Thermodynamics of steam sterilization - how long is saturated steam "effective"

Dear all,

Steam sterilization is driven by the process of saturated steam condensing onto surfaces that need to be sterilized. The condensation of steam causes the pressure to drop (gas to liquid) and therefore, fresh steam is pulled to the area of condensation. This process goes on and on… During this condensation, a huge amount of latent heat energy is transferred to any microorganisms and will lethally destroy all proteins responsible for their reproducing/surviving etc…

But, since condensation of steam only occurs if the surface (on which steam will condensates) is a bit cooler than the steam, this process will eventually stop as the temperature of the surface will eventually reach the same temperature of the steam.

Does this mean that after let’s say 60 minutes (when the temperature of steam and surface is completely equal, no more “steam sterilization” (high lethality) is happening, but only further heat sterilization is applied (less lethality is delivered)?

Has anyone every determined this transition point where steam stops condensing, and becomes in temperature equilibrium with all surfaces.

Thanks for your feedback!

There are a few answers to your question, and few comments. First of all steam will continue to condense and to evaporate even when the temperature is at equilibrium strangely enough…unless you have superheated steam. Superheated steam can be tested for using appropriate steam/pressure calculations and tables (which are a pain), but also through qualification of the steam supply. To asnwer your questions, if the steam is saturated, then you will continue to have condensation/evaporation even at equilibrium.
NOTE: Saturation means that the steam likes being in the steam phase just as much as the water phase, thus it teeters back and forth instantaneously, and thus creates the kill needed. Furthermore,water on an item after steam sterilization cycle is complete does NOT mean that sterilization was effective. Rather, if water is present, there might be some evidence that it never came up to equilibration temperature, and also the water could have acted as a heat shield to the area covered by the water. You want the item to be dry when removing it from the autoclave. Also, a 60 minute cycle is pretty long. One recommendation for “overkill approach” is to be around 24 minutes for hard goods and 45 minutes for bottles. (here is one reference The overkill approach assumes worst case (10^6)reduction of bioburden, and using the most resistance bugs to kill (D-value of 2). Thus overkill is the longest cycle. Other approaches to steam sterilization will result in a shorter duration, as far as I know.

Condensation will cease when the component surface is at an identical temperature to the steam surrounding it. The moisture produced by this heating and despite any further condensation and re-evaporation that may occur as the result of pressure variations, is sufficient to provide the conditions for sterilization. Other than a change of pressure, there is no mechanism that is capable of providing a drying action in a saturated steam environment and even then the evaporation of water will result in heat being extracted from the component to the environment, which can only result in more condensation. Sterilization requires the presence of temperature and moisture, but does not require a continuous supply of condensate throughout the process.

I would argue that the presence of latent heat results in components being heated quickly but does does not contribute to the lethality. This is borne out by BI’s being inactivated in aqueous solutions at the same rate as in a saturated steam environment and that the lethality calculation is based on temperature and not enthalpy.

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It sounds like what KeithS is saying, is that there is always a driving force for the steam to condense on the good (even 60 minutes into a cycle). His argument is that there are pressure variations. So if there is perfect equilibrium (my original statement) there would be no phase change. But the equilibrium is never perfect, but there is always a driving force however small and instantaneous (correction to my post) and so these slight changes in environment causes the saturation to continue throughout the cycle (even over 60 minutes per your original post).

Keith, what is your experience with water condensate at the end of a cycle? My experience is that for aqueous loads, this is obviously expected, but for dry loads, water in a tube can mean steam never got through the tube and thus didn’t get up to temperature. Also freestanding water after a load, is a potential source of microbial contamination (from the environment after sterilization).

Regarding my comment about water/dryness in a load, Monopok’s post indicated he is sterilizing a dry load (he referenced surfaces). A dry goods load with water pooled on it might indicate that it never got up to temperature and that condensation was constantly forming on the piece (because it was cooler than the steam environment), thus steam condenstate needs to be observed, but it also doesn’t necessarily mean the autoclave cycle failed (per KeithS’s comments).

Keith anything else?

Jared, I have had the situation whereby a stopper hopper weighing @ 50 kg had a very high equilibration time due to standing water and it simply was not possible to reconfigure the load to remove it. The only solution was to extend the hold time and provide an explanation for auditors as to why this was the case.

Where water has been an issue in tubing, I have dealt with some exceptionally complex items and have always managed to configure the tubing to drain sufficiently to avoid diiificulties.

Standing water on a load item can result from an excess of condensation on the the item or can come condensation from another item that has dripped upon it. The presence of such water would not make me automatically believe that the item has not been sterilized, but would provide concerns about bacterial growth through the packing, post processing.

It is mayble little off topic, but I must say that overkill cycle is 15 minutes or 12 minutes (PDA), sterilization phases 24 minutes and 45 minutes are too long…for real production site bioburden, Fo 15 give us about 30log reductions…

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Keith. I’ve seen this as well for a pre-vac cycle. I think they first confirmed the air was out of the chamber by looking at the pre-vac cycles (pressure/vacuum pulses), and then they modified the last pulse so that it was less vigorous, this helped the load meet the 30 second equilibration time. Although I think your approach is better, to fail the 30 second hold time, but to explanation why it didn’t meet equilibration time - showing that the cycle is still under control, and steam penetration is complete.