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To Autoclave, or Not to Autoclave…

Read the examination of the autoclave environment that all components of an autoclave load must endure. If you’d like to skip down to the list of what can and cannot be autoclaved, click here.

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Withstanding Elevated Pressures

At the foundational level, autoclaves use steam (high temperature) under pressure to create a sterile environment by killing pathogens inside the autoclave chamber by the completion of the sterilization cycle. Creating this environment requires a tremendous amount of pressure in the chamber. A key component to understanding what can and cannot be autoclaved is whether or not the material can withstand the pressure of an autoclave’s sterilization cycle.

Pressure ramping required to remove entrapped air, a common barrier to proper sterilization, will ramp to over 30 PSI inside the chamber. This pressure creates a net difference of over 15 PSI when compared to standard atmospheric pressure of 14.7 PSI. To better conceptualize the amount of pressure, the simple pressure exerted on the autoclave door for the most common autoclave door sizes:

Chamber Size	Chamber Pressure	Amount of Force (Chamber Door)
10″ cylindrical (common tabletop)	15 PSI	1,178 lbs.
20″ x 20″	15 PSI	6,000 lbs.
26″ x 26″	15 PSI	10,140 lbs.
24″ x 36″	15 PSI	12,960 lbs.
26″ x 63″	15 PSI	24,570 lbs.

According to the CDC, the two most common sterilization temperatures are 121°C (250°F) and 132°C (270°F). It is essential to understand that special cycles, dependent upon the material load type, load sensitivity to temperature, and pathogen danger level, maximum cycle temperature could differ slightly. However, changes to the temperature parameter are not low enough to change the list of autoclavable items.

With that foundation, let’s take a look at some material make-up of low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Both of these types of plastic are commonly used to make plastic containers and bins that are not rated as autoclave safe. The reasoning is simple. According to material selection platform SpecialChem, LDPE has a melting point between 105°C (221°F) and 115°C (239°F) and HDPE has a melting point between 120°C (248°F) and 140°C (284°F). While these are just two examples, it points to the chemical limitations of materials that are not suited for the temperatures inside of an autoclave chamber during a sterilization cycle.

Damage to the Sterilization Load and Autoclave Chamber Floor

At this pressure and temperature, load containers and instruments are likely to fail and ruin the load being sterilized. Not only can this disrupt the sterilization process inside the laboratory as the autoclave is cleaned and inspected, but the ruined material, glassware, and instruments could also cost hundreds or even thousands of dollars to replace. In addition, this could cause damage to the inside of the chamber and could void the manufacturer’s warranty on the autoclave chamber.

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We answer the top questions being asked about autoclaves and the sterilization process.

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At Beta Star, our mission is to provide simple and reliable sterilization solutions. Delivering sterilization simply begins with a consistent understanding of steam sterilization solutions through the use of autoclaves, also called steam sterilizers. In preparing for this article, our team dove into Google’s online search results to identify seven of the most frequently asked questions related to autoclaves and the sterilization process they execute. 

1) Why is the standard autoclave temperature set to 121^oC?

Sterilization of media, the centric purpose of autoclaves, is designed to reduce the presence of live pathogens that could contaminate the development of pharmaceutical drugs, invalidate the results of research institutions, or allow outside pathogens into vivarium laboratories.

The death of the pathogen, whatever type it may be, is completed by the destruction of the structural and metabolic cell components required for replication. Without the ability to replicate or infect, the pathogen is no longer a threat to the environment around it. While specific pathogens or specific environments may require higher temperatures, 121^oC is widely held as the minimum temperature for efficient sterilization. 121^oC is necessary, because at 100^oC (the boiling point of water under unadulterated atmospheric conditions) endospores are not killed and thus sterilization cannot be fully achieved. In fact, Geobacillus stearothermophilus is used as an indicator to confirm that a successful sterilization cycle was completed. This answer is further vetted in conjunction with question number five.

2) Why are autoclaves being used for media preparation?

