Simulated cycle profile generated from the selected preset. Switch to Manual or Paste mode for real thermocouple data.
What the number means. F₀ is the number of minutes at 121.1°C that would kill as many spores as your whole temperature profile did. It is the sum of 10((T − 121.1) / z) × Δt over every interval, with z = 10°C and Δt in minutes. PASS means that sum reached your target. Dry heat (FH) and depyrogenation (FD) use the same sum with their own Tref and z.
Why it differs from the hold time. Every minute counts in proportion to its lethality rate: 1.0 at 121.1°C, about 0.1 at 111.1°C, about 0.01 at 101.1°C, and 10 at 131.1°C. Heat-up and cool-down above roughly 110°C add noticeably, so F₀ is usually higher than the hold alone. A hold that sits even 1°C low delivers about 21% less per minute.
Common mistakes. Using the chamber or drain temperature instead of the coldest point in the load. Pasting time in seconds instead of minutes. Changing z away from 10°C and still comparing with an F₀ target. Logging points too far apart, which makes the trapezoidal sum inaccurate. Assuming a temperature reading proves sterility: F₀ assumes saturated steam is in contact with the load, so air pockets lower the real kill even when the probe reads 121°C.
| Time (min) | Temp (°C) | Lethality Rate | Cumulative F |
|---|
| Organism | Mode | D-value | Temp | ATCC |
|---|---|---|---|---|
| G. stearothermophilus | Moist heat | 1.5–2.0 min | 121°C | 7953 |
| B. atrophaeus | Dry heat | ≥ 2.5 min | 160°C | 9372 |
| C. sporogenes | Moist heat | 0.4–0.8 min | 121°C | 7955 |
| C. botulinum | Moist heat | 0.2–0.25 min | 121°C | — |
| Endotoxin (LPS) | Depyro | — | 250°C | — |
D-value = time at stated temperature to reduce population by 1 log (90%). Standard BI population: 10⁶ spores/unit.
Autoclave F₀ is the equivalent exposure time at the reference temperature of 121.1°C (with z = 10°C) that an autoclave cycle actually delivers. A 15-minute hold at 121.1°C with instantaneous come-up and come-down gives F₀ = 15 min. Real autoclave cycles accumulate F₀ during ramp-up and cool-down too, so the delivered F₀ is always higher than the hold-only value; the integral F₀ = Σ 10((T − 121.1) / 10) × Δt is computed from the thermocouple trace at the coldest point in the load.
An F₀ cycle is a steam sterilization cycle whose integrated lethality meets a target F₀ value. Common targets: F₀ ≥ 8 minutes for terminal moist-heat sterilization of pharmaceutical loads (European Pharmacopoeia minimum), F₀ ≥ 12 minutes for the FDA overkill approach (12-log reduction of a 10⁶ bioburden with D121 = 1 min), and F₀ ≥ 15–30 minutes for conservative SIP of fermenter vessels where cold spots are harder to characterise. References: USP <1211>, Ph. Eur. 5.1.1, PDA Technical Report 1.
F₀ (F-zero) is the equivalent time in minutes at 121.1°C that delivers the same sterilizing lethality as the actual time-temperature profile. It integrates lethality over the entire cycle including heat-up and cool-down phases using the formula: F₀ = Σ 10((T − 121.1) / 10) × Δt. A standard autoclave cycle of 121°C for 15 minutes delivers an F₀ of approximately 15 minutes, but the actual F₀ will be slightly higher due to lethality contributed during ramp-up and cool-down.
The European Pharmacopoeia specifies a minimum F₀ of 8 minutes. The FDA overkill approach requires F₀ ≥ 12 minutes (12-log reduction of a 10⁶ population with D121 = 1 min). The standard reference cycle is 121°C for 15 minutes (F₀ = 15 min). Many pharmaceutical SOPs use 121°C for 30 minutes (F₀ ≈ 30 min) for maximum safety margin.
The overkill approach assumes worst-case bioburden (10⁶ organisms with D121 = 1 min), requires F₀ ≥ 12 min, and is validated via the half-cycle method. It does not require routine bioburden monitoring. The bioburden-based approach uses actual bioburden data to calculate Fbio = D × (log N₀ + 6), allowing shorter cycles for heat-sensitive products. It requires ongoing bioburden testing and more complex validation.
Validation requires: (1) thermocouple mapping to identify cold spots in the chamber or vessel, (2) biological indicator (BI) challenges using G. stearothermophilus spores placed at cold spots, (3) F₀ calculation from thermocouple data demonstrating the target lethality is achieved at the coldest point, and (4) documented IQ/OQ/PQ protocols. For SIP, all drain points, valve seats, and filter housings must independently reach ≥121°C.
Depyrogenation is the destruction of bacterial endotoxins (pyrogens) using dry heat, typically 250°C for ≥30 minutes. It is validated using FD with Tref = 250°C and z = 46.4°C, targeting ≥3-log reduction of endotoxin. Validation uses spiked endotoxin indicators (≥1,000 EU) placed in the depyrogenation tunnel or oven. It applies to glassware, metal parts, and container components — not liquids or plastics.
For moist heat (steam): Geobacillus stearothermophilus (ATCC 7953), D121 = 1.5–2.0 min, population 10⁶ spores/unit. For dry heat: Bacillus atrophaeus (ATCC 9372), D160 ≥ 2.5 min. For depyrogenation: purified endotoxin indicators (≥1,000 EU) demonstrating ≥3-log reduction. BIs are placed at identified cold spots during validation runs.
The integral itself is identical: F₀ = Σ 10((T − 121.1) / z) × Δt with z = 10°C. What differs is where and how heat is delivered. A steam autoclave heats a load fairly uniformly, whereas steam-in-place (SIP) heats a vessel and its dead legs in situ via jacket or direct steam with slower, uneven ramps. SIP validation must confirm that every cold spot — drain points, valve seats, filter housings, sample lines — independently reaches ≥121°C, and the delivered F₀ is computed from the slowest-heating (coldest) location.
The cumulative F₀ is the number of minutes at 121.1°C that would deliver the same kill as your whole temperature profile. PASS means it reached the target you set. The lethality rate L = 10((T − 121.1) / 10) is how many equivalent minutes one real minute is worth at temperature T: 1.0 at 121.1°C, about 0.1 at 111.1°C, about 0.01 at 101.1°C and 10 at 131.1°C. That is why heat-up and cool-down add to F₀, and why a hold just 1°C below 121.1°C delivers about 21% less lethality per minute.
Each D-value of exposure at 121.1°C removes one log of the organism, so the log reduction a cycle delivers is F₀ / D121, and the expected survivors per unit are N₀ × 10−F₀/D. The overkill approach assumes N₀ = 10⁶ and D121 = 1 min: F₀ = 12 min gives a 12-log reduction and a SAL of 10⁻⁶. With a real, lower bioburden or a less resistant organism the same SAL needs less F₀, which is the basis of bioburden-based cycles.
The most common are: (1) using the chamber, drain or set-point temperature instead of the coldest point in the load, such as the centre of the largest liquid container; (2) entering time in seconds instead of minutes, which inflates F₀ 60-fold; (3) changing z away from 10°C and still comparing the result with an F₀ target; (4) logging readings too far apart, so the trapezoidal sum misses the real shape of the curve; (5) forgetting that heat-up and cool-down contribute, or crediting them when the validated cycle only counts the hold; and (6) treating a temperature reading as proof of sterility. F₀ assumes saturated steam is in contact with the load, so trapped air lowers the real kill even when the probe reads 121°C.