Fh value Calculation in Dry heat sterilization

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This article contains the Fh value calculation, Difference between F0 and Fh value, What is mean by Dry heat sterilization and Depyrogenation.

What is Mean by Fh value and its Calculation :

There are different types of sterilization process i.e. Moist Heat sterilization (Steam sterilization), Dry Heat Sterilization, Ethylene Oxide Sterilization, Gamma rays sterilization and Depyrogenation, etc.

Adopting right method for sterilization depends on the material being sterilized and the purpose of sterilization.

Steam sterilization is done under high temperature and high pressure in moist conditions. Hence it takes less time to sterilize.

Whereas, dry heat sterilization is done under high temperature and in dry condition, hence it takes more time to sterilize or remove/Reduction of all types of microorganisms.

Most of these techniques carry their own effectiveness values.

In F0 value article, we established an exposure equivalency for steam sterilization (compared the ideal and actual exposure times).thus, the F0 value is called Equivalent Exposure Time.

Here, we will discuss about Fh value calculator which is in analogy with the above. But in terms of dry heat, the Fh value is called Heat Penetration Factor.

Though the F0, Fh, Fd values are similar in structure, their application is different.

Simply put:

Value Values used in
F0 Moist Heat Sterilization/Steam Sterilization
Fh Dry Heat Sterilization
Fd Depyrogenation (discussed further…)

Logically speaking, sterilization is successful when the product contact parts are free of viable microorganisms.

Now coming to the z-value. if you know, We assumed it as 10°C for the F0 value in Moist heat Sterilization or steam sterilization when the exact microorganism was unknown.

Similarly, in the Fh concept, the z-value should be assumed as 20°C.

When it comes to moisture-sensitive items, steam sterilization is not suitable and may damage the accessories or affect on drug products/Drug substances.

In such cases, Dry Heat Sterilization is the more appropriate option given that the materials withstand higher temperatures than steam sterilization conditions.

Formula For Fh Value calculation :

The following formula is used to calculate the Fh value in dry heat sterilization.

Fh Value Calculator
Formula: Fh Value (Heat Penetration Factor)

Where,
Δt – Sterilization Hold Time in Minutes
T – Average Temperature of all probes during a sterile hold in °C
z – Temperature Coefficient in °C

The term next to Δt in the above equation is lethality rate.

For steam sterilization, the term right to “T” was 121.1°C whereas, in dry heat sterilization, it is 170°C and the z-value should be assumed as 20°C for dry heat Sterilization.

 

      Difference Between F0 and Fh Value

F0 Value Fh Value
It is equivalent Exposure Time It is heat Penetration factor
Assess the effectiveness of Steam Sterilization Assess the effectiveness of Dry Heat Sterilization
The assumed z-value is 10°C for sterilization range of 100 to 130°C The assumed z-value is 20°C for sterilization range of 160 to 200°C
Theoretical Requirement
121.1°C at 30 min. of sterile hold time		 Theoretical Requirement
170°C at 32 min. of sterile hold time
Mitigates micro-organisms, especially living endospores Removes bacterial endotoxins

What is mean by Dry Heat Sterilization (DHS) ?

Working Principle :

Dry heat sterilization uses conduction as a mode of heat transfer. The heat is absorbed by the outside surface of an item, then passes towards its inside center. It will eventually reach the temperature required for sterilization.

In other words, the dry heat oxidizes the cell contents and thereby kills the organisms.

A blend of temperature and time is required to kill microorganisms by heat. In general, an increase in temperature will result in a decreased time for bacteria-kill.

Items that can withstand high temperatures can be sterilized with Dry Heat sterilization like anhydrous (moisture-less).

Uses of DHS:

Dry heat sterilization is particularly suitable for materials that can tolerate high temperatures and are damaged by moisture or high pressure.

Since dry heat does not cause corrosion of metal instruments and glassware, it is appropriate for sterilizing them, but it can’t be used to sterilize fabrics, plastics, or rubber.

Lastly, DHS is for materials that must remain dry. For example, glass bottles that contain pharmaceutical powders.

There are two types of dry heat sterilization:

  1. Natural Convection

  2. Forced Convection

Differences of these two systems as Tabulated Below :

Natural Air Dry heat sterilization (DHS) Forced Air Dry heat sterilization (DHS)
Oven-type coils transfer the heat from the bottom of the chamber by gravity convection. Blower-driven system circulating hot air across the chamber at high velocity.
More of a traditional setup. An advanced setup.
Requires more time to attain the sterilization set temperatures. The rate of convection is high due to the use of a blower. Hence achieves the sterilization set temperature in a quick time.
Inconsistent temperature distribution inside the chamber. Uniform temperature distribution across the chamber.

What is mean by Depyrogenation ?

The process that removes or inactivates the pyrogens (disables their multiplication) is called depyrogenation.

In aseptic filling, depyrogenation is mainly utilized for sterilizing vials. It can also be used for sterilizing assembled and packaged materials, as heat conduction is not dependent on contact with steam or water.

In Depyrogenation case, the temperature is even more than DHS and the Fh value is then replaced by the Fd value.

Fd value is a function to measure the effect of the depyrogenation. If we say Fd value to be 3, the effect achieved is equivalent to the exposure of 250°C at 3 mins.

Meaning, as F0 value is for Steam Sterilization, Fd value is for Depyrogenation.

The theoretical requirement for Depyrogenation is 250°C at 30 min. with the z-value assumed as 46.4°C . Though this is an ideal condition, a successful depyrogenation would also establish when the system achieves ≥ 3 log reduction.

Whereas, Dry Heat Sterilization occurs at 170°C for 32 min.

Parameter Comparison for Dry Heat sterilization and Depyrogenation

Parameter Dry Heat Sterilization Depyrogenation
Value Fh value Fd value
Formula difference T -(minus) 170°C T-250°C
z-Value z = 20°C z = 46.4°C

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