Moist Heat or Steam Sterilization Working Principle

Sterilization is the practice of “destroying completely all forms of microbial life. Steam autoclaves achieve sterilization by exposing loads to a combination of moisture and heat through direct contact with steam. Microbial death occurs because proteins and nucleic acids in the cell wall rapidly denature at high temperatures. 

Steam sterilization occurs in three distinct phases: Purge, Sterilization, and Exhaust. During the Purge phase, steam enters the autoclave chamber (steam has a lower density than air and rises to the top of the chamber) and displaces or “pushes out” the existing ambient air. Complete air removal from the chamber is essential for achieving sterilization because air has an adverse effect upon steam penetration of porous loads.

In addition, steam and air don’t mix well, which can result in variations in temperature within the chamber thus compromising the sterilization process. 

After all of the air is purged from the chamber, the temperature rises to the sterilization set-point and the Sterilization phase begins. The autoclave maintains this set temperature for a time adequate to eliminate all microbial life. 

Finally, the sterilization cycle enters the Exhaust phase where the chamber is exhausted of steam and returned to ambient temperature and pressure.

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Resource Person: Heba Awadallah

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