Steam Principles, Cycle Types and Sterilizer Anatomy

Principles of Steam and Heat Sterilization

In the sterile processing department, technicians are responsible for the critical journey of surgical instruments through cleaning, decontamination, inspection, assembly, packaging, sterilization, and storage. To ensure patient safety, technicians must always follow medical device manufacturer instructions for use and facility policy. Steam sterilization is the most common method used for heat-tolerant items. It destroys microorganisms primarily through the denaturation and coagulation of cellular proteins. Saturated steam sterilization depends on four essential parameters acting together: time, temperature, saturated steam, and pressure.

Saturated steam carries a relative humidity of 100%, and the steam entering the sterilizer must have a dryness fraction between approximately 97% and 100%. Steam becomes superheated when its temperature rises above the saturation temperature at its pressure; low moisture content alone does not make it superheated. Superheated steam behaves like dry heat, which significantly reduces sterilization efficacy. Pressure inside the sterilizer does not kill microbes on its own; its specific physical purpose is to achieve the high temperatures above the normal boiling point of water needed to destroy pathogens. Common operating temperatures for steam sterilizers are 250 degrees Fahrenheit, 270 degrees Fahrenheit, and 275 degrees Fahrenheit. The biological indicator organism used to challenge and monitor steam sterilization processes is Geobacillus stearothermophilus.

Dry heat sterilization is an alternative thermal method that sterilizes primarily by oxidizing cellular components rather than coagulating proteins. Dry heat requires significantly higher temperatures and longer exposure times than steam processing. Dry heat is often used for heat-stable, moisture-sensitive materials such as powders and oils, but other compatible items may be processed when their manufacturers' instructions and the sterilizer's validated cycle permit it. The biological indicator organism used to monitor dry heat sterilization is Bacillus atrophaeus.