Why the autoclave is best for sterilisation

Sterilisation may be defined as the statistically complete destruction of all microorganisms including the most resistant bacteria and spores.

This is a condition that is difficult to achieve and hard to prove. While there are many chemicals, inorganic and organic, that kill microorganisms they may not be totally effective and can leave undesirable or toxic residues.

Ultraviolet and Ionising radiations are also effective biocides, disrupting or modifying the DNA to prevent replication, but Ultraviolet will not produce the effective results and easy validation that moist heat (steam) sterilisation can provide. If sterility is an absolute requirement then today's scientists turn, as their predecessors did, to steam.

Microorganisms tend to become more active as the temperature of their surroundings rises, - most, but not all, die at above 80oC. In the case of Prions the temperature and time requirements for deactivation are much higher. Steam molecules condense on cooler microorganisms, and transfer 2500 joules per gram of steam, very efficiently heating the microorganisms to the temperature at which they are destroyed.

Other methods of heating suffer from the much lower heat transfer of hot dry gases and boundary layer effects, which can insulate and protect the microorganisms. For maximum effect the Steam must be saturated, and this condition, and the temperature and pressure of the steam are easily monitored, facilitating proof that sterilisation has occurred. By employing steam sterilisation techniques a high level of sterility can be achieved and the most popular piece of equipment used in laboratories and hospitals is the steam steriliser or autoclave.

With the introduction of sophisticated validation techniques to ensure compliance with UKAS and similar standards (and ultimately to ensure that sterilisation is always being achieved in the autoclaving process) , it has become increasingly important to ensure that an autoclave meets the precise needs of its particular application(s). A sterilisation cycle for media, for instance, will require different parameters from a typical discard cycle, or a cycle for wrapped instruments and fabrics. Thus, such features as load sensed process timing, vacuum, heated (or cooled) jackets, and internal fan systems have thus become increasingly important.

Astell Scientific, a UKAS accredited company, offers square andcircular-section autoclaves in capacities from 23 to 1000 litres.

All models can be supplied with a comprehensive selection of application-specific options, providing remarkable flexibility in today's quality conscious laboratory.

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Astell Scientific Limited is based in Sidcup, Kent, UK. www.astell.com

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