Food Irradiation

Irradiation of food has the potential to decrease the incidence of foodborne disease and makes possible the replacement of toxic and environmentally harmful chemical fumigants such as methylbromide, ethylene oxide, and propylene oxide. Irradiation can also increase the shelf life of certain food items and decrease losses from spoilage and pests. Decreasing losses is important in the context of global storage of food supplies. Although it remains controversial, food irradiation is widely supported by various international and national medical, scientific, and public health organizations, as well as groups involved with food processing and food services. Many countries have started to irradiate food, including France, the Netherlands, Portugal, Israel, Thailand, Russia, China, and South Africa. However, in the United States, only 10% of herbs and spices and less than 0.002% of fruits, vegetables, meats, and poultry are currently irradiated (18,68-70).

The technology of food irradiation involves use of high-energy radiation in any of three approved forms: gamma rays, electron beams, or x-rays. Gamma rays can be generated by either of two approved radionuclide sources, cobalt-60 or cesium-137, which give off high-energy photons, called gamma rays, that can penetrate foods to a depth of several feet. The radioactive substances emit gamma rays all the time, and massive concrete walls are needed to contain them. Foods to be irradiated are brought into a chamber on conveyor systems and are exposed to the rays for a defined time period. Although some fear that foods become radioactive, since gamma irradiation does not emit neutrons, foods are not made radioactive by the procedure (71,72).

Electron beam (e-beam) technology uses a stream of high-energy electrons propelled from an electron gun. No radioactivity is involved, but shielding is needed to protect workers from the electron beam (72).

The newest technology is x-ray irradiation, an outgrowth of e-beam technology, and is still being developed. The x-ray machine is a more powerful version of the machines used in many hospitals to take radiographs. To produce the x-rays, a beam of electrons is directed at a thin plate of gold, producing a stream of x-rays coming out on the other side. Like gamma rays, x-rays can pass through thick foods and require shielding for worker safety. Four commercial x-ray units have been built in the world since 1996 (73).

The absorption of gamma rays, x-ray photons, or electrons produces ion-ization. Water is the principal target for the radiation since it is the largest component of most foods and microorganisms. Normally, approximately 70% of the radiation-induced ionization occurs in cellular water, and the target organisms are inactivated because of secondary reactions, not because of a direct effect on bacterial DNA. However, others have proposed that DNA damage is the mechanism by which irradiation acts (68,74-76).

Radiation doses used in the irradiation process are measured in units of grays (Gy) or kilograys (kGy), with 1 Gy equal to 100 rads. Doses can be divided into three groups: low dose (less than 1 kGy); pasteurizing dose (1 to 10 kGy) used for pasteurization of meats, poultry, and other foods; and high dose (more than 10 kGy) for sterilization or for reduction of the number of microbes in spices. Some bacterial spores may be more resistant to irradiation than vegetative cells and require doses substantially higher than those used in pasteurization. In general, inactivation of viruses also requires higher doses of radiation than doses used to sterilize pests in plants or for pasteurization (18,77-79).

In the United States, the Nuclear Regulatory Commission (NRC) regulates facilities that utilize radioactive sources. To be licensed, the facility must have been designed with multiple fail-safe measures, and must establish extensive and well-documented safety procedures and worker training. The occupational risk in working in areas where food irradiation takes place is minimal if safe work practice guidelines are followed. Outside the United States, a small number of fatal incidents have been documented in which a worker bypassed multiple safety steps to enter the chamber while the radioactive source was exposed, resulting in a severe or even lethal radiation injury (73).

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