Autoclaves are used to prepare many different types of media for their final purpose. They are used to provide sterile media that may be consumed in different applications, such as in vivarium laboratories and food & beverage manufacturing. Media is also studied in microbiology laboratories, tested in innovative research institutions, and distributed for life-saving applications like pharmaceutical medicine or orthopedic facilities. The most important aspect of the autoclaves that deliver sterilization, are their ability to give a biological blank slate to the user. From this blank slate:

• Media (water, food, etc.) may be consumed safely without contaminants
• Studies may be completed without the altering presence of an unknown or uncontrolled pathogen
• Research tests can be completed and replicated, thus ensuring adoption and acceptance of the test results
• Providing life-saving applications, such as orthopedic devices or COVID-19 testing kits, that are packaged free from pathogens

3) Who invented the autoclave?

The first autoclave-like pressurized vessel, similar to what is now known as a pressure cooker, was built by Denis Papin in 1679! However, it would take another 200 years before the autoclave as we know it would be built. In 1879, Charles Chamberland, working Louis Pasteur (the inventor of the pasteurization process) created the first autoclave for medical and scientific purposes. Want to see a cool photo of a 115-year-old autoclave? Check out the photo of an autoclave built in 1905 by Northwestern Steel and Iron Works.

4) What are autoclave bags used for?

Autoclave bags are used to hold biohazardous items until it is time to be sterilized in an autoclave. The high-density propylene that they are typically made of can withstand temperatures up to 135^oC, the high end of nearly all autoclave cycle parameters.  When searching for autoclave bags to use at your facility, it is recommended that the manufacturer of those bags meet the following regulations:

• Federal DOT Regulation 49 CFR 173.197
• ASTM D1922
• ASTM D1709

5) How does autoclave sterilization work?

Sterilization in an autoclave (often aptly referred to as a steam sterilizer) works through the manipulation of four aspects vital to effective sterilization: time, temperature, pressure, and steam. They all play an important role in the process of effective sterilization:

• Time: Proper time ensures that the pathogens have been in the presence of a sterilizing environment (steam at temperature) for a long enough time to facilitate the proper killing of all pathogens.
• Temperature: Proper temperature of the steam that comes in contact with all pathogens, a minimum of 121°C, ensures the proper destruction of the pathogen’s ability to reproduce.
• Pressure: Manipulating the pressure in an autoclave’s prevacuum cycle ensures that no air pockets are left remaining in the sterilizing chamber. Air pockets provide insulation to pathogens from the sterilizing environment and are removed during pressure ramping so that the pathogens do not escape proper sterilization.
• Steam: Transfer of heat is most efficiently done using steam. The steam fills the chamber so that it can come in contact with the entirety of the sterilization load.

6) What are autoclaves used for?

At a foundational level, autoclaves are used to execute the steam sterilization process. They are designed to kill 99.9999% of all pathogens after a successful sterilization cycle is run. This is commonly referred to as a 6-log reduction in living pathogens. For the respective industries that we operate in, autoclaves are commonly used for:

• Biomedical / Pharmaceutical: In these laboratories, autoclaves are frequently used for the research development of drugs and vaccines.
• Common Laboratory (Research): in these laboratories, research tools and instruments are the most frequently autoclaved items.
• Vivarium: to maximize throughput, bulk autoclaves are used in vivarium facilities to sterilize cages and bedding for the protection of research subjects.

7) Can autoclaves kill viruses?

Autoclaves are capable of killing all types of pathogens, including viruses through the effective use of the four tools for sterilization mentioned in the answer to question 5. This has become especially poignant as the world grapples with the COVID-19 pandemic. Killing viruses are important in some applications for sterilizing medical tools and PPE before they are discarded.

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Sources

(Question 1) Science Direct
(Question 1) National Center for Biotechnology Information (NIH)
(Question 3) Smithsonian Institute
(Question 4) Tufpak
(Question 5) CDC

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