Bioreactors are used for carrying out biochemical processes which employ microbes, fungus, plant cells or mammalian cell systems for production of biological products. The bioreactors provide a controlled environment for the production of metabolites which can help to achieve the optimal growth of microbes. The term fermentor is used as synonym to bioreactors.

Classification of bioreactors

There are numerous types of bioreactors - batch, sequence, continuously stirred tanks, anaerobic contact processes, anaerobic filters, etc.

1. They can be conveniently classified into three major types based on the presence or absence of oxygen and requirement of stirring.
• Non stirred non aerated bioreactors are used for production of traditional products such as wine, beer, cheese etc.
• Non stirred aerated reactors are used much rarely.
• Stirred and aerated reactors are most often used for production of metabolites which require growth of microbes which require oxygen. Most of the newer methods are based on this type of bioreactors.

2. Based on mode of operation, the bioreactors can be classified into three types.
• Batch reactors
• Fed batch
• Continuous e.g.: chemo stat

3. Based on the method of growing of microbes, bioreactors can be either
• Suspended or
• Immobilized
The Petri dish is the simplest immobilized bioreactor. The large scale immobilized bioreactors are used for commercial manufacturing of metabolites. They include
- Moving bed
- Fibrous bed
- Packed bed
- Membrane

4. On the basis of the microbial agent used, the bioreactors can be classified into
• Those based on living cells
• Which employ enzymes

5. Based on the process requirements, bioreactors can be classified into
a. Aerobic
b. Anaerobic
c. Solid state
d. Immobilized

I. Aerobic fermentation

These reactors should have adequate provisions for supply of sterile air and also need a mechanism of stirring up and mixing the medium and cells. These can be either

a. Stirred tank or
b. Air lift type
Generally, they are either closed type or batch reactors. Some special cases use continuous flow reactors also.

1. Stirred tank bioreactor

This is the conventional mixing reactor which is made of either glass or stainless steel. The stirrer can be either at the top or bottom of the reactor. The dimensions of the reactor depend on the amount of heat to be removed from the vessel. Baffles in the centre of the tank prevent formation of vortex and effective mixing of the ingredients.

• Low investment needs
• Low operating costs

• Foaming is often a problem. But this can be overcome using proper antifoaming agents. However, this has to be exercised with caution since some antifoaming agents inhibit the growth of microbes.

2. Air lift bioreactors

The stirred tank bioreactors lack well defined flow of air. In these, air is pumped from below. This creates the bubbles in the medium which rises up through the draught tube by buoyancy and drags the surrounding fluid up. The air that is used to lift up is sufficient to stir up the contents.

• Low friction
• Less energy requirements
• The mechanical parts are easy to construct. There is no need of special aseptic seals.
• Scaling up is easier
• Metabolic performance does not drastically reduce on scale up.

• Capital needed is more
• Difficulty of sterilization
• Efficiency of mixing is low

II. Anaerobic fermentation

These reactors do not require aeration except in a few where initial preparation of inoculums requires aeration. Once the fermentation starts off, the gas released from the media is sufficient to provide mixing.
In case of enzyme production, the recovery has to be strictly under anaerobic conditions since for most of the enzymatic activity is sensitive to the presence of oxygen.

III. Immobilized cell bioreactors

These are based on immobilized cells.
• Useful fro manufacture of intracellular enzymes.
• When the extracted enzymes are unstable
• For preparing low weight products which are released into the medium.
• Reduction of pollution
• Allow continuous operation of bioreactors
• Suitable for production of amino acids, organic acids etc.

Commonly fluidized bed reactors and hollow fiber membrane bioreactors are used

1. Fluidized bed reactors

These reactors can utilize high density of particles and reduce bulk fluid density.

• Heat and mass transfer are efficient
• The mixing of the media between the liquid, solid and gaseous phases are effective.
• The reactor requires less energy.
• Low shear rates and hence suitable for cells which are more sensitive to friction like the plant cells and mammalian cells.

2. Hollow fiber membrane bioreactors

These reactors have hollow fibers are made from cellulose acetate, acrylic polymers, polysulphone etc.

• Extracellular products can be separated from cells at the same time.
• The productivity is high.
• Scale up is easy since several parallel fiber units can be added.

• Sometimes, the pores get plugged.
• Cell growth around the lumen can sometimes distort and rupture the fibers.
• Nutrients and products can diffuse through the membrane and limit the growth of microbes.
• If the toxic products happen to accumulate in the fiber it may inhibit the growth of microbes.

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