A Fermenter/bioreactor is a vessel used to provide optimum conditions for the controlled growth of microorganisms/ cells by regulating agitation, temperature, and aeration.The main components of fermenter are agitators, spargers, baffles, cooling jacket, pH probes, temperature probes, acid probes, dissolved oxygen probes (DO probes) etc. Bioreactors can be operated in a batch, fed-batch or semi-continuous or continuous bases. The design of the fermenters must be fulfilling the following criteria

i. Optimum cell biomass and higher product yield by providing optimal and controlled conditions.
ii. Aseptic environment for the growth of microorganisms for number of days.
iii. Provision of regulating pH and temperature, mixing and aeration.
iv. It must have proper sampling inlet and outlet ports.
v. A wide range of fermentation processes should be possible.
vi. It must require minimum use of labour in operation, harvesting, cleaning and maintenance.
vii. Power consumption should be minimum for the fermentation process.

Body of fermenter is generally constructed of glass or stainless steel. The preference of glass material is due to its certain properties like smooth surface, visibility, non-toxic nature, corrosion proof and easy to sterilize.
That is why glass fermenters are used for small scale processes but the major disadvantage is its cost and breakable nature. Therefore, for large scale processes, stainless steel is preferred due to corrosion resistance and unbreakable nature.

Size of Fermentors:
The size of fermentors ranges from 1-2 litre laboratory fementors to 5,00,000 litre or, occasionally, even more, fermentors of upto 1.2 million litres have been used. The size of the fermentor used depends on the process and how it is operated.

Main components of fermenter
i. Agitator or Impeller
ii. Stirrer glands and bearings
ii. Baffles
iv. Spargers

i. Impellers: These are used for the uniform mixing of heat, O2, microbes and suspended solid particles. Agitators are of four types
Disk Turbines are series of rectangular vanes arranged in vertical plane around circumference. Disk turbines are most successful because these are able to breakdown fast air stream without getting flooded with air bubbles.
Vaned disks are series of rectangular vanes attached to blades of a marine propeller on the agitator shaft.
• Open turbines In this, the vanes are attached directly to a bass on the agitator shaft.
Marine propellers These are the most successful in filamentous, fungal and algae cultures.

ii. Stirrer glands and bearings
The types of seal assembly or stirrer glands and bearings which are commonly used are given below
Packed Gland seal: In this, shaft is sealed by several layers of rings of asbestos or cotton yarns pressed against the shaft. These have been replaced by mechanical sealed bearing due to operational problems. These were mainly used in large scale fermenter.
Mechanical seal is more durable and less likely to be an entry point for contaminants or leakage point for organism and products. These are used in both small and large fermenters. The seal is made up of two parts one is stationary and other rotates on the shaft.
Magnetic drive In this impeller shaft does not pierce the vessel, therefore, this type of seal has least chances of contamination. Although these are expensive but more durable and successful.

iii. Baffles are metal strips usually 1/10th of the vessel diameter attached to vessel and its function is to produce turbulence in flow and minimize microbial growth along the fermenter walls. These are usually four in number but with increase in volume of vessel baffles number can also be increased.

iv. Sparger’s function is to provide proper aeration to the media for optimum growth of microorganisms. There are mainly three kinds of spargers
1. Porous sparger
2. Orifice sparger
3. Nozzle sparger

1. Porous sparger is made up of sintered glass; ceramics like material. It is usually used in non-agitated vessels. The problem of getting blocked due to microbial growth is very common in porous sparger.
2. Orifice sparger The perforated pipes are arranged below the impeller in the form of crosses or rings approximately 3/4th of the impeller diameter. The air holes are drilled on the under surface of the tubes making up the ring or cross.
3. Nozzle sparger is a single open or partially closed pipe which provides stem to the air bubbles. These have advantage of causing a lower pressure and preventing blockage. These are most common in laboratory and industrial scale fermenter. The pipe is positioned centrally below the impeller and it is away from impeller.

Combined sparger-agitator In this type of sparger, air is introduced through a hollow agitator shaft and

Types of Fermentor:
The fermentor (bioreactor) types used extensively in industries are
(i) Stirred Tank Fermentor:
Stirred tank fermentors consists of a cylindrical vessel with a motor driven central saft that supports one or more impellers.
(ii) Airlift Fermentor:
In airlift fermentor the liquid culture volume of the vessel is divided into two interconnected zones by means of a baffle or draft tube. Only one of the two zones is sparged with air or other gas and this sparged zone is known as the riser.
(iii) Bubble Column Fermentor:
A bubble column fermentor is usually cylindrical with an aspect (height-to-diameter) ratio of 4-6. Gas is sparged at the base of the column through perforated pipes, perforated plates, or sintered glass or metal micro-porous spargers.

• The bioreactor is modular in nature and carries out all the processes of fermentation in a single contained environment.
• Bioreactor plays a core role in bioprocess.
• Stirred tank bioreactors are commonly used in fermentation
• Due to simple technology and higher yield solid state bioreactors are widely used in industries.
• Ethanol fermentation is done by saccharomyces cerevisiae in bioreactor.
• Organic acids e.g. acetic acid and butyric acid are formed in bioreactor by the Eubacterium limosum.
• Thienamycin an antibiotic also produced in bioreactor.