Irradiation of food
What is food irradiation?
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 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:
Which foodborne diseases could be prevented with irradiation?
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.
What is the actual process of irradiation?
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.
How does irradiation affect foods?
The foods are not changed in nutritional value and they are not made dangerous as a result of the irradiation. The high energy ray is absorbed as it passes through food, and gives up its energy. The food is slightly warmed. Some treated foods may taste slightly different, just as pasteurized milk tastes slightly different from unpasteurized milk. If the food still has living cells, (such as seeds, or shellfish, or potatoes) they will be damaged or killed just as microbes are. This can be a useful effect. For example, it can be used to prolong the shelf life of potatoes by keeping them from sprouting. The energy can induce a few other changes. At levels approved for use on foods, levels of the vitamin thiamine are slightly reduced. This reduction is not enough to result in vitamin deficiency. There are no other significant changes in the amino acid, fatty acid, or vitamin content of food. In fact, the changes induced by irradiation are so minimal that it is not easy to determine whether or not a food has been irradiated.
Irradiated foods need to be stored, handled and cooked in the same way as unirradiated foods. They could still become contaminated with germs during processing after irradiation, if the rules of basic food safety are not followed. Because the irradiated foods have fewer microbes of all sorts, including those that cause spoilage, they may have a longer shelf life before spoiling.
The safety of irradiated foods has been studied by feeding them to animals and to people. These extensive studies include animal feeding studies lasting for several generations in several different species, including mice, rats, and dogs. There is no evidence of adverse health effects in these well-controlled trials.
How does irradiation affect disease-causing microbes?
When microbes present in the food are irradiated, the energy from the rays is transferred to the water and other molecules in the microbe. The energy creates transient reactive chemicals that damage the DNA in the microbe, causing defects in the genetic instructions. Unless it can repair this damage, the microbe will die when it grows and tries to duplicate itself. Disease-causing organisms differ in their sensitivity to irradiation, depending on the size of their DNA, the rate at which they can repair damaged DNA, and other factors. It matters if the food is frozen or fresh, as it takes a higher dose to kill microbes in frozen foods.
The size of the DNA "target" in the organism is a major factor. Parasites and insect pests, which have large amounts of DNA, are rapidly killed by extremely low doses of irradiation, with D-values of 0.1 kiloGray or less. It takes more irradiation to kill bacteria, because they have a somewhat smaller DNA, with D-values in the range of 0.3 to 0.7 kiloGray. Some bacteria can form dense hardy spores, which means they enter a compact and inert hibernation state. It takes more irradiation to kill a bacterial spore, with D-values on the order of 2.8 kiloGray. Viruses are the smallest pathogens with that have nucleic acid, and they are in general resistant to irradiation at doses approved for foods. This means that they may have D-values of 10 kG or higher. The prion particles associated with bovine spongiform encephalopathy (BSE, also known as mad cow disease) do not have nucleic acid at all, and so they are not inactivated by irradiation, except at extremely high doses. This means that irradiation will work very well to eliminate parasites and bacteria from food, but will not work to eliminate viruses or prions from food.
Which foods can be irradiated?
At low doses, irradiation could be used on a wide variety of foods to eliminate insect pests, as a replacement for fumigation with toxic chemicals that is routine for many foods now. It can also inhibit the growth of molds, inhibit sprouting, and prolong the shelf life. At higher doses, irradiation could be used on a variety of different foods to eliminate parasites and bacteria that cause foodborne disease. Many foods can be irradiated effectively, including meat, poultry, grains, and many seafoods, fruits and vegetables. It is likely to have greatest application for raw foods of animal origin that are made by mixing materials from many animals together, such as ground meat or sausage.
However, not all foods are suitable for irradiation. For example, oysters and other raw shellfish can be irradiated, but the shelf life and quality decreases markedly because the live oyster inside the shell is also damaged or killed by the irradiation. Shell eggs can sometimes be contaminated on the insides with Salmonella. However, irradiation causes the egg whites to become milky and more liquid, which means it looks like an older egg, and may not serve as well in some recipes. Alfalfa seeds used in making alfalfa sprouts can sometimes be contaminated with Salmonella.
Using irradiation to eliminate Salmonella from the seeds may require a dose of irradiation that also interferes with the viability of the seeds themselves. Combining irradiation with other strategies to reduce contamination with germs may overcome these limitations.
