Moist Heat Sterilization: Principle, Advantages, Disadvantages

Of all the methods available for sterilization, moist heat in the form of saturated steam under pressure is the most widely used and the most dependable method. Moist heat has better penetrating power than dry heat and, at a given temperature, produces a faster reduction in the number of living organisms. Steam sterilization is nontoxic, inexpensive, rapidly microbicidal, and sporicidal. It rapidly heats and penetrates fabrics.

Sterilization is defined as killing or removal of all microorganisms including bacterial spores.

Moist heat sterilization using autoclave is commonly used for the sterilization of biohazardous trash, heat, and moisture resistant materials such as aqueous preparation (culture media). This method is also used for the sterilization of surgical dressings and medical devices.

The most common type of steam sterilizer in the microbiology laboratory is the gravity displacement type. Another type of autoclave is vacuum/gravity assisted.

Gravity Displacement type Autoclave
Gravity Displacement type Autoclave

Principle of Moist Heat sterilization

Moist heat destroys microorganisms by the irreversible denaturation of enzymes and structural proteins. The temperature at which denaturation occurs varies inversely with the amount of water present. Sterilization in saturated steam thus requires precise control of time, temperature, and pressure.

Pressure serves as a means to obtain the high temperatures necessary to quickly kill microorganisms. Specific temperatures must be obtained to ensure microbicidal activity. Minimum sterilization time should be measured from the moment when all the materials to be sterilized have reached the required temperature throughout.

The recommendation for sterilization in an autoclave is 15 minutes at 121°C (200 kPa). The temperature should be used to control and monitor the process; the pressure is mainly used to obtain the required steam temperature.

Alternative conditions, with different combinations of time and temperature, are given below.

1 1 atm =  325 Pa

corresponding pressure
Minimum sterilization time
126-129250 (~2.5 atm)10
134-138300 (~3.0 atm)5

In certain cases (e.g. thermolabile substances), sterilization may be carried out at temperatures below 121 °C, provided that the chosen combination of time and temperature has been validated.

Monitoring of steam sterilization process

Like other sterilization systems, the steam cycle is monitored by mechanical, chemical, and biological indicators. Steam sterilizers usually are monitored using a printout (or graphically) by measuring temperature, the time at the temperature, and pressure.

Chemical indicators are affixed to the outside and incorporated into the pack to monitor the temperature or time and temperature. Autoclave indicator tapes are commercially available and a change in color of the tape suggests proper sterilization.

Temperature-monitoring probes should be inserted into representative containers, with additional probes placed in the load at the potentially coolest and least accessible parts of the loaded chamber. The conditions should be within ±2 °C and ±10 kPa (±0.1 atm) of the required values. Each cycle should be recorded on a time-temperature chart or by other suitable means.

Biological Indicators

The effectiveness of steam sterilization is monitored with a biological indicator using an envelope containing spores of  Geobacillus stearothermophilus (formerly Bacillus stearothermophilus; e.g. ATCC 7953 or CIP 52.81) for which the D-value (i.e. 90% reduction of the microbial population) is 1.5-2.5 minutes at 121 °C, using about 106 spores per indicator (this is based on a worst-case scenario that an item may contain a population of 106 spores having same resistance as that of Bacillus stearothermophilus). After sterilization is over the strip is removed and inoculated into tryptone soy broth and incubated at 56°C for 5 days. No growth of Geobacillus stearothermophilus indicates proper sterilization.

Table: list of commonly used biological indicators (BIs)

Spores of BacteriaD Value
Geobacillus stearothermophilus
(most common)
Bacillus coagulans0.3
Clostridium sporogenes0.8-1.4
Bacillus atropheus0.5

Positive spore test results are a relatively rare event and can be attributed to operator error, inadequate steam delivery, or equipment malfunction.

Advantages of Steam Sterilization Method

  1. Nontoxic to patient, staff, environment
  2. Cycle easy to control and monitor
  3. Rapidly microbicidal
  4. Least affected by organic/inorganic soils among sterilization processes listed
  5. Rapid cycle time
  6. Penetrates medical packing, device lumens

Disadvantages of Steam Sterilization Method

  1. Deleterious for heat-sensitive instruments
  2. Microsurgical instruments damaged by repeated exposure
  3. May leave instruments wet, causing them to rust
  4. Potential for burns

References and further readings

  1. CDC:Guideline for Disinfection and Sterilization in Healthcare Facilities

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

20 thoughts on “Moist Heat Sterilization: Principle, Advantages, Disadvantages

  1. Pls I need to know if it is possible to sterilize paper,cotton wool,rubber and cloth in oven

    1. Please think about it seriously. What will the temperature inside oven? Will these material be able to withstand this temperature extreme? When there are heat labile materials, we use either gases eg. ETO or chemicals etc to sterilize them, depending on the nature of the material.

    1. Thank you for your query. We do not sterilize the spores, we sterilize the materials that might contain Bacterial spore. Depending up on the nature of the material (moisture content, type of liquid etc), we can either use Moist heat (Autoclave) or Hot Air oven (Dry heat method) or any other sterilizing techniques.

    1. Pyrogen are fever causing substances. Endotoxin (lipopolysaccharide) is an exogenous pyrogen, various cytokines act as endogenous pyrogen.

  2. what is difference between vertical and horizontal autoclave? Which is superior and why? which is more preferred to sterilize surgical instruments and implant?

    1. Dear Pradip
      Thank you so much for your comments. Both the autoclave works in the same principle of Moist heat sterilization so we can not tell one is superior than other in terms of sterilization achievable. Which type will be convenient for particular hospital/laboratory depends on their purpose. Generally speaking vertical autoclave are suitable for small research/diagnostic laboratory, if the size of vertical autoclave increases, there will be problem in loading/unloading. Horizontal autoclave are generally bigger, more versatile and are easier to operate, load/unload etc. Manufacturers’ send sterilized implants and implantable devices in individually wrapped package. If some health care facility has to sterilized such implants, steam under pressure (autoclave is the preferred method). Type of autoclave can be selected on the basis of the factors discussed above.

  3. once the temp and pressure achieve(121c for 15lbs ),we count the time for 15 min.why 15 minutes ,why not 14.8 or 13 min ?

  4. Why loading pattern is required for sterilisation,based on the volume of the equipment to be filled load can be fixed for sterilisation?

  5. If an autoclave is making the dresses and load to have more moisture than normal after sterilization. What may be cause factor

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