Autoclave Steam Sterilization Cycles, Part 6: F0 Cycle

Autoclave efficacy is highly dependent on time, temperature, and pressure, and these parameters can be manipulated and optimized to create specific sterilization cycles for each application. To help your facility get the most out of your autoclave, we introduce the sixth article in our Steam Sterilization Cycles series: The F₀ Cycle (pronounced f-sub-zero or f-sub-oh).

The F₀ Cycle is especially useful for sterilizing large liquid volumes with, say, greater than 1 Liter of fluid. The term “F₀ ” is defined as the number of equivalent minutes of steam sterilization at 250°F (121°C) delivered to a load (product)¹. For example, if a cycle has an F₀ value of 12, the sterilization effectiveness of that cycle is equal to 12 minutes at 250°F (121°C) regardless of the process temperature and time used in the cycle.

Note: The theory and equations behind the F₀ Cycle can be a bit intimidating to some readers and they go beyond the scope of this blog; however, we will do our best to explain just the important facts regarding the F₀ Cycle.

Steam Sterilization Cycles: The F₀ Cycle

The basic concept behind F_0­ is that microbes begin being destroyed prior to hitting the sterilization set point (e.g. 250°F.), and therefore, true sterilization can actually begin before this set point is reached. So, to correct this lapse in achieving an accurate reading, the F₀ Cycle essentially is designed to give you “credit” for the sterilization that happens while your load is coming up to temperature.

Before we explain this concept further, let’s review the process steps of a Liquids Cycle that utilizes a Load Probe.

1. Press the Cycle “Start” button.
2. Steam is injected into the chamber until the chamber temperature reaches the sterilization set-point (usually 250°F or 121°C).
3. The chamber temperature² is maintained at slightly above the sterilization set-point until the load temperature reaches the sterilization set-point. This portion of the cycle is referred to as the “lag time” because the load temperature is “lagging behind” the chamber temperature.
4. Once the load temperature has reached the sterilization set-point, only then can the sterilization timer start.
5. When the timer completes, the chamber is exhausted slowly to prevent boil-over and the cycle ends.

Figure 1 shows a typical time-versus-temperature profile of both the chamber temperature and the liquid load. There are two things to notice about this graph:

1. The shaded area is the sterilization phase during a standard Liquids Cycles, and
2. The lag time.

For large liquid loads (upwards of 1 liter per container), the lag time in step 3 can take a relatively long time. For instance, depending on the autoclave configuration and the volume of the liquid, it could take the load 30-60 minutes to reach sterilization temperature—and this is the real problem. This extended lag time results in longer-than-necessary cycles that can cause problems for busy labs (e.g. caramelized broth, long wait periods, etc.).

To correct this, one way to increase the speed of a cycle and/or maintain the integrity of the broth is to get credit for long lag times by using the F₀ Cycle. As mentioned earlier, and as shown in the shaded area in Figure 2, the F₀ Cycle gives you “credit” for the sterilization that happens while your load is coming up to temperature.

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Here’s how:

During the lag time, your load has not yet reached the sterilization temperature set-point (e.g. 250°F), however,the temperature is high enough for some sterilization to occur.  So, for example, if the load is at 232°F for 15 minutes, that creates the sterilization equivalent of being at 250°F for 1.5 minutes. This is known as the F₀ value. The F₀ value tells us the equivalent amount of sterilization (in minutes) that would have been completed had the load been at 250°F.  (As temperature increases, so does sterilization/kill rate.)

By calculating the F₀ values every few seconds over the length of a cycle and adding them up, a total F₀ value for the cycle is determined. Once the desired F₀ value is reached, the cycle will have achieved sterilization, regardless of whether the load ever reached the original set sterilization temperature. Therefore, the F₀ cycle becomes extremely useful for autoclaving large volumes of liquid in a timely manner, as quite a bit of “sterilization credit” is accumulated during the lag time for these larger loads.

Using the F₀ Cycle at Your Facility

Before enlisting the F₀ Cycle at any facility, be sure you fully understand when and how to use this advanced cycle. Done correctly, this cycle could help improve overall efficiencies and lead to more accurate sterilization processes.

To get an overview of steam sterilization, check out our video here:

Feeling stuck? At CSS, it is our mission to help you get the most out of your steam sterilizers so you can properly sterilize your unique load types. As such, we encourage you to visit the Chamber Blog for more useful information; to catch up on this series (Part 1, Part 2, Part 3, Part 4; Part 5); or to contact our team of sterilization experts to ask questions and learn more.

Stay tuned for more articles in this ongoing series—we’ve got a lot to share!

¹ Calculated using a z-value of 10°C.