Autoclaving equipment using Steam/Air mixture

Hi all,

Currently we autoclave our Component Prep equipment using a steam/air mixture. Our autoclave settings are 122 for 30min therefore an overkill cycle. Our criteria are consistently met along with BI challenge, equilibration time etc etc.

However we recently included a Bowie-Dick test out of interest which failed, due to steam/air mixture i presume. What are the implications of this?
Is there any further tests we can include or need to include to eliminate any doubt?

Thanks Fin

Steam/air mixture autoclaves are intend for liquid products, where is a need for contra pressure to provide integrity of containers. As the container is heating, internal pressure is raising so there is a need for contra pressure which is provided but compressed air. The steam is mixing with air with fans. Bowie Dick test is designed for steam autoclaves to indicate presents of air. If you inject air in the chamber Bowie Dick test will failed. Bowie Dick test is for pre vacuum cycles autoclave. The heat transfer is less efficient for steam air autoclaves and this type of autoclaves are for liquid products where is a gol temperature in containers.

Hi Zeljko,

Thanks for the reply. I understand what steam/air autoclaves are used for and why our bowie-dick test failed. However what is the risk of sterilisng equipment in a steam/air cycle if we are consistently getting favourable results? Can there be any hidden risks etc?

There are many factors to be taken into account. Generaly, steam/air autoclaves are less effective. This does not mean that the item is not sterile, but the question that SAL can be realistically achieved. In any case smaller than a conventional vacuum cycle with steam. Why? The largest amount of energy is released when the steam is on the boundary conditions condenses . In this case, steam and air will deliver less energy than just steam. Secondly, what is the load? Is this worst case with filter and silicone hose or just basket and pistons and cylinders? And third, I am sure that you must perform empty chamber temperature mapping because if tha fan is not working propertly, you will have cold zones and distribution of steam and heat is not uniform. Distribution (mixing) of steam and air is very critical. If you dont have steam in your load and if you have hot air even at 121oC your cycle will be critical. I do not say that your load is not safe because regular bioburden is not termoresistant like B.stearothermophilus but your cycle 122 / 30 minutes is less effective than in regular autoclave that using just steam.

The risk in not using a pre-vac cycle for dry goods sterilization is that steam might not contact all surfaces. This could be the situation with harder-to-sterilize items, such as tubes and narrow mouth containers. Without steam (moist heat) contact, it could be dry heat for which longer times and higher temperatures are required. To verify that you have moist heat sterilization conditions, challenge all of the most difficult to sterilize areas with a biological indicator in a half cycle as part of you cycle validation process.

If there are no pre vacuum cycles, then this autoclave is not for usage for porous loads. If there are no pre-vaccum cycles then air can not be eliminate from load. Steam is heating air and hot air is not effective ( dry heat sterilization is at 170oC for 1h) so 121oC for 15, 20 minutes is not effective at all.

Hi, Thanks again for replys. This is not a dry heat cycle. it is a steam/air mixture. Our results have consistently established thourough heat distribution/penetration. Our worst-case load contains a buffer tanks with lots of silicone tubing which we have challenged on many occasions and has passed on thermal profile, BI challenge, CI challenge and theoretical SAL calculation. Our criteria of 122 for 30 min goes above and beyond what is required. Therefore can we not be confident of sterility?
There are 3 x prevacums pulled however steam is introduced mixed with air therefore it can not be totally free of air. The rototherm is predominately used for sterilising liquids which is the reason for this approach.
Furthermore our worst-case load with silicone tubes does come out with condensate in the the tubing which would indicate steam is reaching/penetrating there more intricate areas.

There are many factors that can affect the properly sterilization. But for silicone tubing I suggest at least five prevacum cycles. It depend of course from diameter of the tubes, are they open or closed, are they long or short. If you have good results for termal profile they should be good for BI too.

Do we need to perform steam quality tests near steam mixture sterilisers