The autoclave sterilisation process: phases, cycles and parameters
— Equipo JP Selecta
Autoclave sterilisation is a fundamental process for the efficiency of laboratories and healthcare centres, eliminating microorganisms and spores through saturated steam under pressure, reaching temperatures of 121 to 134 °C for a defined period of time.
A standard autoclave sterilisation process usually comprises the initial loading and air purge, the plateau phase (the actual thermal sterilisation), steam evacuation or depressurisation and, finally, drying to keep the material free of moisture.
What is autoclave sterilisation?
Autoclave sterilisation is a highly efficient physical decontamination method. It works by subjecting loads to the action of saturated steam under pressure, causing the irreversible coagulation and destruction of the structural proteins of viruses, bacteria and highly resistant spores. The sterilisation autoclave works by isolating the material inside a sealed pressure chamber, where steam condenses rapidly on the cold surfaces and releases its high heat of vaporisation, acting as the perfect destructive agent.
The phases of the autoclave sterilisation cycle
For an autoclave to be completely safe and effective, every programme runs a precise sequence of thermodynamic stages:
- Chamber loading and air purge: for steam to saturate the sample effectively, all the air inside must first be removed, since air acts as a thermal insulator. This extraction is carried out by atmospheric gravity purging or, in advanced equipment, by precise fractionated vacuum pulses.
- Heating and plateau phase (sterilisation temperature and time): the unit injects steam progressively until it reaches the programmed set-point pressure and temperature. The "plateau" is the strict period during which the entire load is held at that lethal temperature in order to destroy the bioburden.
- Steam evacuation / depressurisation: once the plateau time has elapsed, the exhaust valve releases the internal steam in a controlled manner to bring the pressure back down to atmospheric pressure without damaging the materials.
- Drying (vacuum or gravity, depending on the model): this consists of removing residual moisture. Depending on the technology, heat is applied with deep vacuum stages to ensure that instruments and porous materials come out completely dry.
- Cooling and unloading of the material: the chamber and the load dissipate thermal heat down to a safe temperature that allows the door to be unlocked and the sterilised load to be handled without risk.
Types of autoclave sterilisation cycles
Autoclave sterilisation cycles must be selected rigorously according to the type of packaging and the thermal nature and physical state of the load.
Standard cycle with temperature ramps
This type of programme uses progressive heating and cooling ramps to control thermal transmission. It is the recommended procedure for standard solid loads, wrapped or unwrapped metal laboratory instruments and stable textile materials that must avoid damage from violent thermal shock.
Short or flash cycle
The flash or high-speed cycle is characterised by applying high temperatures (typically 134 °C at 2 bar of pressure) for a very short exposure time (between 3 and 5 minutes). It is designed exclusively for unwrapped solid material that must be immediately available at the workbench, such as surgical or dental instruments.
Fo cycle (control by accumulated lethality)
This is an advanced control system based on the accumulated lethality value (F0) instead of operating with fixed, preset time/temperature combinations. Using an independent probe in the load, the machine intelligently calculates the exposure time required, which is very common in pharmaceutical validation and the food industry in order to avoid thermal degradation at the core of the samples.
Other cycles (liquids, biological waste, textiles)
Depending on the model, there are parameterised cycles for more complex items:
- Liquids: these use slow cooling protocols or integrated regulated counter-pressure systems. They are essential to prevent sudden boiling and the spillage of liquids or culture media in pressurised bottles as atmospheric pressure drops.
→ In this case you need autoclaves for liquid sterilisation.
- Biological waste: cycles designed to destroy pathogens or laboratory biowaste (a prion cycle at 134 °C for extended times such as 18 or 20 min) that ensure the total denaturation of proteins.
- Textiles and porous material: specific "Class B" processes that combine deep preliminary fractionated vacuum phases to extract the air trapped microscopically between fibres or internal hollow cavities, ensuring that steam penetration is absolute. → In this case you need Autoclaves for sterilisation without fractionated vacuum drying or vertical fractionated models.
Key parameters of the sterilisation cycle
Whichever cycle is selected, its effectiveness depends on three parameters that the equipment must control and record at all times: temperature, pressure and plateau time. The following table summarises the usual ranges in laboratory autoclaves.
| Parameter | Usual range | Note |
|---|---|---|
| Temperature | 121-134 °C | Depending on load type and cycle |
| Pressure | 1-2 bar | Saturated steam |
| Plateau time | 3-30 min | Depends on temperature and load |
| Drying | Vacuum or gravity | Removes residual moisture |
Validation and process control in autoclave sterilisation
In highly regulated environments, it is not enough to assume that the equipment worked correctly. The process must undergo thorough validation, certifying under IQ/OQ/PQ protocols (Installation, Operational and Performance Qualification) that the machine responds consistently under industrial and quality standards (such as the EU GMP directive or the FDA).
Physically, the cycle is validated through the use of biological and chemical indicators that verify the lethality of the chamber.
In addition, tools such as the Bowie-Dick and Helix tests verify steam penetration and the effectiveness of air removal in hollow and porous loads, technical subjects that we will cover in detail in a future article.
To achieve this level of technical confidence, J.P. Selecta manufactures laboratory autoclaves with programmable sterilisation cycles compliant with the most demanding international standards, integrating PID control, digital cloud logging and USB connectivity. This technology always ensures meticulous control of the data and guarantees full traceability of every process.