Meals that have been bombarded with the radiation are served to transplant patients – and patients in intensive care units – in many hospitals. Some countries irradiate supermarket foods. Similar food is also eaten by American and Russian astronauts during their missions in space.
Food irradiation is designed to kill bacteria that cause food poisoning – a particularly grave threat to weak hospital patients, or astronauts who are cut off from medical help. Irradiation also helps to prevent food from going bad while it is being stored. Although at the low recommended doses it does not give indefinite preservation.
The process has been known since 1921 when an American scientist discovered that X-rays could kill a parasite, Trichinella spiralis, which can contaminate pork.
Irradiation now is done by exposing food on a conveyor belt to the radio active isotopes caesium 137 or cobalt 60 in the lead shielded chamber with walls 5 feet (1.5m) thick. The isotopes give off electromagnetic ionising radiation in the form of gamma rays. Alternative methods used X-rays or beta rays, both forms of radiation.
Doses of radiation
The effect of radiation is measured in units known as Grays (Gy). Doses less then 1kGy are used to kill parasites in meat. Low doses are also used to kill or sterilise insects in cereals, cocoa beans and other groups. They prevents stored crops such as potatoes and onions from sprouting, and slow down the ripening of some fruits.
Medium doses from 1-10kGy – can extend the shelf life of food by reducing spoiling organisms in meat, fish, fruit, vegetables and spices. They also killed food poisoning bacteria such as salmonella, in raw poultry and shellfish. About half the poultry sold contains live in salmonella, which are usually killed by cooking. However, some of the bacteria can survive if the poultry is not cooked right through.
For complete sterilisation of foods, high doses of above 10kGy are used. Animals on sterile diets regularly eat food irradiated up to 25kGy with no observed ill effects. Sterile diets are used when scientists are seeking side effects in various trials – and want to eliminate food poisoning.
Food irradiation is governed by international agreement through United Nations agencies.
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