Irradiation is the process of exposing a material to ionizing radiation whose source is photons ( γ -rays, x rays), or high energy electrons. Commonly, γ -rays are produced by radioactive isotopes such as cobalt-60 and cesium-137. These isotopes have been approved for use in food because the γ -rays they produce have insufficient energy to induce radioactivity in foods. Conversely, electron beams are produced by electron accelerators, such as Van de Graaff generators or linear accelerators. The power limits for these machines are also regulated to no more than 10 million EV, to assure that they cannot induce radioactivity in foods. Finally, x rays are produced by the collision of high energy electrons, produced by linear accelerators, with a metal target. The amount of ionizing energy absorbed by a material is the dose, and this dose is measured in Grays (1 Gray , abbreviated Gy, equals 1 J/kg) or kiloGrays (kGy).
Irradiation destroys bacteria primarily by disrupting the chromosomal DNA in individual cells. A second mode of action is the "near-miss" theory, where a photon of energy passes very close to the DNA and forms a peroxide radical (from oxygen in the air) in the cell cytoplasm. The peroxide radical then oxidizes part of the DNA.
Purposes of Food Irradiation
The purpose of food irradiation is to improve the quality of the food being irradiated, either from a microbiological, physical, or organoleptic perspective. Very low doses (< 1 kGy) are used to prevent sprouting in potatoes or delay the ripening of fruits. These doses can also be used to disinfect foods by killing insects in grains and fruits or inactivating parasites (trichinae) in meat. Slightly higher doses (1–5 kGy) can be used to pasteurize foods. Radiation pasteurization , or radurization, significantly reduces or eliminates bacteria of public health significance in the food while also decreasing the total number of bacteria in the food. As with pasteurization by heating, this reduction in the total number of bacteria in a food also results in an increase in the shelf life of the food. Finally, very high doses, on the order of 25 kGy or greater, can be used to sterilize foods. In practice, most of the current interest and research has focused on the low doses commonly used in pasteurization.
One of the main interests in food irradiation is that it can be used as a "cold" pasteurization technique. Many food-borne disease bacteria, including Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes, are very sensitive to irradiation. Although there are some variations in sensitivity among the previously listed bacteria, a dose range of 1.5–3.0 kGy is sufficient to eliminate all these bacteria as they naturally occur in foods.
An advantage of irradiation is that it can penetrate packaging material. Foods can be prepackaged and then irradiated, eliminating the potential for contamination during packaging. In addition, irradiation can be used as a replacement for many food additives that are used to inhibit bacterial growth. Irradiation can also be used as a quarantine treatment for imported fruits and vegetables, which eliminates the need for the use of toxic fumigants.
Consumer concerns about food irradiation fall into two broad categories: the first relating to the technology and the second relating to the quality of the food. The terms "radiation" and "radioactivity" have negative connotations to many individuals. There is still lingering doubt in the mind of some consumers regarding induced radiation. The radiation sources that the U.S. Food and Drug Administration (FDA) has approved for food irradiation cannot make the food radioactive. Another consumer concern relating to the quality of the irradiated food is that of nutrient loss. Irradiation does, in fact, reduce the levels of vitamins in foods, especially the B vitamins. Thiamine is particularly sensitive to irradiation, and substantial losses of this vitamin can occur in irradiated foods at high doses. However, vitamin losses occur with many food processes, including cooking and canning. There is no question that an irradiated food which is cooked will have a slightly lower vitamin content than an identical food which has not been irradiated prior to cooking. However, it is not anticipated that every food will be irradiated, and even if an individual specifically limited his or her consumption to irradiated products, there would still be sufficient vitamins in that individual's diet. The American Dietetics Association (ADA) has reviewed the nutritional changes caused by food irradiation and is on record as supporting the technology.
The ultimate consumer concern with any new food process is, "Is the food safe to eat?" A joint committee of the Food and Agriculture Organization (FAO), the International Atomic Energy Agency, and the World Health Organization (WHO) evaluated all available studies on the wholesomeness of irradiated food. The final report concluded that the irradiation of any food commodity causes no toxicological hazard and hence toxicological testing of foods treated in this manner is no longer required. Many toxicological studies have been conducted over the years, but one of the most comprehensive was a multigenerational study conducted in the United States during the 1970s, in which five different species of animals and insects were fed irradiation-sterilized (58 kGy) chicken as a major component in their diet. The results of these studies, including all the original histology slides, were completely reevaluated by the FDA during its consideration of the petition to irradiate poultry. After more than two years of evaluation, the FDA concurred with the conclusions of the original study and issued regulations on poultry irradiation. In addition, during this same time period the American Medical Association (AMA) reviewed the toxicological data; it has publicly supported food irradiation.
Brynjolfsson, Ari (1985). "Wholesomeness of Irradiated Foods: A Review." Journal of Food Safety 7:107–126.
Tauxe, Robert V. (2001). "Food Safety and Irradiation: Protecting the Public from Foodborne Infections." Emerging Infectious Disease 7(3, Supp.): 516–521. Also available from http://www.cdc.gov/ncidod/eid/vol7no3_supp/tauxeG.htm .
World Health Organization (1980). Wholesomeness of Irradiated Food. (Summary of the final draft of a joint FAO/IAEA/WHO expert committee.) Geneva: World Health Organization.
World Health Organization (1994). Safety and Nutritional Adequacy of Irradiated Food. Geneva: World Health Organization.