How can I tell if the food has been irradiated?
A distinctive logo has been developed for use on food packaging, in order to identify the product as irradiated. This symbol is called the "radura" and is used internationally to mean that the food in the package has been irradiated. A written description may also be present, such as "Irradiated to destroy harmful microbes". It is not required to label a food if a minor ingredient of the food, such as a spice, has been irradiated itself.
Are consumers ready to buy irradiated foods?
Many consumers are quite willing to buy irradiated foods. This is particularly true if the purpose of the irradiation is clearly indicated. Consumers are interested in a process that eliminates harmful microbes from the food and reduce the risk of foodborne disease. In test marketing of specific irradiated foods, consumers have shown that they are willing to buy them. Typically at least half will buy the irradiated food, if given a choice between irradiated product and the same product non-irradiated. If consumers are first educated about what irradiation is and why it is done, approximately 80% will buy the product in these marketing tests.
Would irradiation replace other foodborne disease prevention efforts?
Irradiation is not a short cut that means food hygiene efforts can be relaxed. Many steps need to be taken from farm to table to make sure that our food supply is clean and safe. Irradiation is a major step forward, but it does not replace other important efforts, including efforts to improve sanitation on the farm and in the food processing plant. For irradiation to be effective, the food that is to be irradiated already needs to be clean. The more initial contamination there is, the higher dose of irradiation it would take to eliminate possible pathogens, and the greater the change in the taste and quality of the food. The protection of irradiation will be overcome if the contamination levels are too high.
The same is true for pasteurized milk. To be pasteurized, milk must be produced in regulated dairy farms, and must be of Grade A quality. Milk that is less than Grade A is not pasteurized for direct sale as milk. Thus, irradiation of food is an important additional step for added safety in the whole farm-to-table continuum of food safety measures.
Is irradiation of food just like pasteurization of milk?
Irradiation has the potential to be used like milk pasteurization in the future. We have confidence in the safety of pasteurized milk for several reasons. The milk is graded and tested to make sure that the milk is clean enough to pasteurize in the first place. Careful industry standards and regulations monitor the effectiveness of the pasteurization process. The pasteurization occurs just before the milk goes into the carton, so the chance of re-contamination after pasteurization is nearly zero. Similar strategies and designs can make food irradiation as effective as milk pasteurization.
Currently, pasteurization is applied to foods (like milk) that already meet a defined cleanliness standard, and is applied at a dose that gives a standard defined effect. As the irradiation of food becomes commercialized for various foods, similar standardization will be required.
What radioactive waste is generated?
Cobalt 60 is manufactured in a commercial nuclear reactor, by exposing non-radioactive cobalt to intense radiation in the reactor core. Cesium 137 is a by-product of the manufacture of weapons-grade radioactive substances. Thus the supply of these two substances, like that of other radioactive materials used in medicine, science and industry, is dependent on the nuclear industry.
The food irradiation facilities themselves do not become radioactive, and do not create radioactive waste. The cobalt sources used in irradiation facilities decay by 50% in five years, and therefore require periodic replacement. The small radioactive cobalt "pencils" are shipped back to the original nuclear reactor, where they can be recharged for further use. The shipment occurs in special hardened steel canisters that have been designed and tested to survive crashes without breaking. Cobalt is a solid metal, and even if somehow something should break, it will not spread through the environment. Cobalt 60 may also be disposed of as a radioactive waste. Given its relatively short half life (5 years) and its stable metallic form, the material is not considered to be a problematic waste.
In contrast to metallic cobalt, cesium is a salt, which means it can dissolve in water. Cesium 137 sources decay by 50% in 31 years, and therefore are not often replaced. When they are replaced, the old cesium sources will be sent to a storage site in the same special transport canisters. If a leak should occur, there is the possibility that the cesium salts could dissolve in water and thus spread into the environment. This happened at a medical sterilizer facility in Decatur, Georgia in 1992, when a steel container holding the cesium cracked, and some cesium leaked into the shielding water tank.
What about the effect of irradiation on food packaging materials?
The food to be irradiated will often already be in its final package. This raises the question about whether the irradiation has any effect on the packaging that might be transferred to the foods. Many modern packaging materials have simply not been tested. Testing and approving a wider array of packaging materials is critical to the successful commercialization of irradiated foods.