Starting today, we will begin a series on filter sterilization. First, let’s discuss the fundamental concepts of moist heat sterilization for filters.
Moist heat sterilization is a method that uses steam under controlled pressure and temperature. Its efficacy depends on the ability to deliver the moist heat process.
The principle of moist heat sterilization is the denaturation of microbial proteins and nucleic acids, leading to cell death.
This denaturation begins with the cleavage of hydrogen bonds within the molecules. When hydrogen bonds break, the internal structures of proteins and nucleic acids are disrupted, resulting in the loss of their original functions. However, if the number of broken hydrogen bonds does not reach the critical threshold for microbial death, the molecules may revert to their original form, and the microorganism will not be killed.
To effectively denature proteins, such as in autoclaving, the steam must have sufficient temperature and exposure duration, which is critical for sterilization efficacy.
Sterility Assurance Level
Sterility Assurance Level (SAL), also known as the probability of microbial survival, represents the degree of sterility assurance provided by a sterilization process. It is expressed as the probability of a non-sterile unit in the product. For example, SAL = 10-6 means that no more than one unit in 10⁶ sterilized products contains viable microorganisms.
The time required to achieve a specific SAL depends on the heat resistance parameter of the microorganism (D-value) and the initial microbial bioburden (N₀).
The figure below shows a microbial death rate curve, where the slope represents the D-value. The heat resistance parameters of Biological Indicators (BI) are significantly higher than those of the actual bioburden.
Standard Sterilization Time F₀ Value
F₀ refers to the equivalent sterilization time at 121℃ delivered by a steam sterilization cycle to the sterilized load. It is commonly used to evaluate the lethality of different sterilization cycles.
The reference temperature for moist heat sterilization is defined as 121℃. Using Geobacillus stearothermophilus, a highly heat-resistant microorganism, as the biological indicator, sterilization cycles at various temperatures and times are converted to the equivalent time (in minutes) at 121℃ that provides the same lethality. This value is the F₀ value.
The formula is expressed as:
F₀ = D₁₂₁℃ × (lg N₀ − lg N)
Where:
• D₁₂₁℃ represents the heat resistance parameter of the microorganism at 121℃.
• N₀ represents the initial microbial bioburden.
• N represents the sterility endpoint (10-6).
For example, if a pharmaceutical product contains 100 heat-resistant microorganisms (N₀) with a D-value of 1 minute, achieving an SAL of 10-6 requires a moist heat sterilization time (F₀) of 8 minutes.
Overkill Method
Heat-resistant and/or porous materials, such as filters, can withstand prolonged steam sterilization. The overkill method is typically employed to enhance sterility assurance. Under this method, the F₀ value delivered to the sterilized load must be at least 12 minutes.
The overkill method assumes an initial microbial population and heat resistance higher than those typically found in practice. Since most microorganisms have relatively low heat resistance, overkill sterilization cycles provide a very high level of sterility assurance.
Filter Sterilization Cycle
Filters are generally composed of polymers, which have good thermal insulation properties. Therefore, longer sterilization times are usually required to ensure adequate heat penetration. Additionally, a key challenge in filter sterilization validation is identifying the coldest spot within the filter assembly. Consequently, filter sterilization typically requires a higher degree of overkill.
Conventional sterilization cycles are often set to twice the actual system hold time to inactivate the biological indicators used in validation (typically Geobacillus stearothermophilus with a D-value of 2 minutes and a population of 10⁶). If a 12-minute sterilization cycle is used during validation, complete inactivation of the biological indicator must be confirmed; therefore, the routine sterilization cycle should be 24 minutes.
In practice, the sterilization cycle for filters is typically defined as:
Once the system reaches the minimum temperature of 121.1℃, the sterilization hold time shall be no less than 30 minutes.