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Steam Sterilization and cGMP.

Typical laboratory bench top autoclave that would require autoclave validation with the FDA, WHO & EU regulators.

Autoclave Validation / Qualification is manadatory for all machines used for biological sterilization, in the biomedical and pharmaceutical industries within the FDA, WHO & EU controlled areas. Sterilization can be accomplished by either physical or chemical means. The principal physical means is autoclaving; other physical methods include boiling and dry heat. Chemicals used for sterilization include the gases ethylene oxide and formaldehyde, and liquids such as glutaraldehyde .  Of all these sterilants, autoclaving is the fastest, most reliable and hence; most commonly used within the FDA, WHO and EU zones of influence. It must always be remembered that it is also one of the easiest processes to get wrong. This is why regulators will nearly always scrutinize and ask about validation methods used in autoclave qualification / Qualification activities.

Autoclave validation / Qualification must follow the routine validation / Qualification document string of;


Temperature and Time Relationship.

This graphic depicts a typical autoclave that autoclave validation within FDA, WHO and EU; applies to.

Autoclaving is the most effective and most efficient means of sterilization. All autoclaves must go through the GMP process of autoclave validation / Qualification during which, the various programs are verified as comforming to the requirements detailed in the User Requirement Specification (URS). They operate on a time/temperature relationship. These two variables are extremely important. Higher temperatures ensure more rapid killing. Some standard temperature/pressures employed are 115ºC/10 p.s.i., 121ºC/15 p.s.i., and 132ºC/27 p.s.i. Longer times are needed for larger loads, large volumes of liquid, and more dense materials. Autoclaving is ideal for sterilizing biohazardous waste, surgical dressings, glassware, many types of microbiologic media, liquids, and many other things.  When proper conditions and time are employed, no living organisms will survive a trip through an autoclave.

The thermal resistance of specific microorganisms is characterised by “D”–values and “Z”–values.  A D-value is the time in minutes, at a specific temperature, to reduce the surviving microbial population by 1 – log.  A Z-value is the temperature change required to result in a 1-log reduction in D-value.  Other time measurement variables pertaining to thermal resistance are F-values and Fo-values.  An Fo-value is the number of minutes to kill a specified number of microorganisms with a specified Z-value at a specific temperature.  An Fo– value is the number of minutes to kill a specified number of microorganisms with a Z-value of 10° C (50° F) at a temperature of 121.1° (250°F).

It is not unusual to find people thinking 121° C is the temperature for sterilisation.  In the early days of steam sterilisation a standard temperature was used in order that studies could be accurately compared, the temperature chosen was a nice round figure of 250deg F (121.1° C).  The Fo-value can be determined as per the following

Fo = 10 (T – 121.1)/10

Where T = temperature (° C) and Fo = equivalent sterilization time (minutes)

So given a Bioburden of 1215 CFU, with a D-value of 1.6 min/log at 121.1°C and a required SAL of 10-6.

Then: Log (1215) = 3.08

Loge reduction = 3.08 log + 6 log = 9.08 log.

Ideal Cycle at 121.1°C (250°F) = (9.08 log)(1.6 min/log) = 14.53 minutes.

Biological Indicator in Autoclave Validation / Qualification.

This graphic depicts a typical biological indicator that may be used in autoclave validation.

Moist heat sterilization (or autoclaving) is conducted by supplying dry, saturated steam under pressure to an autoclave. The energy (heat) from the condensation of steam on the items in the sterilizer will kill the present microorganisms by irreversible damage of cell components.


The effectiveness of a moist heat sterilization process increases considerably when air is removed before adding steam to the chamber. Obtaining a vacuum can be difficult, resulting in limited capability of the steam to penetrate into cavities of instruments etc. The use of biological indicators during autoclave validation / Qualification is therefore recommended for monitoring allowing the conditions at different points in the sterilized goods to be assessed.


Biological indicators include preparations of selected microorganisms (bacterial spores) with high resistance towards specific sterilization methods. The bacterial spores are deposited on a carrier, e.g. filter paper, which is wrapped in a suitable primary package, making the system ready for use and with defined resistance characteristics. The inactivation of the biological indicator indicates an effective sterilization process. Whether inactivation has been obtained is determined by cultivation after exposure.

Steam Quality In Autoclave Validation / Qualification.

