Sterilisation by irradiation

Certain types of irradiation are used to control the growth of microorganisms. These include both ionising and non-ionising radiation.

The most widely used form of non-ionising radiation is ultraviolet (UV) light. Wavelengths around 260 nm are used because these are absorbed by the purine and pyrimidine components of nucleic acids, as well as certain aromatic amino acids in proteins. The absorbed energy causes a rupture of the chemical bonds, so that normal cellular function is impaired. You will recall from Chapter 11 that UV light causes the formation of thymine dimers (Figure 11.21), where adjacent thymine nucleotides on the same strand are linked together, inhibiting DNA replication. Although many bacteria are capable of repairing this damage by enzyme-mediated photoreac-tivation, viruses are much more susceptible. UV lamps are commonly found in food preparation areas, operating theatres and specialist areas such as tissue culture facilities, where it is important to prevent contamination. Because they are also harmful to humans (particularly the skin and eyes), UV lamps can only be operated in such areas when people are not present. UV radiation has very poor penetrating powers; a thin layer of glass, paper or fabric is able to impede the passage of

The world's biggest UV wastewater treatment plant was opened in Manukau, New Zealand in 2001. Its 8000 UV lamps are able to treat up to 50000 cubic metres of water per hour.

the rays. The chief application is therefore in the sterilisation of work surfaces and the surrounding air, although it is increasingly finding an application in the treatment of water supplies.

Ionising radiations have a shorter wavelength and much higher energy, giving them greater penetrating powers. The effect of ionising radiations is due to the production of highly reactive free radicals, which disrupt the structure of macromolecules such as DNA and proteins. Surgical supplies such as syringes, catheters and rubber gloves are commonly sterilised employing gamma (y) rays from the isotope cobalt 60 (60Co).

Gamma radiation has been approved for use in over 40 countries for the preservation of food, which it does not only by killing pathogens and spoilage organisms but also by inhibiting processes that lead to sprouting and ripening. The practice has aroused a lot of controversy, largely due to concerns about health and safety, although the first patent applications for its use date back nearly a hundred years! Although the irradiated product does not become radioactive, there is a general suspicion on the part of the public about anything to do with radiation, which has led to its use on food being only very gradually accepted by consumers. In this respect Europe lags behind the USA, where during the 1990s a positive attitude towards irradiation of food both by professional bodies and the media has led to a more widespread acceptance of the technology. Gamma radiation is used in situations where heat sterilisation would be inappropriate, because of undesirable effects on the texture, taste or appearance of the product. This mainly relates to fresh produce such as meat, poultry, fruit and vegetables. Irradiation is not suitable for some foodstuffs, such as those with a high fat content, where unpleasant tastes and odours result. Ionising radiations have the great advantage over other methods of sterilisation that they can penetrate packaging.

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