Sterilization effect – Do you distinguish F0 Value, D Value and Z Value for moist heat sterilization?

27/10/2026

Definition of D Value: The time required to reduce the microbial population by one log unit at a base temperature,the time needed to kill 90% of microorganisms (one-log reduction) at a given temperature. The D value is a heat resistance parameter of microorganisms. Different microorganisms have different D values under varied conditions, as shown in the table below:
Sterilization method Microorganism Temperature / ℃ Medium or sample D Value / min
Steam sterilization Geobacillus stearothermophilus 105 5% Aqueous glucose solution 87.8
Steam sterilization Geobacillus stearothermophilus 121 5% Aqueous glucose solution 2.4
Steam sterilization Geobacillus stearothermophilus 121 Water for injection 3
Steam sterilization Geobacillus stearothermophilus 105 5% Aqueous glucose solution 1.3
Steam sterilization Geobacillus stearothermophilus 135 Paper 16.5
Steam sterilization Geobacillus stearothermophilus 160 Glass plate 18 s
Definition of Z Value: The Z value is the temperature rise required to reduce the D value of a specific microorganism by one log unit. The Z value reflects temperature dependence, normally expressed in degrees Celsius. A larger Z value means the microorganism is less sensitive to temperature changes. The Z value is used to convert and calculate microbial lethality at different temperatures.
Definition of F0 Value: For moist heat sterilization, the reference temperature is defined as 121 ℃, with Geobacillus stearothermophilus as the biological indicator and a Z value of 10 ℃. The F0 value is the equivalent time (in min) at which the lethality achieved at temperature T with Z=10 ℃ equals that achieved at 121 ℃ with Z=10 ℃. In other words, F0 converts the sterilization effect at any temperature into an equivalent lethality at 121 ℃.
F0 formula:
F o = ΔtΣ10 (T-121/Z)
“Δt”: Time interval for temperature measurement at temperature T
“T”: Temperature of the load during sterilization hold time
“Z”: Temperature coefficient, the temperature change needed to achieve a one-log change in D value
Factors affecting F0 Value
1.Temperature: As shown in the formula, F0 varies exponentially with product temperature T. Even minor temperature deviations exert a significant impact on F0. It is required to measure the actual temperature inside the sterilized load.
2.Size, shape and heat penetration coefficient of the item to be sterilized
3.Viscosity of the solution and fill volume of the item to be sterilized
4.Quantity and loading arrangement of the items to be sterilized
5.Bioburden level of the item to be sterilized
As stated in the GMP Guide for Pharmaceuticals, moist heat sterilization cycles are generally classified into overkill approach and bioburden-based approach (also called survivor probability approach / moderate lethality approach) according to the bioburden level and moist-heat resistance of the items. Both approaches can achieve the required sterility assurance level for products and materials.
1.A sterilization cycle designed by the overkill approach is defined as: “A sterilization cycle delivering a minimum F0 of 12 minutes to the load.” In EU regulations for terminally sterilized products, overkill is defined as “moist heat sterilization at 121 ℃ for 15 minutes”.
2.Bioburden-based approach: The cycle time shall be designed to achieve a probability of a non-sterile unit (PNSU) below 10-6.
F0 determination is of great significance for verifying sterilization efficacy, so it is essential to validate the reliability of sterilization methods.