In a mixture of air and steam, the presence of air will cause the temperature to be lower than expected. The total pressure of a mixture of gases is made up of the sum of the partial pressures of the components in the mixture.  This is known as Dalton's Law of Partial Pressures. The partial pressure is the pressure exerted by each component if it occupied the same volume as the mixture.

Consider a steam/air mixture made up of ¾ steam and ¼ air by volume. The total pressure is 4 bar.  Therefore the steam only has an effective pressure of 3 bar as opposed to its apparent pressure of 4 bar. The mixture would only have a temperature of 134°C rather than the expected saturation temperature of 144°C.  This could render autoclaving ineffective where a minimum temperature is essential in order to kill bacteria. It is therefore of paramount importance during the autoclave validation / Qualification task to validate that all air has been removed from the chamber

None Condensable Gasses

The Non-Condensable Gas Test demonstrates that the attainment of sterlisation conditions in all parts of a steriliser load (particularly for porous load items) is not impaired by the presence of non-condensable gases. 

The measurement of non-condensable gases is made by cooling a steam sample with an efficient condenser, using water siphoned from a tank at 200ml per minute. Minimum requirements are: one metre head and water temperature below 28 degrees centigrade. Pressurised water is not required.  When the sampled steam is condensed any non-condensable gases present are released and separated from the cooled condensate into sight glass columns.


Dryness Value test:

To ensure and to test that an acceptable amount of moisture is present in the steam supply. For little amount of moisture there is a chance of superheating may occur. Even too little moisture may prevent sterilizing conditions in the chamber. Steam with a dryness fraction of 0.99 consists of 99% steam and 1% water. Similarly, steam with a dryness fraction of 0.95 consists of 95% steam and 5% water. The dryness value of the steam should be equal to or greater than 0.9 for porous loads or 0.95 where metal loads are processed.


Superheated Steam

There are quite a few reasons why superheated steam is not as suitable for use in steam autoclaves.  In heat transfer applications, steam with a large degree of superheat is of little use because it:

a)             Gives up little heat until it has cooled to saturation temperature.

b)             Creates temperature gradients over the heat transfer surface as it cools to saturation temperature.

c)             Provides lower rates of heat transfer whilst the steam is superheated.

d)             Requires larger heat transfer areas.


This graphic depict the formulae used in the calculation of steam dryness as required for autoclave validation.

This graphic depicts the formula for the calculation of steam superheat as used in autoclave validation.

How Many Thermocouples?

this graphic depicts the formula used in autoclave validation for the calculation of heat transfer.

Positioning of the thermocouples (t/c's) during autoclave validation / Qualification or indeed in any GMP temperature mapping exercise is all about appreciating what is adding or subtracting heat from the room or cabinet being qualified.  

In the case of temperature mapping during autoclave validation / Qualification, heat is added in the form of pressurized wet steam, anything that can affect the distribution of the incoming steam, can affect uniformity of temperature. Conversely anything that can take heat away from the chamber can affect temperature uniformity.

Lets me say at this stage if you want to be pedantic and put t/c’s down the drain, the mapping exercise will probable fail. However you are there to verify that product will be sterilized, and product is never placed down the drain. Only the designated product containment area has to be verified.

If this is new installation, then get hold of the Factory Acceptance Test (FAT). In the FAT the chamber is subjected to detailed temperature transfer studies.

Even distribution of the in coming steam can be verified by placing a thermocouple sensor (t/c) in each of the eight corners in the autoclave and one in the cabinet centre. (9 t/c’s)

Cooling due to heat loss will be maximum the further away you are from the steam inlet and the closer you are to metal that will conduct heat out of the chamber. That is usually, the door, or doors if double sided.  The drain is also a heat sink that conducts heat out of the chamber.  One t/c should be placed as close to the drain as product would be, when the autoclave is in normal use and another placed alongside the cabinet product temperature probe.  This gives us an additional 2 t/c’s, bringing the total for a standard sized autoclave to 11 t/c’s.

This is normally considered sufficient for 1.5 to 2.5 m3 autoclaves. Any bigger and I would concentrate on heat loses i.e. add t/c’s to the top and bottom of the doors and or end wall.

It is most important to understand that it is impossible for autoclave validation / Qualification to be successfully executed while using none validated steam.

Your steam must be validated for – superheatdrynessnone condensable gases.
Another GMP essential is to carry out pre and post mapping, calibration of your thermocouples.  These should be calibrated against test standard instruments whose calibration is traceable to national standards, and for which you have valid current calibration certification.



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