WHAT IS THE RPM?

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


Type rest of the post here.