Autoclave (steam steriliser)

An autoclave is a sealed pressure chamber that is filled with saturated steam. Raising the pressure above atmospheric lets the steam reach temperatures well above 100 °C, typically 121 °C (at about 1.05 bar gauge) or 134 °C (at about 2.04 bar gauge). When steam touches a cooler surface it condenses and releases a large amount of latent heat, which heats the load quickly and coagulates the proteins of microorganisms.

A typical laboratory cycle for culture media is 121 °C for 15 minutes, measured once the coldest part of the load has reached temperature. Larger liquid volumes and dense loads need longer exposure, which is why each load pattern is validated by temperature mapping and biological indicators.

Air is the enemy of steam sterilisation. Air trapped in the chamber or inside the load lowers the temperature reached at a given pressure and blocks steam from contacting surfaces. Autoclaves therefore remove air either by gravity displacement (steam pushes air out through a drain at the bottom) or by mechanical pre-vacuum pulses before steam is admitted.

In a pharmaceutical QC lab, two separate autoclaves (or two validated cycles) are commonly kept: one for sterilising clean media and materials, and one for decontaminating used cultures and waste, so that clean and contaminated items never share a load.