of both economic and technical success of their fermentation. After all no two fermentors carrying out the fermentation process are the same! This situation will be most critical when the fermentation is carried out on a large scale. In considering stirring and RPM. the impeller or the paddles influence must be considered. Adding an additional impeller will increase the efficiency of the agitation of the fermentation broth but at the additional expense on the load of the stirrer which will increase the strain on the stirrer system

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Saturday, July 23, 2011


I have mentioned earlier that fermentation can be carried out in any vessel or container. The difference is that if fermentation are carried out in such vessels we might not end up with a proper and efficient fermentation process. Inefficiency could arise out of poor mass transfers, monitoring and lack of stringent aseptic control.
The design and building of good fermentors depends on inputs of engineers and microbiologists. It is just risky to just depend on one set of people to design and build a good fermentor.
Yes, while it is true we can build a vessel or structure that seems to be identical with well known fermentors on the market, however, that is where the similarity just ends. Functionally the fabricated fermentor could just be a failure
A few days ago I was surprised by the news that we have successfully designed and built our pilot plant fermentors. The project is the involvement of a few universities and research institutes. Large amount of money was invested in the project and the opening ceremony was even officiated by the respective Minister of the relevant department.
As I said earlier that I am not surprised we can build fermentors but more worried about the fact as to whether in depth studies have ben carried out to ensure the functionality of the fermentor built. It will be a tragic day to find out later that that there might be serious problems ‘overlooked’ in the design and construction of such fermentor.
Building fermentors require the in depth knowledge and experience of experts and specialists in the field of building fermentors. The only data that I can gleam from the article is more towards the capacity of the pilot plant fermentors. Not much or any other important technical detail were provided.
Building a fermentor is a project in science and technology. Hearsays are just not enough to convince people who wish to rent or even buy the fermentors. Don’t be surprised if such multi million dollar projects will just be underused or just become white elephants! If it happens it will be a waste of the taxpayers money.
As mentioned in this blog earlier there are a number of universities and companies which all ready have existing pilot plant facilities,,,,, almost unused and dying to look for clients to use their pilot plants.
It is most surprising the pilot plant built seems to be equipped with standard components as judged by the type of stirrers used. As we know it has always been the case of the type and rheology of the fermentation of the broth and the type of fermentation that dictates the type of fermentors used in the pilot plants. Having similar geometry in the scale up is idealistic but need not always be obligatory.
I have always wondered if the building of the pilot plants were tested and validated by standard methods? If such preliminary studies were not carried out, it will be a disaster awaiting to happen and reputation at stake.

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Friday, July 15, 2011


Of course in the context of our discussions we are talking mostly of the internal or inside surfaces of fermentors and even of stainless steel pipings to and from the fermentors.
In the fermentation industries involved in the production of biopharmaceuticals and innobiologics such stringent sanitary and sterilized conditions could be the make and break of the fermentation process. It could be a successful and profitable fermentation process or it could be a financial disaster.
The provision of smooth surfaces will not only eliminate microbial contaminants or fermentation residues but also will help improve the cleaning and washing of the surfaces. Not only is the cleaning process becomes more efficient but it is even proven that substantial cost savings made in water and cleansing solutions
Smooth finishing of internal surfaces as provided by stainless steel material and structure have always been regarded as the solution to providing the smooth surfaces of fermentors and the ancillary piping to and from the fermentors
However, most of these statements are often not tested. Different fermentation processes show different fluid properties and thus also different washing and cleaning regimes. Smooth or mirror polishing could itself result in smooth surfaces but also provide hidden niches where microbial contaminants and organic deposits could be hidden in the smoothened crevices. Visually we may see the stainless steel as being very smooth but under the magnification of the microscope, it is not as smooth as we expected.
One of the solutions to producing smooth surfaces is of course by electro polishing, in which passing current help to smoothen the rough surfaces
Whatever the outcome any stringent fermentation industries have to carry out their own cleaning validation process to ensure that they are on the right track.

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