Thursday, April 9, 2009

Physical means of achieving sterilisation

Physical means of achieving sterilisation

The principal physical methods of sterilisation are using...

  • ...heat,
    • Dry Heat
    • Autoclaves
    • Tyndallisation and Pasteurisation
  • ...irradiation or
  • ...filtration.


Throughout history, humans have used fire to purify items. The possessions of leprosy victims were burned even in Biblical times. The bacteriological loop is still sterilised this way. Burning, however, is a bit excessive for everyday usage! Heat acts by disrupting membranes and denaturing proteins and nucleic acids.

Dry heat is less effective than moist heat. Clostridium botulinum is a dangerous pathogen associated with canned food. In saturated steam, spores of this bacterium are killed in just five minutes at 121 o Celcius. It takes two hours at 160 o in a dry air oven to kill spores of this bacterium.

There are a number of ways of estimating how effective heat sterilisation can be. The thermal death time is the time microbes must be exposed to a particular temperature before they are all dead. Similarly, the thermal death point is the temperature at which all microbes in a sample are killed. Both are very unsatisfactory, since they depend on many factors. How many microbes were present in the sample? What conditions were present when the estimates were made? There are other important questions.

To overcome these problems, the D-value or Decimal Reduction Time may be used. This is defined as the time taken under specified conditions and at a particular temperature to kill 90% of the microbes in a sample. Only 10% or 1/10 of the original number of microbes survive the decimal reduction time: hence its name.

Dry Heat

Moist heat is more efficient than dry heat in killing microbes but there are a number of situations when it is preferable to use dry heat for sterilisation. It is used to sterilise delicate metal instruments that would corrode if exposed to steam. Glassware and items where steam penetration may present a problem provide further applications for dry heat sterilisation. These include water-repellent powders or petroleum jellies. Ideally these should be sterilised in batches with a large surface area to volume ratio to permit adequate heat penetration during sterilisation.

A typical dry air oven sterilisation regime would be two hours at 160 o Celcius. One hour is allowed for the contents of the oven to reach the desired temperature, and the second hour is to allow for sterilisation.

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