Food irradiation is a technology for controlling spoilage and eliminating food-borne pathogens, such as salmonella. The result is similar to conventional pasteurization and is often called "cold pasteurization" or "irradiation pasteurization." Like pasteurization, irradiation kills bacteria and other pathogens, that could otherwise result in spoilage or food poisoning. The fundamental difference between the two methods is the source of the energy they rely on to destroy the microbes. While conventional pasteurization relies on heat, irradiation relies on the energy of ionizing radiation. The FDA emphasizes that no preservation method is a substitute for safe food handling procedures
Bulk or packaged food passes through a radiation chamber on a conveyor belt. The food does not come into contact with radioactive materials, but instead passes through a radiation beam, like a large flashlight. The type of food and the specific purpose of the irradiation determine the amount of radiation, or dose, necessary to process a particular product. The speed of the belt helps control the radiation dose delivered to the food by controlling the exposure time. The actual dose is measured by dosimeters within the food containers.
Cobalt-60 is the most commonly used radionuclide for food irradiation. However, there are also large cesium-137 irradiators and the Army has also used spent fuel rods for irradiation.
The food irradiation process uses three types of ionizing radiation sources:
Cobalt-60 emits ionizing radiation in the form of intense gamma rays. "Gamma facilities" store it in stainless steel capsules (like "pencils" of cobalt), in underwater tanks. Cobalt-60 has several advantages:
- up to 95% of its emitted energy is available for use
- penetrates deeply
- yields substantial uniformity of the dose in the food product
- decays to non-radioactive nickel
- considered to pose low risk to the environment.
However, its 5.3-year half-life offers disadvantages:
- cobalt-60 "pencils" require frequent replenishment
- treatment of the food is relatively slow.
Cesium-137 is a gamma source that is also used for irradiation. Cesium-137 has a less penetrating gamma beam and a longer half-life, making it more suitable under certain circumstances.
Electron beam facilities generate e-beams with an electron beam linear accelerator. (It works on the same principle as a television tube.) The electrons are concentrated and accelerated to 99% of the speed of light and energies of up to 10 MeV.
Because e-beams are generated electrically, they offer certain advantages:
- they can be turned on only as needed
- they do not require replenishment of the source as does cobalt-60
- there is no radioactive waste
E-beam technology also has disadvantages:
- shallow depth of penetration
- e-beams must be converted to x-rays to penetrate large items such as carcasses
- high electric power consumption
- complexity, and potentially high maintenance
View the Electron Beam Linear Accelerator at the Iowa State University Research Lab Portable Electron Beam Systems
X-ray facilities use an electron beam accelerator to target electrons on a metal plate. Although some energy is absorbed, the rest is converted to X-rays. Like gamma rays, X-rays are penetrating, and can be used on food boxes 15 inches thick or more. This allows food to be processed in a shipping container.
X-rays offer the advantage of high penetration, but share the other e-beam technology disadvantages.
Gamma and X-Rays
Radiation doses vary for different foodstuffs. For the vast majority of foods, the limit is less than 10 kiloGray. The U.S. Food and Drug Administration (FDA) sets radiation dose limits for specific food types:
This table shows radiation dose limits for specific food types. This table has two columns and four rows. The first row contains column headers. The columns from left to right, are: food type and dose(kiloGrays)."
The dose limit for spices and seasons is higher, because they are consumed in very small quantities.
When ionizing radiation strikes bacteria and other microbes, its high energy breaks chemical bonds in molecules that are vital for cell growth and integrity. As a result, the microbes die, or can no longer multiply causing illness or spoilage.
Breaking chemical bonds with radiation is known as radiolysis.
Irradiation of foods must be approved by the U.S. Food and Drug Adminstration (FDA). Some applications also require U.S. Department of Agriculture (USDA) approval. USDA's Food Safety Inspection Service must approve both the process and the facility for irradiation of meat and poultry. USDA's Animal and Plant Health Inspection Service must approve irradiation for plant quarantine protection. FDA approval comes only after extensive testing demonstrates that the proposed dose of irradiation effectively eliminates the pathogen or insect of concern and does not generate toxic or carcinogenic chemicals in the food.
FDA has approved irradiation for several foodstuffs, including spices and herbs, potatoes, pork, poultry and other meats, fruits, and vegetables. It sets the maximum dose based on test results. The USDA may also set a minimum dose to assure effectiveness, for example, to assure the destruction of an insects on plants in quarantine control.
Ionizing radiation also breaks some of the chemical bonds within the food itself. The effects of chemical changes in foods are varied. Some are desirable, others are not. Examples of some food changes are:
- changes in structure of certain foods too fragile to withstand the irradiation, for example, lettuce and other leafy vegetables turn mushy
- slowed ripening and maturation in certain fruits and vegetables lengthens shelf-life
- reduction or destruction of some nutrients, such as vitamins, reduces the nutritional value (the effect is comparable to losses in heat pasteurization)
- alteration of some flavor compounds
- formation of compounds that were not originally present requires the strict control of radiation levels
- generation of free radicals, some of which recombine with other ions.
These effects are the result of radiolysis. Whether the products of radiolysis in food are all innocent from a human health perspective is still debated. However, years of experience in food irradiation has not demonstrated any identifiable health problems.
Safety testing of irradiated foods has taken place since the early 1950's. Irradiated foods have been fed to several species of animals, some up to 40 generations. Additionally, irradiated foods have been evaluated chemically.
FDA must approve any use of irradiation on food and USDA Food Safety Inspection Service must approve the process and the facility if meat and poultry products are involved. USDA's Animal and Plant Health Inspection Service approves use of irradiation for plant quarantine protection.
Several foods have been approved in the U.S. The FDA sets the maximum dose permitted on food based on what was petitioned to assure safety. The USDA sets the minimum dose on some foods to assure the desired effects such as destruction of microorganisms or effect insect quarantine control.
Assessing the safety of irradiated foods has involved investigation in the following areas:
- radiation chemistry
- general toxicology/animal testing
- nutrition of irradiated foods
- microbiology of irradiated foods
No. Food does not come in contact with radioactive material during food irradiation, and cannot be contaminated this way. Radiation that is too energetic, however, can disrupt the energy balance in the nuclei of food atoms, making them unstable (radioactive). This is known as induced radioactivity.
Electron and x-ray beams can be energetic enough to induce radioactivity. To prevent induced radioactivity, FDA limits the energy of the radiation from these sources to less than 4 mega-electron volts. Radiation from cobalt-60 sources is not energetic enough to induce radioactivity.
There are many traditional methods for preserving foods, such as drying, smoking, salt or sugar curing, and canning. These methods generally alter the flavor and chemical composition of the food. More modern methods, such as heat pasteurization and refrigeration or freezing, as well as freeze drying, are also common. Decisions about which method to use for individual foods and circumstances must weigh feasibility, effectiveness, and cost as well as the chemical changes each method causes in the food. The FDA emphasizes that no preservation method is a substitute for safe food handling procedures.
- Center for Consumer Research at the University of California-Davis
- Iowa State University Food Safety Research Project
History of Food Irradiation