Batch and Continuous Sterilization of Media

Batch sterilization reduces contaminant organisms by heating an entire vessel of media at once, using thermal or radiation techniques.

Continuous sterilization, on the other hand, rapidly heats the media using steam condensate without a heat exchanger. Steam is injected into the system via a nozzle, and the media stays in a holding loop for a set time until it becomes sterile.

Continuous sterilization is more efficient than batch sterilization because it heats small portions of the media at a time, rather than the entire system, saving energy.

What is Sterilization?

• Sterilization is the process of removal of unwanted organisms like bacteria, fungi, etc. from the fermentation medium.
• It includes both pathogenic and non-pathogenic microorganisms, as well as bacterial spores and fungal spores.
• Sterilization is a measure that uses strong physical and chemical factors to make all microorganisms inside and outside any object lose their ability to grow and reproduce forever. 
• Sterilization is a necessary condition to obtain pure culture, and it is also a necessary technology in the field of food industry and medicine. 
• Different methods can be used according to different needs, such as medium sterilization generally using moist heat sterilization, and air filter sterilization.
• Commonly used methods of sterilization include chemical reagent sterilization, radiation sterilization, dry heat sterilization, moist heat sterilization, and filter sterilization.
• The types of sterilizations are Batch sterilization, Continuous sterilization, Filter sterilization.
• The contamination can be avoided by sterilizing the fermenter, using pure inoculum to start the fermentation, sterilizing the medium, sterilizing all the components of the fermenter, and maintaining the aseptic conditions.

Why Sterilization in Fermentation is Important? 

The reason is simple: to prevent bacterial contamination. After sterilization, all organisms die, and after inoculation, it can be ensured that the inoculated bacteria are not mixed with miscellaneous bacteria. 

We all know that the purpose of fermentation is to use the respiration of bacteria and other microorganisms to obtain some products that humans need. The product can be used if it has a certain degree of purity. 

If there are miscellaneous bacteria, the product will be impure or even deteriorated first, or the miscellaneous bacteria and the inoculated fungi will compete for nutrients and living space, and the inoculated fungi will multiply, thereby affecting the quality and yield of the product.

Types of Sterilization of Media

There are different types of sterilizations such as Batch sterilization, Continuous sterilization.

Batch Sterilization

• Batch sterilization is the most commonly used and suitable method for liquid media sterilization.
• Generally, the Batch Sterilization process is completed into three steps: heating, holding, and cooling period.
• In the heating step, the steam is directly injected into the fermentation medium, or the vessel is heated electrically by heating jackets / Coils.
• Now the temperature is increased until it reaches the sterilization temperature where it is held for a set period. During this period, most of the unwanted microorganisms are destroyed from the media and Bioreactor.
• Finally, the system is cooled to bring the sterile culture media back to the desired temperature. 
• The autoclave is an instrument that works on the principle of batch sterilization. It is used to kill germs that invade the fermentation media. 
• The highest temperature, which appears to be feasible for batch sterilization is 121°C.
• The advantage of batch sterilization it has lower capital of equipment costs. 
• The disadvantages of batch sterilization are it wastes energy and can overcook the medium and it requires expensive heat requirements than continuous sterilization.
• The long exposure of batch sterilization may lead to degradation of the medium. 
• The medium may lose all its nutritional quality. 
• The heat-labile components like proteins, vitamins get degraded due to overheating.
• It will leave some bacteria a chance to survive, and once the bacteria have formed spores (a kind of dormant body), it is difficult to kill them.

Continuous Sterilization

• Continuous sterilization is the rapid transfer of heat to the medium through steam condensate without the use of a heat exchanger/Coils.
• This method is only used for medium sterilization in large-scale fermentation plants. 
• The process includes sterilization operations such as heating, maintenance, and cooling the medium outside the fermenter before entering the fermentation process. 
• In Continuous sterilization, the culture medium is continuously pumped into a heating tower heated by direct steam, so that it can reach the temperature of 126 ~ 132 ℃ in a short time. 
• Then enter in the maintenance tank (or maintenance tube), keep it at the sterilization temperature for 5 to 7 minutes, and then enter the cooling tube to cool it to the inoculation temperature and directly enter the fermenter that has been sterilized in advance (empty tank sterilization).

• The cooling methods of the culture medium include spray cooling, vacuum cooling, and thin plate heat exchanger.
• There are two types of continuous sterilizers are used for sterilization of media – Indirect heat exchanger and Direct Heat exchanger or steam injectors.
• The function of the holding loop is to sterilize the medium. The heating coil or loop sterilizes the medium and the cooling loop or coil cools the medium to the fermentation temperature. This is the basic process of continuous sterilization.
• The advantages of continuous sterilization over batch sterilization are that the heating time of the medium is short (the predetermined sterilization temperature can be reached in 20-the to 30s), and the nutrient components are less damaged. The quality is uniform. The steam load is balanced in production. Suitable for automatic control. Applicable conditions: large-scale production, the medium does not contain solid particles or less foam, corrosion of the fermenter is reduced. 
• A disadvantage of continuous sterilization has a relatively blind top will kill some useful bacteria.