FACTORS TO CONSIDER WHEN SELECTING THE MOST SUITABLE STIRRER FOR YOUR FERMENTOR

Most fermentors on the market are generally provided with the Rushton type impeller irrespective of the type of fermentation process carried out and the nature of the fermentation broth rheology. It must be bear in mind that the Rushton impeller is not the universal or standard impeller available. There are many choices of impellers available and the experienced or educated fermentation technologist should have the confidence to choose the right impeller for the right fermentation process
Choosing the most suitable stirrer and impeller system is not easy and one could not rely entirely on the ‘advice’ given by the salesman. He has the job of selling and pushing his products to you and will promise you everything. After all a lot of business is based on deception or the ‘second hand car dealer’ bag of lies
The basics of using stirrers and impellers in a fermentor are to provide mixing and homogenous conditions that will enhance the various mass transfer processes. There are many factors to be considered when choosing the right impeller and stirrer such as speed, power consumption, volume of fermentation broth to even the shape, size and geometry of the fermentor
The objective of any mixing is not to overmixed or undermixed the fermentation broth. Instead the stirrer and impeller should be able to provide a ‘comfortable range’ of mixing where the objectives are met without extreme damages or negative effects to the fermentation process.
This knowledge can only be achieved through experience and sound advice acquired from other fermentor users. But what ever, totally relying on the universal Rushton impeller for all your fermentation mixings is not the right move.


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INTRICACIES OF SCALING UP EXERCISES- THE NAKED TRUTH

As important it is to do scale up exercises in fermentation technology especially for new studies involving new fermentation process or modification of existing fermentation process, the exercise itself is not simple and cannot be executed in one or two simple steps involving one or two parameter studies.
Most fermentation technologists in their first exposure of the scale up exercise would just think just having a few fermentors of different sizes or just monitoring some basic ‘rules of the thumb’ such as tip speed studies they will be able to complete the studies to reach the necessary confidence.
It is far from that, as we know it ourselves that the fermentation process is a very complex process involving various phases that affect the behavior of the microorganisms. IT IS NOT JUST A SIMPLE MATHEMATICS OR ENGINEERING CALCULATION OR EXTRAPOLATION!. It is not easy to achieve the ideal state of trying to reproduce the efficiency of the fermentation process obtainable on small scale studies to at those at larger scale involving several magnitudes in the increment of volume
If we look at the various components in the fermentation process in the scaleup the behavior of the various parameters show different results or behavior. In such a way even before scale up studies are done, detailed studies and understanding of the process of the fermentation must be deeply understood.
The trouble is that most textbooks on fermentation technology seem to present the model of scaling up as being to easy and discussing the topics in such a brief account with a sprinkle of a few formulae
Scaling up understanding are often acquired through real experience and through trial and errors. There is no ‘short cut’ about it. You need to adjust as you go on and to have inputs not only from books but research papers in the journals!
WHO EVER SAYS ITS EASY THEN????


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FERMENTATION SUBSTRATES- THE PRELIMINARY CONSIDERATIONS

The raw materials used in fermentation constitute the substrates for the fermentation process. The substrates could be seen as either the raw materials that will be ultimately transformed into the desired fermentation products or they could be regarded as the source of nutrients for the fermentation microorganisms.
The substrates form the bulk of the fermentation broth and often considered as one of the most important component in the cost of the fermentation products. Thus in order to lower the costs of production the search for the most cheapest and economical source of fermentation substrate will be the top most agenda in any proposed fermentation industry.
The search for the most suitable substrate is not only dictated by costs and availability of the substrate but by other factors such as complexity of unwanted reactions that affect not only upstream, midstream but downstream activities. This would also mean the problem of treating its effluent from polluting the environment
THE RIGHT NUTRIENT COMPOSITION
In most cases the search for the right fermentation substrate or its admixture composition started with the detailed nutrient elemental analyses. Generally a single source of substrate do not have the sufficient composition to fulfill all the requirements as the fermentation substrate and a concocted admixture is often created to make it ‘wholesome’ or complete as the ideal substrate
Some of the recipes of the fermentation substrate is determined or created in the laboratory where the necessary type and concentration of media is determined as if it is a recipe. Experiments and complex chemical and elemental analyses would be carried out by using various analytical methods and its suitability by carrying out nutritional and physiological studies involving petri dish to even small fermentors
After all just having a fermentation media complete with all its composition does not always guaranteed the suitability or success for the fermentation
THE RIGHT SOURCE
The source of the fermentation substrate should be cheap economical and easily available. This is a problem usually involving high volume low cost fermentation process. Pure chemicals though idealistic would send the fermentation costs shooting to the sky! And would not be economically viable unless the fermentation product is high cost and low volume
Such source of cheap fermentation substrate for high volume low cost fermentation has only one possible source that is the unwanted cheap agricultural or industrial waste products. After all to the microbes they don’t see it as dining at expensive restaurant; a food or nutrient is just a source of nutrients or elements
It is of utmost importance that the source of waste products for fermentation is not only cheap but continually available in stable supply. Several alternative sources for contingencies must always be taken care of as good logistics procedure


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CUSTOMISED FERMENTORS?

