# Moist heat sterilization: can we speak about F0 in case of liquids sterilization?

Dear all,

I learned that the administered lethality (F0) during a moist heat sterilization process can only be mathematically calculated if saturated steam conditions exist during the sterilization process.

Can we also speak of this administered lethality F0 in case of liquids sterilization (e.g. bottles of WFI)? Since the temperature probe is inside the liquid, no saturated steam conditions exist (because it is a liquid with maybe air inside the head space in case of a gravity displacement cycle).

I have looked and looked for someone to post Z and Tb values for liquid loads. I have been able to find them for saturated steam loads, and also for dry heat loads (depyrogenation).

I have used Z=10 C and Tb = 121.1 C for cycles which sterilize liquids (like a media load). I have never been questioned about it.

Using the above values, I have also confirmed sterilization using Biological Indicators, with passing results. So I felt comfortable in that.

So I’m not sure if it’s not published or well understood. But I also have that question. Please share if anybody has more information, and also the source reference.

I ll try to explain but this is almost science so this space is too small for detailed explanation.
You both are right. Now I ll tell you why:

1. For porous load Fo value is helpful but not enough. Why? You can have high Fo values but if steam is not at boundary phase (saturated steam conditions) sterilization is not effective. It is because ammount of energy delivered to surface is lower than steam is saturated (saturated steam conditions). Remember that dry sterilization is at 190oC for 1hour. Because of that is better to follow temperature , time and equilibration time and steam pressure/temperature correlation.
For liquid load you deliver some energy to liquid and temperature of liquid destroy bioburden. So, there is no need to be saturated steam because you heat liquid and you can heat with everything you want, but fastest way is with saturated steam-of course because of delivered energy.

@JaredCroft Temperature 121.1 oC is taken by agreement - it is 250o Farenheit I think. And z values to, for calculation of Fo. Both values are taken by agreement.

Scientifically, steam and dry heat are very different, and steam and liquid heat are also very different. (latent heat and convection forces)

So scientifically how does one defend using the same Z value for liquid bulks, and a Z value for items which are exposed to steam directly? I’ve asked this question multiple times, have never gotten an answer to satisfy my question. Instead I have used BI results to conclude the cycle was correct.

Remember that Fo calculations are NOT allowed to defend autoclave loads as the final results. BI’s are required for every PQ. Fo calculations are useful to build a case, and to prepare for PQ runs, and to support PQ runs, but they cannot be used stand-alone.

Unless this question can be answered, I would recommend using Z, Tb, and D values as you would for saturated steam load. And make sure BI’s are also used (per requirements)

@JaredCroft
Ok, let discard dry heat sterilization.
WE have liquid container sterilization and steam sterilization of porous load.
For liquid container, we want to compare ammount of heat delivered to each container (or coldest…).
So, how can we do that if we have different values for z, Tb and D? to uniform this comparation, somebody saw that only temperature can have inluence on sterilization effectivity. Other parameters we will take that are the same that we can compare different processes. in the case we have always different z, Tb, D, we will get always differernt values. So, Fo is just ammount of energy that we deliver to each container! No matter what is inside the container. After you have Fo you can make calculation for theoretically SAL and find real SAL for product D value, Tsterilization and othet specific parameters. But, most regulatory bodies will not be satisfied with PNSU only, they ask for Fo too! See EMEA Decision tree for sterilization process or other regulatives.

For porus load is totaly another story- we need conditions that steam is saturated, that steam is in direct contact with surface not via trapped air (equilibration time) and temperature - time request. So, here Fo is not usefull because from Fo we can not see what conditions were. EN 285 and Annex 1 say that we can into account the biological indicators, but they are not decisive parameter.

Regarding porous loads, this is the first we mentioned it, so sorry I lead you astray about the BI’s being the final say - I thought we were just talking about liquid load. Honestly, I have less experience with porous loads, and I wouldn’t want to discuss that with expertise. For porous loads is there a heavy emphasis on the Bowie Dick tests, to show the steam has penetrated a stack of porous items?

But with liquid load - I’m with you. Typical liquid loads might be for example (1) finish product that is terminally sterilized, or else a (2) media (for sterility assurance testing / media fill) that need to be sterile prior to processing in the sterile process simulation tests (again often called media fills). For liquid loads, use TCs to test the liquid bulk conditions. The TCs will have to be placed in the liquid (without touching the walls). Any BI/Ampule will also be placed in the liquid (not near the walls). It is assumed that the coldest spot in a liquid load is in the centre of the liquid load (away from the walls). This takes some manipulation to ensure the TCs and BIs/Ampules are in the dead center of liquid load.

Can you send the EMEA Decision Tree link? I think that would help with the discussion.

Of course, I will send to you EMEA Decision tree…for liquid load, for volume greater than 100ml shoud be performed temperature mapping of the bottle to find coldest location in the bottle. My experience is 2/3 from the top of the bottle.

OK. The decision tree is lists the order of “preference”. Start with the most conservative approach, then go to the next possible approach.

The first tree is summarize as:
Use terminal sterilization if possible (greater than 15 minutes)
Use terminal sterilization next (Fo 8 minutes)
Use sterile filtration next
Do aseptic processing next (sterile train, gowning, etc.).

The second decision tree is as follows:
Use terminal dry heat (120 minutes at 160 C)
Use terminal dry heat (until SAL < 10^-6)