Sterilization 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. Whilst 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) sterilization 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 sterilization has occurred. By employing Steam Sterilization techniques a high level of sterility can be achieved and the most popular piece of equipment used in laboratories and hospitals is the steam sterilizer or autoclave.
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