Autoclave sterilisation is one of the most effective methods of sterilisation. By generating high pressure steam, nasty microorganisms such as spores can be rapidly denatured and killed. Typically, gravity cycles, vacuum cycles and liquid cycles are the three most common cycles that you will encounter when it comes to autoclave sterilisation.

But have you ever wondered about the difference between gravity, vacuum, and liquid, autoclave cycles?

Gravity Autoclave Cycle

Gravity autoclaving, also known as gravity displacement autoclaving, is the most basic form of autoclaving.
The process of gravity autoclaving involves replacing all ambient air in the autoclave chamber and forcing it out of the exhaust valve. This is done so that the steam can sterilise the objects inside the autoclave.
As the steam is pumped into the autoclave chamber it rises to the top because it is less dense than air. This forces the air to the bottom of the chamber where it then exits through the exhaust valve. When the cycle is complete, the steam is released through the drain and the cooling process begins.
Gravimetric vapour pressure is commonly used to process laboratory water, media, regulated medical waste, pharmaceutical products and non-porous items whose surfaces can come into direct contact with steam.

Vacuum autoclave steam circulation

On the other hand, vacuum autoclaving, or pre-vacuum autoclaving, is best used when air is not readily available from the sterilised medium. It commonly sterilises objects such as animal cages, bedding or bandaged surgical kits.
Hot press tanks designed to perform vacuum cycles have a vacuum system. This system alternately injects steam and evacuates the vacuum at the beginning of each cycle. This helps to remove air from the autoclave chamber.
Vacuum extraction helps to completely remove air from the most difficult to reach areas of the object. Once all the air is gone, the steam can quickly penetrate the object, resulting in a fast and efficient disinfection cycle.

Liquid high-pressure steam circulation

Unlike gravity and vacuum cycles, the liquid cycle does not use liquids for disinfection. Instead, it disinfects the liquid itself.
Conventional liquid sterilisation techniques often lead to situations where liquids boil over. This happens when a liquid that has been autoclaved boils over the container that heats it. This usually occurs during the venting phase when the pressure is released too quickly.
A disadvantage of boiling overflow is that a large amount of liquid is lost due to violent overflow of the container. It is therefore important to use an autoclave specifically designed for liquid circulation in order to avoid such losses.
The vapour pressure circulation of liquids avoids boiling by slowly releasing the pressure in the chamber. Because of this, the circulation is also known as the slow-drain method. By releasing the pressure slowly, the temperature of the liquid can be cooled slowly as the pressure drops.
The liquid autoclave cycle is capable of sterilising water, brine and agar.

In summary

There are generally three types of autoclave cycles that you may encounter: gravity cycles, vacuum cycles, and liquid cycles.
In a gravity cycle, steam is allowed to enter the top and sides of the sterilisation chamber. This replaces air, which is less dense. Once this happens, the air is expelled from the chamber through the vent holes and the object is thoroughly sterilised with steam.
At the same time, a vacuum cycle produces steam injection and vacuum extraction to replace air in hard-to-reach areas. Once this step is complete, the sterilisation process proceeds quickly and efficiently.
Finally, the liquid cycle is used to sterilise liquids. It is the preferred method for sterilising liquids as it prevents boiling by slowly releasing pressure and allowing the liquid to cool gradually.