The Steam Sterilization Process

Typically, moist heat sterilization (or autoclaving) is utilized in hospitals for sterilizing the surfaces of various utensils, including hollow items or wrapped goods. The process is performed by supplying dry, saturated steam under pressure into an autoclave.

The heat from the condensation of steam envelops the items in the sterilizer and kills the microorganisms in a quick and simple way. It is achieved by damaging the cells irreversibly by coagulation.

Moist heat sterilization takes a minimum of 3 minutes at 134 °C and a pressure of 3 BAR, or at least 15 minutes at 121 °C and a pressure of 2 BAR. Objects which are sterilized using moist heat are usually non heat-sensitive items, e.g. reusable medical equipment, dental instruments, simple surgical instruments, textiles or surgical equipment with cavities.

The utilization of saturated steam to sterilize pharmaceutical equipment, products, and reagents is also a widely used sterilization technique. This process is highly energy efficient due to the latent heat released when steam condenses on the items. This can be beneficial, especially for pharmaceutical products.

However, creating the vital vacuum can be challenging, insufficient level results in a limited capability for the steam to penetrate cavities in instruments etc. The utilization of testing equipment is recommended for regular routine monitoring in order to permit the conditions at multiple points in the process to be assessed.

Steam Sterilization

Image Credit: Ellab

The Steam Sterilization Process

The steam sterilization process has the clear benefits of being non-toxic and relatively simple to control. The main parameters are:

  • Air removal
  • Drying
  • Steam contact
  • Time
  • Temperature
  • Pressure
  • Moisture

Pressure and temperature measurements are the most crucial factors. Their measurements need to have an accuracy of +/- 0.5 °C or more at the sterilization temperature used, and +/- 1.6 % or better for the pressure over the scale range of 0 to 4 Bar (according to EN285).

Air Removal

Air must be removed from the sterilizer chamber and load prior to operation in order to secure saturated steam conditions. This is performed by using a vacuum system which provides a series of vacuum pulses.

Removing all of the air is technically impossible, but the level should be kept at an absolute minimum (high dilution factor). The main reason for sterilization failures is usually insufficient air removal, leaks in vacuum or a bad steam quality (presence of too many non-condensable gasses).


Making sure that the load is dry enough when leaving the sterilizer is crucial, as it could otherwise be re-contaminated. Proper drying is normally performed by applying a vacuum to the chamber at the end of the cycle, which boils all condensates and transports them away through the vacuum system.

Steam Sterilization

Image Credit: Ellab

Steam Contact

In order to ensure that enough of its stored energy is transferred to the object by the means of condensing, the direct steam contact to the potentially contaminated surface is vital.

For comparison, the amount of energy stored in steam is a lot higher than in dry air or water at the same temperature, which is why the preferred solution is operating with saturated steam.


As bacteria does not die instantly, time is a crucial factor, which is why a minimum time is needed in order to eliminate them all. The parameter is linked closely to temperature as the killing effect (death value/lethality value) is dependent on both.

The correlation is logarithmic, which is why the same killing effect can be acquired by simply adjusting the exposure time and temperature accordingly. The killing effect is shown as the lethality value, which should reach the same value by sterilizing at 121 °C for 15 minutes as one would by sterilizing at 134 °C for 3 minutes.

This means that quite a substantial amount of time can be saved by choosing 134 °C if a product is able to withstand the higher temperatures. The Sterility Assurance Level (SAL) must be considered when choosing the required lethality value for a specific application.

The SAL required differs based on the application, but it is typically defined as sterile around 1/1,000,000, which means that only one out of a million bacteria will have survived the sterilization process.

Steam Sterilization

Image Credit: Ellab

Temperature and Pressure

As temperature is linked to the lethality value directly, this parameter can also be employed to check how well the autoclave performs. Furthermore, when utilizing Moist Heat Sterilizers, pressure can be converted into theoretical temperature which can then be compared to the actual temperature to assess whether the steam is saturated. This stops the risk of air pockets, which could otherwise threaten the process.


Steam moisture has an extremely high impact on destroying proteins by denaturation (coagulation), which is why it is crucial to utilize saturated steam. The steam should be clean, and superheated steam (above its saturation temperature) must be avoided as it will not have enough moisture to ensure proper sterilization if this happens.

