Steam sterilization, primarily achieved through the use of autoclaves, is the most reliable method of sterilization.
It utilizes four main parameters: steam, pressure, temperature, and time.
The ideal steam for sterilization is dry saturated steam with a dryness fraction of at least 97%.
Pressure is essential to achieve the high temperatures necessary for rapid microbial death.
Common sterilizing temperatures are 121°C (250°F) and 132°C (270°F), with minimum exposure times of 30 minutes at 121°C in a gravity displacement sterilizer or 4 minutes at 132°C in a prevacuum sterilizer.
5 Key Parameters Explained
Steam
The quality of steam is crucial in steam sterilization.
Dry saturated steam is preferred as it effectively transfers heat to the load, ensuring uniform heating and rapid microbial inactivation.
The dryness fraction, which indicates the percentage of steam that is in the vapor phase, should be at least 97% to ensure efficient sterilization.
Pressure
Pressure in the autoclave is used to achieve and maintain high temperatures.
By increasing the pressure, the boiling point of water is raised, allowing for temperatures above 100°C to be reached.
This is necessary for the effective killing of microorganisms, including spores.
Temperature and Time
The temperatures typically used for steam sterilization are 121°C and 132°C.
At 121°C, items are usually exposed for 30 minutes in a gravity displacement sterilizer.
At 132°C, the exposure time is reduced to 4 minutes in a prevacuum sterilizer.
The choice of temperature and time depends on the type of item being sterilized, whether it is wrapped or unwrapped, and the specific design of the sterilizer.
Monitoring and Safety
Steam sterilization processes are monitored using mechanical, chemical, and biological indicators.
Mechanical monitors track temperature, time, and pressure, often providing a printout or graphical display.
Chemical indicators are used both inside and outside the pack to confirm exposure to the correct temperature or time and temperature conditions.
Biological indicators, which use spores of Geobacillus stearothermophilus, are the most definitive test for ensuring the effectiveness of the sterilization process.
Specialty Cycles and Equipment
Some autoclaves offer specialty cycles designed to protect delicate items from damage caused by rapid temperature and pressure changes.
These cycles may include longer durations at lower temperatures or special pressure controls to prevent damage to sealed containers.
Steam Sources
Steam for sterilization can be sourced from hospital steam boiler systems or self-contained electric boilers.
Both require a treated water supply to ensure the quality and purity of the steam, which is critical for effective sterilization.
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