Food irradiation: General information & principles

Food irradiation is a promising new food safety technology that can eliminate disease-causing germs from foods. Like pasteurization of milk, and pressure cooking of canned foods, treating food with ionizing radiation can kill bacteria and parasites that would otherwise cause foodborne disease.


Similar technology is used to sterilize medical devices so they can be used in surgery or implanted without risk of infection. The food that NASA astronauts eat has been sterilized by irradiation to avoid getting foodborne illness in space. The effects of irradiation on the food and on animals and people eating irradiated food have been studied extensively. These studies show clearly that when irradiation is used as approved on foods:

• disease-causing germs are reduced or eliminated

• the food does not become radioactive

• dangerous substances do not appear in the foods

• the nutritional value of the food is essentially unchanged

Irradiation is a safe and effective technology that can prevent many foodborne diseases.

Treating raw meat and poultry with irradiation at the slaughter plant could eliminate bacteria commonly found raw meat and raw poultry, such as E. coli O157:H7, Salmonella, and Campylobacter. These organisms currently cause millions of infections and thousands of hospitalizations in the United States every year. Raw meat irradiation could also eliminate Toxoplasma organisms, which can be responsible for severe eye and congenital infections. Irradiating prepared ready-to-eat meats like hot dogs and deli meats, could eliminate the risk of Listeria from such foods. Irradiation could also eliminate bacteria like Shigella and Salmonella from fresh produce. The potential benefit is also great for those dry foods that might be stored for long times and transported over great distances, such as spices and grains. Animal feeds are often contaminated with bacteria like Salmonella. Irradiation of animal feeds could prevent the spread of Salmonella and other pathogens to livestock through feeds.

Three different irradiation technologies exist, that use three different kinds of rays: gamma rays, electron beams and x-rays.

The first technology uses the radiation given off by a radioactive substance. This can be either a radioactive form of the element cobalt (Cobalt 60) or of the element cesium (Cesium 137). These substances give off high energy photons, called gamma rays, which can penetrate foods to a depth of several feet. These particular substances do not give off neutrons, which means they do not make anything around them radioactive. This technology has been used routinely for more than thirty years to sterilize medical, dental and household products, and it is also used for radiation treatment of cancer. Radioactive substances emit gamma rays all the time. When not in use, the radioactive "source" is stored down in a pool of water which absorbs the radiation harmlessly and completely. To irradiate food or some other product, the source is pulled up out of the water into a chamber with massive concrete walls that keep any rays from escaping. Medical products or foods to be irradiated are brought into the chamber, and are exposed to the rays for a defined period of time. After it is used, the source is returned to the water tank.

Electron beams, or e-beams, are produced in a different way. The e-beam is a stream of high energy electrons, propelled out of an electron gun. This electron gun apparatus is a larger version of the device in the back of a TV tube that propels electrons into the TV screen at the front of the tube, making it light up. This electron beam generator can be simply switched on or off. No radioactivity is involved. Some shielding is necessary to protect workers from the electron beam, but not the massive concrete walls required to stop gamma rays. The electrons can penetrate food only to a depth of three centimeters, or a little over an inch, so the food to be treated must be no thicker than that to be treated all the way through. Two opposing beams can treat food that is twice as thick. E-beam medical sterilizers have been in use for at least fifteen years.

The newest technology is X-ray irradiation. This is 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 and dental offices to take X-ray pictures. To produce the X-rays, a beam of electrons is directed at a thin plate of gold or other metal, producing a stream of X-rays coming out the other side. Like cobalt gamma rays, X-rays can pass through thick foods, and require heavy shielding for safety. However, like e-beams, the machine can be switched on and off, and no radioactive substances are involved. Four commercial X-ray irradiation units have been built in the world since 1996.

The dose of irradiation is usually measured in a unit called the Gray, abbreviated Gy. This is a measure of the amount of energy transferred to food, microbe or other substance being irradiated. 10 kiloGrays, or 10,000 Grays, is the same as an older measure, the megaRad. A single chest X-ray has a dose of roughly a half of a milliGray (a thousandth of a Gray). To kill Salmonella., fresh chicken can be irradiated at up to 4.5 kiloGrays, which is about 7 million times more irradiation than a single chest X-ray. To measure the amount of irradiation something is exposed to, photographic film is exposed to the irradiation at the same time. The film fogs at a rate that is proportional to the irradiation level.

The killing effect of irradiation on microbes is measured in D-values. One D-value is the amount of irradiation needed to kill 90% of that organism. For example, it takes 0.3 kiloGrays to kill 90% of E. coli O157, so the D-value of E. coli is 0.3 kGy. These numbers can be added exponentially. It takes two D (or 0.6 kGy in the case of E. coli) to kill 99% of the organisms present, 3 D (or 0.9 kGy) to kill 99.9% and so on. Thus, once you know the D-value for an organism, and how many organisms might possibly be present in a food, the technician can estimate how much irradiation it will take to kill all of them. For example, if you think that a thousand E. coli O157 could be present in a food, then you want to be able to treat with at least 4 D, or 4 x 0.3 kGy, or 1.2 kGy. The D-values are different for each organism, and need to be measured for each organism. They can even vary by temperature, and by the specific food.

The energy of e-beams and of x-rays is measured in the amount of energy developed by the electron gun, and is measured in electron volts (eV). The usual apparatus runs at 5 to 10 million electron volts (MeV).



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