# What is F0 value in moist heat sterilization process

This article describes the D value, Z value and F0 value which is calculated in moist heat sterilization process

## What is mean by D value, Z value and F0 Value.

**What is D value ?**

**As per EU Annex 1** : The value of a parameter of sterilization (duration or absorbed dose) required to reduce the

number of viable organisms to 10 per cent of the original number.

**As per US FDA sterile guide :** The time (in minutes) of exposure at a given temperature that causes a one-log or 90 percent reduction in the population of a specific microorganism

**As per PMDA :** A value indicating the extinct rate of microorganism. The time or radiation dosage required to achieve inactivation of 90% of a population (one tenth of the survival rate) of the test microorganism under stated exposure conditions.

**Example : 1)** If population of microbes is 1000 and it will be reduced to 100 i.e. 90 percent /One log reduction at specific temperature i.e. 121 degree . The time required to reduce one log reduction/90 % of population from the existing size.

**2)** D value of 1.5 means it takes 1.5minutes to reduce 1 log (to 10%) @121 degree Celsius

**3) **D-value of 2.0 means more resistant while a D-value of 1min means less resistant.

**Another way to calculate the D-value is by using the following formula.**

D = t / (log C_{o} – log C)

where, t – exposure time in minutes

C_{o} – initial number of micro-organisms

C – final number of micro-organisms

Approximately D-value varies by 10 times for variation of 10°C. This doesn’t mean that D-value varies by 1 time for 1°C variation. The rationale for this is beyond the scope of this topic. To keep things simple, only note that “**Variation in 1°C requires a variation of D-value by 25-30%**”.

Even a small temperature incline will impact steam sterilization. At the same time, it may be insignificant in dry heat sterilization.

### What is Z value ?

Z-value – The temperature difference that leads to a 10-fold change in the D-value of the biological indicators. Refers to the temperature change required to produce a 1 log reduction in D value.

The z-value is considered as 10°C for the temperature range from 100 to 130°C for steam sterilization. Generally, bacteria that form spores have a z-value range of 10 to 15°C while the non-spore-forming ones have a range of 4 to 6°C.

The D-value is measured in minutes while the z-value is a temperature coefficient measured in °C.

### What is F0 value?

F0 value is the equivalent exposure time at 121.1°C to that of the actual exposure time at a variable temperature calculated with a temperature coefficient of the destruction of 10°C.

where,

Δt – Time interval between two temperature readings

T – Temperature at time t of the product under sterilization

z – Temperature coefficient (assumed as 10°C)

#### Testing F0 Value And Its Meaning

Consider sterilization hold time of 30 min. at constant 121.1°C.

Put this value in below equation

Solving above equation we will get, F0 = 30 min. (*This is an ideal condition for steam sterilization*)

But what if the temperature is increased or decreased from 121.1°C (ideal condition)?

#### Example 1 (Below Ideal Condition)

Consider sterilization hold time of 30 minutes at constant 110°C instead of 121.1°C, in a similar manner as above,

Solving above equation we will get, F0=2.33 min.

Hence, 30 minutes of sterilization at 110°C is lethally equivalent to 2.33 min. of sterilization at 121.1°C. *(Compare this sentence with the definition of F0 value for easy understanding)*

The crux of the example is,

121.1°C **→** 30 min. (Expected condition)

110°C **→** 2.33 min. (Actual condition compared to the expected condition)

Compared to the expected condition, the effective sterilization cycle time was only 2.33 instead of 30 minutes because of the lowered temperature of 110°C.

Now, let us understand “how long will it take to complete the sterilization?” for this condition. Meaning, if we maintain temperature 110°C constantly, how much hold time is required to achieve an F0 value of 30 min.

By reverse calculation, we can do this.

Let’s fix the F0 value to 30 minutes. The unknown part then would be Δt (sterile hold time).

Data we have now is F0=30 minutes and Temperature T=110°C

Putting these values in below equation

Solving above equation we will get, Δt=386 min.

**When the temperature of the system under sterilization is maintained at a constant 110°C, the time of 386 min. (i.e. Sterile Hold Time) would be required to achieve the lethal effect of 121.1°C at 30 min.**

#### Example 2 (Above Ideal Condition)

Consider sterilization hold time of 30 minutes at constant 125°C instead of 121.1°C, in a similar manner as above,

Solving above equation we will get, F0=73.64 min.

Hence, 30 minutes of sterilization at 125°C is lethally equivalent to 73.64 min. of sterilization at 121.1°C. *(Compare this sentence with the definition of F0 value for easy understanding)*

Simply put,

121.1°C **→** 30 min. (Expected condition)

125°C **→** 73.64 min. (Actual condition compared to the expected condition)

Compared to the expected condition, the effective sterilization cycle time was 73.64 instead of 30 minutes because of the elevated temperature of 125°C.

Let us understand “how long will it take to complete the sterilization?” for this condition. Meaning, if we maintain temperature 125°C constantly, how much hold time is required to achieve an F0 value of 30 minutes.

By reverse calculation, we can do this. Let’s fix the F0 value to 30 min. The unknown part then would be Δt.

Data we have now is F0=30 minutes and Temperature T=125°C

Putting these values in below equation

Solving above equation we will get Δt = 12.24 min.

**When the temperature of the system under sterilization is maintained at a constant 125°C, the time of 12.24 min (i.e. Sterile Hold Time) would be required to achieve the lethal effect of 121.1°C at 30 min.**