A fermenter is an apparatus that maintains optimal conditions for the growth of microorganisms, used in large scale fermentation and in the commercial production of antibiotics   and hormones.The main function of the fermenter is to provide a controlled environment  for the growth of a pure culture or of a defined mixture of microorganisms or animal cell to obtain the desired product. The first true large scale fermenter aerobic fermenter were used in Central Europe in the 1930s for the production of compresed yeast. Many different types of fermenter are described till date but commonly used are those having a stirred upright vessel ranging in size from 0 dm³- 1000 dm³.

       The fermentation process is carried out in a strict aseptic condition. So while constructing of a fermenter, selection of materials is 

       important. The material should be capable withstand repeated stem sterilization conditions.      

Different parts of fermenter:

1) Fermenter Vessel:

A suitable vessel is used to carry out the whole fermentation process. The vessel is capable of being operated aseptically and meet the containment requirements. The vesse is designed in such a way that it requires minimal labour operation and maintainence. It has smooth internal surfaces and a similar geometry. The cheapest material which enables satisfactory results is used. Depending upon the fermentation process two types of vessels are used. For small scale fermenration, glass vessels are used and for industrial scale stainless steel vessels are used.

Glass vessel:          

Glass vessels are smooth which makes it non toxic, corrosion proof. It also makes it easy to examine the interior of the vessel. The large vessels are usually borosilicate battery jars with a round or flat bottom and a top flanged carrying plate. These vessels requires autoclaved sterilization. The diameter of glass fermenter is nearly 60cm.

Stainless steel vessel:

 Stainless steel is the most satisfactory material for large scale fermentations. These vessels can be sterilized in situ and hence have the capability to withstand pressure and corrosion. The corrosion resistance of stainless steel is depends upon the thin hydrous film on the surface of the metal. This film is stabilized by chromium and is continuous, non porous, insoluble and self heating. Corrosion resistance can be improved by tungsten, silicone and other elements. 

2) Heating and cooling apparatus:

During fermentation process, heat is generated due to microbial activity and mechanical agitation. The heat generated by both these processes should be ideal. To maintain this temperature, heat is to be either added to or removed from the system. Heat is generally provided by placing the fermenter in a thermostatically controlled bath, or by use of internal coils. To remove excess heat from the system, silicone jackets are used. They consists of double silicone mat with heating wires between the mats. Once a certain size is exceeded, the surface area covered by the jacket becomes too small to remove heat, in such case cold water is circulated on the internal coils to maintain the accurate temperature.

3) Sealing assembly:

 Sealing assembly provides most satisfactory sealing to the stirrer shaft, which is required to provide agitation. This assembly can be operated aseptically for long periods. Three types seal assembly are used:

Packed gland seal: The shaft is sealed by several layers of packing rings of asbestos, pressed against the shaft by a gland. The packing rings are regularly replaced and checked to prevent unsatisfactory heat penetration.

Mechanical seal: The seal is consists of two parts, the stationary part in the bearing and the other rotating on the shaft. The two components are pressed together by springs. Stem condensate are used to lubricate and cool seals during operation.

Magnetic drives: It consists of two magnets, one driving and another driven. The driving magnet is held in bearing on the outside of head plate and connected to the drive shaft. The internal driven magnet is placed on one end of the impeller shaft and held in bearings on the inner surface of the headplate.

4) Baffles:

 Baffles are metal strips nearly one tenth of the vessel diameter and attached radially tp the wall. They are incorporated into agitated vessels of all sizes to prevent vortex and to improve aeration capacity. Baffles maintain a gap between them and the vessel wall to enable scouring action thus minimising microbial growth on the walls of the fermenter.

5) Impeller:

Impeller is used for agitation. Agitation is required to ensure that a uniform suspension of microbial cells is acheived in an homogeneous nutrient medium. The impeller achieves a number of mixing objectives like suspension of solid particles, bulk fluid and gas phase mixing etc. Impellers are classified as:

Disc turbines: It consists of a disc with a series of rectangular vanes. the vanned disc disc again has a series of rectangular vanes attached vertically on the underside. Air from the sparger hits underside of the disc and is displaced towards the vanes where the air bubbles are broken up into smaller bubbles.

Variable pitch open turbine: It also contains vanned disc, the vanes of which are attached to the blades of a marine propeller on the agitator shaft. The air bubbles in this turbine do not hit any surface before dispersion by vanes or blades.  

6) Sparger:

Sparger is used for aeration. The purpose of aeration is to provide sufficient oxygen to the microoraganisms for metabolic requirements. Sparger introduces air into liquid in the fermenter. Three types of sparger are used:

Porous sparger: It is used on the laboratory scale. It is made up of sintered glass or ceramics and is used in non agitated vessel.

Nozzle sparger: It is a single open or partially closed pipe which provides stem to the air bubbles. The single nozzle sparger are mostly used because they causes a lower pressure and does not get blocked.

Combined sparger - agitator: It introduced air thriugh a hollow agitator shaft and emitting it from the holes drilled in the disc and connected to the base of the main shaft. It gives good aeration in a baffled vessel when the agitator is operated at a range of rpm.

7) Feed ports:

Feed ports are slicone tubes connected to the nutrient reservoir. They are used to add nutrients and acid/alkali in the fermenter. They are sterilized in situ with stem after connection has been completed and before any additions are made.

8) Foam control:

Foam minimization is important in fermentation. Excessive foam results in wetting of filters thus resulting in contamination. A foam sensing and controll unit is used to control foam. A probe is inserted through the top plate of the fermenter. It is set at a defined level above the broth surface. when the foam rises and touches the probe tip, a current is passed through the circuit of the probe. The current actuates the pump and an antifoam is released within seconds.

9) Valves:

       Valves are required to controll the flow of gases and liquids in a variety of way. Five types of valves are used.

Globe valves: They are suotable for general purposes on stem or a water line for use when fully opened or fully closed. They do not regulate flow.

Butterfly valves: They are used in large diameter pipes operating under low pressure. They are not suitable for aseptic operation.

Ball valves: They are suitable for aseptic condition. the can handle mycelial broths and can be operated under high temperature.

Diaphragm valves: They are used for flow regulation and for stem services within pressure limits.

Safety valves:

Safety valve is incorporated in every air or stem vessel and pipe layout which is subjected to work under pressure. These valves ensures that the pressure never exceeds the safe upper limit of the specified value. In simple safety valves, a spindle islifted from its seating against the pressure of gas. Once the pressure falls below the value set by the spring, the spindle return to its fermenter.