BIOLOGY Volume 2 - A Guide to General Biology - 2004

12. MICROBIOLOGY AND BIOTECHNOLOGY

12.10. Large-Scale Production

12.10.4. Fermenter Design and Operation

Fermenters, or bioreactors, are vessels in which microorganisms are cultivated in liquid or solid media. The process taking place inside a fermenter is called Fermentation. Originally, the term fermentation referred exclusively to anaerobic processes, but today it is used in a broader sense to encompass both aerobic and anaerobic processes. Fig. 12.16 illustrates a typical fermenter. It is a rather complex piece of engineering equipment, so let us take some time to examine its design. Do not forget the scale-up challenges outlined in the previous section. As a rule, the contents of operating fermenters are mixed by one method or another. For example, in The production of ICI Single-Cell Protein (prutin), mixing is achieved by pumping air at high speed through the bottom of the vessel. The product may be the Cells themselves (biomass) or a useful cellular metabolite. All operations must be carried out under sterile conditions to prevent culture contamination. Furthermore, it is essential to ensure that all inlet and outlet ports of the fermenter can be maintained in a sterile state. The fermenter and medium are sterilized prior to use, either together or separately. The starter cultures of the Organism to be used in the fermentation process are stored in an inactive form (e.g., frozen). The sample is then thawed and activated, grown up to a sufficient volume using aseptic techniques (inoculum buildup), and subsequently introduced into the fermenter (inoculation). Inside the fermenter, the organism grows and multiplies utilizing the nutrient medium.

Fermenters are typically constructed from high-grade stainless steel to prevent corrosion and the leaching of toxic metal salts into the medium. All equipment, Materials, and air used must be sterile. Equipment is sterilized using pressurized steam. Steam must have access to all surfaces, which in turn should be as smooth and polished as possible, free of cracks and crevices where microorganisms could accumulate. The medium is sterilized prior to inoculation by passing steam through a cooling system. Air is sterilized by filtration.

For safety reasons, the exhaust air leaving the fermenter must also be sterilized, if only to prevent the escape of genetically engineered microorganisms.

Class="center">

Fig. 12.16. A typical fermenter. Vessel capacity can range from 1 dm³ (experimental scale) to 500,000 dm³ (commercial production). Shape and construction materials also vary, although fermenters are most commonly built as cylindrical stainless-steel vessels.

Key:

A. Air inlet. In most cases, fermentation is aerobic and requires large volumes of sterile air supplied through a specialized device called a sparger. Air bubbles help agitate the contents, provide oxygen for aerobic Respiration, and facilitate the removal of volatile waste products.

B. Impeller, present in most fermenters. Agitation increases The rate of oxygen dissolution, maintains diffusion gradients of oxygen and nutrients into cells and products out of cells, prevents clumping of cells or fungal mycelium, and ensures heat exchange between the medium and cooling surfaces. The impeller shaft must be robust and sterile.

C. Impeller blades, typically flat and vertically positioned.

D. Alkali and antifoam inlets. These are connected to sensors that monitor pH and foam formation within the fermenter. Aeration and agitation generate foam (primarily due to the presence of Proteins), which can obstruct the withdrawal of contents through the outlet port. When foam contacts the antifoam sensor, it closes an electrical circuit that triggers a pump to inject an antifoam agent. Other sensors are electrically linked to different "effectors." Whenever the fermentation medium becomes too acidic, alkali is added to maintain a constant pH level.

E. Outlet port. Because the Contents of the fermenter are under pressure, a pressure gauge and a safety valve are fitted here.

F. Access port. Located at the top of the fermenter, this opening is used for adding the medium and inoculum (microorganisms). It also provides access for cleaning the fermenter.

G. Baffle — a vertical rib on the inner wall of the fermenter that enhances mixing and prevents The formation of vortices as the culture rotates, thereby improving oxygen transfer efficiency.

H. Cooling jacket. This removes excess heat generated during culture growth.

I. Sampling port. This allows samples to be drawn during the fermentation process for continuous monitoring.