In the previous blog, I have discussed the design of the fermentor and the concept that there is no such thing as a ‘standard fermentor’ although the various fermentors used almost similar system requirements.
When we say that we are ‘customising’ a particular fermentor it means that the fermentor is designed and built for a specific fermentation process. It is not designed for the use of other fermentation process.
The concept of a standard fermentor means that it is a general fermentor that can be used or modified for various types of fermentation process. The concept of the ‘ standard fermentor’ is to simplify it so that it is easily understood for those who are in their beginning to learn fermentation or to use fermentor in their novice stage.
But as you advanced in your research and understanding of the limitations and capabilities of fermentors you tend to prefer to ‘ customise’ your fermentor so as to provide specific solutions to your problems.
In certain cases of customizing such as in the use of novel bioreactors, the fermentors do not at all looked like the standard fermentor. The use of standard design fermentors would not be able to provide the answers to your problems.
In certain cases, extreme modifications are made in customizing the fermentor.
This is one of the example where issues arise during scaling up or scaling down studies. The exercise is looked as a simple exercise where the use of similar geometry seems to be the easiest way to find the answers. No wonder such studies do not really help or give the accurate answers…….



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AIR IN AUTOCLAVING

The presence of air or gases could be a complicating factor during autoclaving process. The volume that makes up the internal environment of the autoclave is filled with air. Air itself is a bad conductor of heat. So the presence of air in the autoclaving could affect the efficiency of the autoclaving process, unless these air are removed before the start of the autoclaving process.
The removal of air from within the autoclave could be carried out by heating the autoclave until active steaming is carried out. Once this is achieved the valve could be closed and the autoclaving process is started.
What is not often appreciated is that air is not only present in the autoclave but are also present in the spaces of the bottles, test tubes, fermentors and containers to be autoclaved. It is important therefore that this consideration be taken into account.
Sufficient time and heating must be given to drive these air out too or else the parameters for the sterilization cycle be modified if needed.


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PRESSURE AND AUTOCLAVING

One of the essential components in the use of autoclave is pressure. Autoclaves are usually operated at a higher pressure than the environment. High pressure is needed in order to achieve the sterilizing temperature of 121 degrees centigrade. Without pressure water can only boil at 100 degrees centigrade under normal atmospheric pressure. ( Of course at higher elevation such as on mountain tops, where the atmospheric pressure is less water boils at lower temperature!)
Generally pressure of about 15 psi is required for autoclaving. This pressure is achieved by heating the water in the sealed compartment of the autoclave. High pressure is due to the built up of the steam as the water is heated. There is a pressure safety valve in autoclaves that the high pressure is not overshot. ( This explains the regular hissing sound produced during autoclaving as excess pressure is regulated)
While it is very important to build the pressure to the correct value before autoclaving is initiated, it is also important that at the end of a sterilization that the pressure is brought back to normal values before the autoclave is opened to remove the sterilized items.
Heating and cooling processes take time. Most users are impatient for the time it takes to cool and lower the pressure. Impatience often results in users trying to speed up the release of pressure by opening the release valve too quickly. This action would result in the boiling over of the contents of the flasks and test tubes which could result in the wetting of the cotton plugs and increase the possibility of contamination.
Opening the doors to quickly could result in the sudden release of internal pressure by the autoclave. This could result in scalding of the body as the hot steam rushes out. Release of pressure must be done slowly and only when it reaches zero is the door of the autoclave opened

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THE AUTOCLAVE- APPROACH WITH CAUTION!

The autoclave is one of the standard equipment in any fermentation activity or in the microbiology laboratory. Some of the autoclaves occur as independent units on its own while there are those which are in built into the fermentor for in situ sterilization. Irrespective most of these autoclaves are used for sterilization purposes. This is especially so in pure culture fermentation work where there is the need to remove the occurrence of unwanted microorganisms at the onset of fermentation. The autoclave is also used at the end of the fermentation run for sterilization or disinfection to avoid biohazards.
There are many options available in disinfection or sterilizations besides autoclaving. Yet in most cases in fermentation, autoclaving is the preferred method of sterilizations. Despite its wide usage there are many of us who take the autoclave for granted without understanding its principles of operations and even its limitations. In my years of autoclaving, everybody seems to remember the steps in the operation of the autoclaves without taking time to understand its limitations. The users have more faith in the operation of the autoclaves as 100% ‘fool proof’ and that there would be no problems with its operation or the efficiency of the sterilization process carried out using the autoclaves.
Thus it is often not surprising that often the quality of sterilization is poor and that the autoclave is not working to their expectations. By then it is often too late….. .
One of the biggest issues in having autoclaves is that often the autoclave in the laboratory have to accommodate all kinds of users (from students, post graduates, post docs and even laboratory assistants and attendants) Too many users with too many types of materials to be autoclaved who may vary from having to little knowledge to excessive pseudo knowledge in the proper use of autoclaves.
At some point, these users are exposing themselves to various health hazards from explosions to even being scald alive! REMEMBER! THE AUTOCLAVE IS IN REALITY A BOILER WHICH WRONGLY USED COULD RESULT IN HIGH PRESSURE AND HOT STEAM!
So many years observing autoclaving have seen many common mistakes made mostly attributed to poor understanding of the autoclave operation to the dangerous ‘don’t care attitude’
Common mistakes often observed have seen plastic trays used to hold the stuff to be sterilized becoming lumps of melting plastic, Melting agar flushed down the pipes resulting in blockages, over spilling of media and even projectiles of bolts and nuts!
Yet again and again lessons are not learnt as proven by the mistakes which occur repeatedly.
It is very very important that those using fermentors and autoclaves are properly trained in using the autoclaves to ensure their very own safety! Those that use the autoclaves need proper training and be supervised and to pass examinations on the use of autoclaves!


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