Bioreactors: stirred-tank and sparged-tank

A very large flask, with controls

If you want to bake bread for a whole town, you do not use a kitchen oven. You use a large oven with dials and sensors. A bioreactor is such a machine for cells.

A bioreactor is a vessel in which raw materials are converted into specific products, such as proteins and enzymes, by microbial, plant, animal or human cells. NCERT says that large volumes of culture, from 100 to 1000 litres, can be processed in one. A bioreactor gives the optimum conditions for the product: the right temperature, pH, substrate, salts, vitamins and oxygen.

The stirred-tank reactor

The most common bioreactors are of the stirring type. The vessel is cylindrical, or has a curved base, so that the contents mix well. A stirrer, also called an agitator, turns inside the vessel. The stirrer does two jobs. It makes the mixing even, and it makes oxygen available throughout the culture.

The sparged stirred-tank reactor

In another design, sterile air is bubbled through the reactor. To bubble, or sparge, means to blow a gas through a liquid in the form of small bubbles. The air is sterile, so that it does not bring in other microbes. The bubbles rise through the culture and give oxygen to the cells. The vessel still has a stirrer, so it is called a sparged stirred-tank reactor.

The parts of a bioreactor

A bioreactor has the following.

• An agitator system, which mixes the culture.
• An oxygen delivery system, which gives oxygen to the cells.
• A foam control system. Mixing and bubbling make foam, which can spoil the culture, so a substance that breaks the foam is added.
• A temperature control system, which keeps the culture at the temperature that is best for the cells.
• A pH control system, which keeps the pH steady, because the cells change it.
• Sampling ports, through which small volumes of culture can be taken at intervals. The workers can then test the culture as it grows.

All of these are needed to keep only the wanted cells growing in a sterile environment. This is the bioprocess engineering of the first lesson of this course.

Some numbers for A-Level and IB

The amount of air for a bioreactor is often given as vvm, the volume of air for each volume of culture in each minute. A value between 0.5 and 1.5 is common in teaching examples. NCERT does not give such numbers. The air flow is the volume of the culture times the vvm. The product of one batch is the volume times the amount of product in each litre.

You should see that both the air flow and the product grow in proportion to the volume of the culture. A larger bioreactor needs more air in proportion.

Try it in Python

The program works out the air flow and the product of one batch.

Practise with new numbers

Common mistakes

• Mixing up the two designs. The simple stirred-tank reactor mixes with a stirrer. The sparged one also bubbles sterile air.
• Forgetting that the air is sterile. Ordinary air would bring in other microbes.
• Leaving out the foam control. Foam is a real problem when a culture is stirred and bubbled.
• Saying that the sampling ports are for adding food. They are for taking small samples of the culture.
• Thinking that a bioreactor only holds the cells. It controls the conditions.

Remember these

• A bioreactor gives optimum conditions: temperature, pH, substrate, salts, vitamins and oxygen. NCERT mentions 100 to 1000 litres.
• Stirred-tank: a cylinder, or a curved base, with a stirrer for even mixing and oxygen. Sparged stirred-tank: sterile air is bubbled through as well.
• Parts: agitator, oxygen delivery, foam control, temperature control, pH control and sampling ports.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is a bioreactor, and what conditions does it control?

2. What are the two jobs of the stirrer?

3. What is meant by a sparged reactor?

4. Why is a foam control system needed?

5. A bioreactor holds 400 L of culture and is given 0.75 L of air for each litre in each minute. What is the air flow?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 9, Biotechnology: Principles and Processes.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 17.3.