Biotechnology - Yu.O. Sazykin 2006

General Biotechnology
Main Stages of the Biotechnological Process
Sterilization of Nutrient Media

Industrial media (typically liquid, complex, or less frequently synthetic) are sterilized using thermal Methods (saturated steam).

The heat RESISTANCE OF MICROORGANISMS is determined by multiple factors, notably the species of the microorganism. It is well established that spores are significantly more heat-resistant than vegetative Cells. The efficiency of thermal sterilization is also affected by the initial Cell count in the medium: the lower the count, the more readily the sterilization effect is achieved. Consequently, the microbial load in the medium must be reduced prior to sterilization.

Temperature and exposure time are critical parameters in thermal sterilization. The higher the temperature, the faster the sterilizing effect is achieved. To evaluate the effectiveness of the sterilization process, various methods are employed: physical (monitoring temperature and steam pressure), chemical (using melting points or color change indicators), microbiological (inoculation onto standard media), and bioindicator methods (utilizing Bacillus stearothermophilus).

In addition to destroying contaminating microorganisms, thermal sterilization can also degrade thermolabile Components of the medium, such as Vitamins, Enzymes, and Certain Amino Acids. To mitigate this degradation—which compromises the quality of culture media—processes are optimized by increasing the temperature while simultaneously shortening the exposure time.

Thermal sterilization of liquids is performed using two approaches: batch and continuous. In the batch method, sterilization takes place directly within the fermenter. The entire volume of liquid (medium) is heated to the sterilization temperature, held for a specified duration, and subsequently cooled to the required setpoint. This method is straightforward and well-suited for smaller equipment; however, its drawbacks include a significant temperature gradient across the vessel volume and potential undertreatment (dead-leg issues) in stagnant zones. In the continuous method—which is more advanced and efficient—sterilization is carried out in dedicated units.

As a result, temperatures can be elevated to 130—150 °С, while the sterilization time is reduced to 3—10 min, which has a highly favorable impact on the quality of the medium.

The drawback of this approach is the extended piping network required, which increases the risk of secondary contamination.