Documentation Requirements

Daily Routine Control

Chemical and biological indicators are used in the hospital Central Sterilization Service Departments (CSSD) to control the sterilization process as a daily routine control of air removal autoclaves that make use of vacuum.

Yet, these indicators are only able to supply highly subjective results, and the purchase and running costs can be a huge strain on a budget. Therefore, alternative electronic devices have been available on the market for some time now, permitting the validation of autoclaves or washers/disinfectors in addition to daily routine controls.

These devices measure and assess critical physical parameters of the process and enable parametric release of the loads based on the findings. The Sterilization Guidelines which were developed by experts that passed in 2017 for the first time, officially admitted this as a possibility.

Parametric release is based on the measurements of critical physical parameters by an independent electronic device. The technique provides a much quicker release than biological tests and is also more accurate.

After the process has finished and data is read, The test result is available virtually immediately, while biological tests require long-term incubation. This is extremely beneficial, for instance in the case of urgent orders involving the sterilization of surgical sets.

Regular Qualification and Validation

Further to the daily routine control of sterilizers every morning and the batch control of every load, all sterilizers need regular qualifications and their processes regular validations as a regulatory requirement.

The requirements for testing or qualifying steam sterilizers are dependent on the use and the country. In Europe, standardized cycles are recommended with sterilization temperatures of 134 °C for 3 minutes or 121 °C for 15 minutes.

There are a number of other measurements which could be considered critical in addition to achieving minimum and maximum temperatures for set time periods.

These include the equilibration time, which is the difference between the first sensor achieving the set temperature and the last sensor to do so – and also the spread of temperatures during the sterilization period and the deviation of individual temperature sensors over the sterilization period.

The calibration of temperature sensors is another key factor when qualifying steam sterilizers. It is vital that it can be established that the sensors being utilized are within an acceptable accuracy (at least 0.5 °C). This means calibrating prior to the validation run, a process known as Pre Calibration.

After the validation process the accuracy is checked to make sure that the sensors are still within the predefined tolerances, this process is known as Post Calibration or Verification.

If the post calibration shows that the accuracy was not within the required limits, the validation study is considered failed and must be repeated after possible correction of the cause.

It is also crucial to check the sterilizer for possible leaks. If air can penetrate and enter the chamber during operation, the insurance of having saturated steam available is completely lost, which is why a leak test is mandatory.

Multiple leak tests are required before and after mounting the feed through system on the autoclave when working with wired systems.

Using the Appropriate Equipment

Using extremely versatile and reliable equipment is crucial for patient safety, as it provides smooth and compliant processes. The differences between various equipment and methods can be detrimental and so should not be taken lightly.

There are a number of pieces of equipment that are deemed acceptable for routine control and/or validation of steam sterilization processes, but they each come with a certain set of pros and cons which should be considered.

Daily Routine Control

Due to the aforementioned objective result and effective process, electronic devices are highly recommended for Bowie Dick testing. Some devices can carry out both routine- and batch control to eliminate the requirement for both types of indicators (chemical and biological).

Choosing a device which can generate and print auditable reports is also important, as they provide a clear-cut view of the process and result. Selecting an electronic Bowie Dick test device that can run several cycles in a sequence can be highly advantageous as most simple models have to cool down for 90 minutes before use, which results in downtime or expensive backup devices.

Regular Qualification and Validation

According to the European norm, a minimum of 12 measuring points is needed for a single sterilizer validation run with a volume of less than 2 m3. It is a known fact that wireless loggers, are more accurate, stable and repeatable due to the RTD sensor design.

They also drift far less over time. The most important factor may be the price, while wired cable systems may be cheaper to begin with, from an investment point of view, they need a lot more resources to operate.

A wireless data logging system on the other hand, is a larger investment initially, but saves users considerable resources in the long run as they are far quicker to operate. Working with a wireless system generally saves operators a noteworthy amount of time.

It should also be noted that cables require a feed through system for the thermocouples to access the chamber – a setup that requires additional costs/resources and introduces the risk of leakage.

Steam Sterilization

Image Credit: Ellab

This information has been sourced, reviewed and adapted from materials provided by Ellab.

For more information on this source, please visit Ellab.


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