What is an Autoclave?

Among the many aspects of modern life that we take for granted is the sterilization of materials.

This very important hygienic procedure was developed less than two centuries ago by Louis Pasteur.

Another important step in the history of sterilization was the invention of the autoclave, a laboratory instrument still used today.

Let's find out together.

How an Autoclave Works

The father of the autoclave is considered to be Charles Chamberland, a collaborator of the aforementioned Louis Pasteur, who was the first to hypothesize the use of a hermetic sealing mechanism already known for sterilization purposes.

This mechanism is precisely the autoclave which literally indicates a type of hermetic closure in which the difference in pressure between the inside and outside of the container facilitates the seal. 

Technically, the pressure cooker we use in the kitchen is also an “autoclave,” but today this term mainly refers to modern moist heat sterilizers that use a seal.

The autoclave is therefore a laboratory instrument consisting of a hermetically sealed environment within which sterilization is carried out by means of saturated steam.

There are other hot air sterilization methods which we have detailed here.

As regards the autoclave, however, the process is divided into three phases.

• Vacuum generation: the removal of air from inside the autoclave to ensure the effectiveness of the sterilization process. The expelled air is then pumped out of the autoclave, replaced by pressurized saturated steam, and the environment is sealed with the autoclave itself. 

• Heating: thanks to saturated steam, it heats every tiny component of the instruments to be sterilized, reaching the necessary temperature.

• Sterilization: the duration of the actual sterilization process, which varies depending on the temperature reached by the autoclave and the number of germs present on the objects being sanitized. The standard duration is over 15 minutes at a temperature of 121°C, but there are many exceptions.

What is an autoclave used for?

The autoclave is a commonly used instrument in most laboratories because sterilization is a preparatory procedure for most operations.

In addition to serving as a preparatory step for scientific research, the autoclave is also essential in medical practice for the sterilization of medical devices, such as operating chamber instruments or those found in a dental office.

Finally, sterilization is also required in all non-medical activities that involve the body, such as a tattoo parlor.

Autoclaving practices are described by the international standard EN 13060.

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FDM Ovens for Hot Air Sterilization

Saturated steam autoclaving is not the only sterilization method available.

Dry hot air sterilization is an alternative method that meets different needs.

FDM – Environment Makers has been manufacturing laboratory instruments for over 70 years and, when it comes to dry hot air sterilization, we have two different products that perform this function.

On one hand, there are FDM ovens and dryers, specialized in temperature control and capable of reaching extreme values ​​such as 300°.

On the other hand, there are FDM environmental chambers that reach 180° (perfect for the standard procedure) but have many other functions including relative humidity control and can be equipped to carry out most climatic tests.

For any further questions or concerns, please do not hesitate to contact us.

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Reference Standards

• EN 285 — Sterilization – Steam sterilizers – Large sterilizers. European standard for large-capacity steam autoclaves (> 60 L) for healthcare and industrial applications.
• EN 13060 — Small steam sterilizers. Standard for small clinic/practice autoclaves, classes N, S, and B.
• ISO 17665-1 — Sterilization of health care products – Moist heat – Requirements for development, validation and routine control.
• USP — Sterilization of Compendial Articles. General chapter of the US Pharmacopoeia on the sterilization of pharmaceutical products.
• ISO 11138 (series) — Biological indicators for sterilization processes (Geobacillus stearothermophilus for saturated steam).
• ISO 11140-1 — Chemical indicators – General requirements, includes Bowie-Dick test for class B autoclaves.
• PED 2014/68/EU — Pressure Equipment Directive, applicable to autoclaves as pressure vessels.

Authoritative Sources

• CEN – European Committee for Standardization. It manages the EN 285 and EN 13060 standards harmonized with EU Regulation 2017/745 on medical devices.
• ISO – International Organization for Standardization. Publishes the ISO 17665/11138/11140 family of standards on sterilization.
• USP – United States Pharmacopeia. General Chapter
• Istituto Superiore di Sanità. Italian guidelines for the application of hospital and industrial sterilization.

Frequently Asked Questions about the Autoclave

What is the difference between class N, S and B autoclaves according to EN 13060?

The EN 13060 standard classifies small autoclaves into three classes based on the load they can handle. Class N (Naked) sterilizes only unwrapped solid instruments; typical cycle time 15-20 minutes at 134°C, not suitable for hollow or fabric instruments. Class S (Specified) treats single wrapped and simple hollow instruments as specified by the manufacturer. Class B (Big/Hospital), equivalent to large EN 285 autoclaves, sterilizes any load: porous instruments (fabrics, gauze), hollow (type A lumen cavity), and multiple wrapped instruments. It requires fractionated pre-vacuum (3-4 pulses at −0.9 bar) and daily Bowie-Dick testing according to ISO 11140-4 to verify steam penetration. The standard B cycle: 134°C for 4 minutes or 121°C for 20 minutes.

What time and temperature parameters guarantee sterility?

Saturated steam sterilization requires a temperature-time-humidity combination (100% saturated steam) with a log10 decimal reduction. Standard ISO 17665-1 cycles are: 121 °C for 15-20 minutes (F₀ = 15 min) with a pressure of 1.1 bar; 134 °C for 3-4 minutes (prion 18 minutes according to WHO) with a pressure of 2.1 bar; and 115 °C for 30 minutes for temperature-sensitive products. The Sterility Assurance Level (SAL) 10⁻⁶ requires a reduction of at least 12 logs of the Geobacillus stearothermophilus microbial population (D₁₂₁ = 1.5-3 minutes). The F₀ integrates the equivalent time at 121 °C with a z-value of 10 K and is the reference parameter for cycle validation.

How is an autoclave validated according to ISO 17665-1?

The qualification includes IQ/OQ/PQ. TheIQ verifies installation: PED certificate, class 0.6 pressure gauge calibration, traceability of class A T sensors (PT100 ± 0.15 °C), software documentation ≥ 21 CFR Part 11. TheOQ performs thermal mapping in an empty chamber with 12-30 SIT/ACCREDIA calibrated Type T thermocouples distributed according to EN 285 Annex D, with criteria: band ± 2 °C, equilibrium time ≤ 15 s at 134 °C, saturated steam with NCG < 3.5%. The PQ uses real loads (worst-case load) with thermocouples at the coldest point and biological indicators (BI) Geobacillus stearothermophilus 10⁶ CFU; 3 consecutive cycles with ≥ 6 log reduction. Revalidation annual or after modifications. Cycle parameters must be recorded in real time with an audit trail.

When is an autoclave needed for sterilization vs. decontamination in a BSL laboratory?

BSL-2/BSL-3/BSL-4 biosafety laboratories require double-door (pass-through) autoclaves for mandatory decontamination of biological waste before disposal. Typical cycle: 121°C for 60 minutes for liquids, 134°C for 20-30 minutes for contaminated solid waste (bottles, pipettes, PPE). Thermally slow loads (5-10 L liquid bottles) require a liquid cycle without initial vacuum with controlled cooling to prevent boil-over, or overpressure (air ballast) with +0.3 bar above saturation. BSL-3 autoclaves must have HEPA ventilation, biotight sealing on the dirty-side door, and 100% traceability of cycle parameters according to the WHO biosafety manual. Prion inactivation requires a 134°C/18-minute cycle with 1N NaOH pretreatment according to Creutzfeldt-Jakob guidelines.