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Everything you need to know about microbial technology. Some of the most frequently asked questions are as follows:-
1. What is a shake flask and what do you understand by shake flask volume?
The flask which is used for shaking particular liquid for particular purpose e.g. for aeration or mixing is called shake flask.
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They are of following types:
(A) Standard shake flask or the Erlenmeyer flask
(B) “Flying saucer” shake flask
(C) Shake flask with baffles
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(D) Flat bed “Thompson” or “Roux” bottle as shown in Fig. 22.1.
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The lower the volume of medium in a shake flask, the better will be the OTR (Oxygen Transport Rate). The minimum volume that can be practically obtained (e.g. 50 ml in a 250 ml shake flask) should give the best OTR and hence the best results. This will also be dependent on sample volume. Very low volumes can only be used for short-term fermentations, otherwise the medium will evaporate and the nutrients would become too concentrated for the culture to perform satisfactorily.
2. How does an airlift bioreactor or fermentor help you to avoid damaging cell shear?
Some cell lines are so fragile in culture that any type of mechanical impeller will shear them. Air lift fermentor also known as an air tower fermenter or air bioreactor uses the sparged air to gently circulate cells and growth media without damaging them. Due to the action of air lift fermentor many air bubbles are generated at the bottom of vessels which aerate and agitate the culture medium. Thus resulting mixing of culture as well as aeration.
The air bubbles lift to the top of the medium and the air passes out through an outlet. The cells and the medium which lift out of the draft tube move down outside the tube and are recirculated. The oxygen supply is quite efficient but sealing up presents certain problems. Fermentors of 2-90 litres are commercially available but 2000 litres fermentors are being used for the production of monoclonal antibodies.
3. What is fed batch fermentor? Explain.
There are mainly two types of fed batch fermentor:
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(1) Fixed volume fed batch
(2) Variable volume fed batch.
In fixed volume fed batch, the limited substrate is fed without diluting the cultures. The culture volume can also be maintained practically constant by feeding the growth limiting factors without affecting the culture volume. Variable volume fed-batch is one in which the volume change with the fermentation time due to the substrate feed. In this way volume change is dependent on the requirement, limitation and objective of the operator.
4. What is immobilized cell reactor? Explain.
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Ethanol fermentation is an immobilized cell reactor using Saccharomyces cerevisiae. The fermentation set up comprise of a column packed with beads of immobilized cells. The immobilization of S. cerevisiae simply performed by enriched cells cultured media harvested at potential growth phase. They are fixed with loaded ICR (Immobilized cell reactor) carried out at initial stage of operation and the cell are entrapped by calcium alginate.
The production of ethanol is steady after 24hrs of operation. ICR is a plug flow tubular column constructed with a nominal diameter of 5cm, ID of 4.6cm, Plexiglas of 3mm wall thickness and 85cm length. The medium is sent to ICR column from a feed tank located above the column. A variable speed master flex pump model L/S easy load (Cole-Parmer, Vermon Hills, IL, USA) may be used to transfer feed medium from a 20 litre polypropylene autoclavable nalgene carboy. The carboy serves as a reservoir.
5. What are stirring and-mixing?
Diffusion takes place by discontinuous as well as continuous movements. Stirring and mixing are going to trigger diffusion of nutrients. Two rivers can flow together for miles side by side and do not mix properties. Blobs and Mediterranean water can be traced unmixed in the Atlantic for years. Eventually molecular diffusion has very long time scale.
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Turbulent stirring stretches the two fluids and increase the amount of contact between the fluids which permits mixing to take place much more quickly as the molecular diffusion is due to randomly exchanging molecules in many more places. This is because that the surface where two fluids touch has a larger area. To conclude stirring without diffusion is not mixing and can be reversed.
