What is the advantage of steam sterilization (autoclaving) compared to hot air sterilization?

Sterilization requires a lot of energy. The energy of steam is several thousand times greater than that of air. Think about the operating temperature difference between a wet/dry sauna and the feeling of heat: in a wet sauna, a certain degree of heat already feels very burning, while the same temperature (or even higher) is much more tolerable in a dry sauna.

The advantage of sterilization with an autoclave is that the process takes place in a much shorter time thanks to the high energy of the steam. Another advantage of steam sterilization is the relatively low sterilization temperature (121oC and 134oC), instead of the 160-180oC used in hot air sterilizers. The use of hot air sterilization is often limited by the temperature tolerance of the devices.

Sterilization of liquids and culture media is not even possible with a hot air sterilizer, because the liquid evaporates (boils) in the open system. Liquids can only be heated to the sterilization temperature in a high-pressure device, i.e. in an autoclave.

In order for the devices or samples to become truly sterile, they must be kept at a certain temperature for a minimum time specified in the protocols. The most common autoclaving protocol is holding at 121°C for a minimum of 20 minutes. Another commonly used method is sterilization at 134°C for 3 minutes.

The traditional method of producing steam in an autoclave and its challenges:

In most autoclaves that work with traditional technology, the steam is produced in the chamber itself. In this case, there is always water at the bottom of the chamber, and this water is heated by the heaters located at the bottom of the chamber.

The main enemy of steam sterilization is air trapped in the autoclave. Where air remains, the steam cannot release the high energy required for sterilization. If the air in a hollow sample, e.g. in an empty glass container, in a pipette tip box, or inside a porous material, then the minimum time and temperature requirements stipulated in the protocol are not met, so we can be sure that our sample has not become sterile! Sterility is ensured by the so-called can be verified with biological indicators.

The arrangements shown in the picture represent the old (traditional) method, and their main disadvantage is that the removal of air is very difficult. The water occupies the lowest point of the device, so the air can only be removed from the device at a higher point. Since air is heavier than steam, the natural movement of air is downward in the chamber, so some of it remains inside the device.

In autoclaves with such heating, even a vacuum pump cannot achieve perfect results (remove air 100%), because after the first – relatively good – vacuum cycle, the second one will be less effective due to the continuous evaporation of the water at the bottom of the chamber.

But how can this problem be overcome? On the following page, we present the technical solutions used by today's modern autoclaves to achieve perfect sterility. >>