Thursday, April 9, 2009

Tyndallisation and Pasteurisation

Tyndallisation and Pasteurisation

Autoclaving may be too harsh a process to sterilise certain of the growth media used in diagnostic microbiology. To overcome this, Tyndallisation may be used. The medium is boiled on the first day, and held for ten minutes and then is allowed to cool. This kills the vegetative microbes in the medium but permits spores to survive and then to germinate. On the second day of the process the medium is once again heated for ten minutes, killing any microbes that have germinated. The process is completed with a second overnight incubation and heating.

The gastronomic qualities of food may be destroyed if it is held for too long at a high temperature. Remember what happens to the soft boiled egg in which you want to dip your soldiers! Yet certain foods may be the vector for life-threatening infections. To make milk safe for consumption Pasteurisation is used.

The older method was to LTH - low temperature holding. This involved holding milk at 62.8 o Celcius for at least thirty minutes. This was found to be inadequate to kill Coxiella burnetii the cause of Q-fever. Now HTST pasteurisation - High Temperature Short Time - is used. Milk is exposed to a temperature of 71.7 o Celcius for 15 seconds.


Ultraviolet light

Ultraviolet light at 260 nm causes the formation of pyrimidine dimers in DNA. This leads to genetic damage to cells and their ultimate death. Ultraviolet irradiation is an effective method of sterilising work surfaces and air but it does not penetrate glass and thus is used for sterilising surfaces.


X-rays are too unpredictable to be of use in the routine sterilisation of microbes, despite their efficient germicidal properties.


Gamma-irradiation can be used in a process referred to as 'cold sterilisation'. Gamma rays can penetrate objects with reasonable efficiency. It has a controversial use in the decontamination of certain foods. Gamma irradiation impairs the flavour of certain foods. This, coupled with public resistance to the practice, probably means that it will not become widespread in the near future.


Heat-labile solutions may be sterilised by filtration. Formerly, earthenware candles, sintered glass filter and asbestos pads were used. Today nitrocellulose filters have replaced the older filtration methods. When nitrocellulose filters were first introduced the size of pores was variable and they probably worked as much by adsorption of microbes as by filtration. Now, average pore size is much better controlled.

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