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Describe irradiation.

Food Irradiation:  The Treatment of Foods with Ionizing Radiation

Kim M. Morehouse, Ph.D., Research Chemist
U.S. Food and Drug Administration
Center for Food Safety and Applied Nutrition
Office of Premarket Approval

{As published in Food Testing & Analysis, June/July 1998 edition (Vol. 4, No. 3, Pages 9, 32, 35)}

Food irradiation is a means of food preservation that has been in development since the early part of the twentieth century. If applied properly, irradiation can be an effective way to reduce the incidence of foodborne disease and treat a variety of potential problems in our food supply. It is capable of improving the safety and quality of many foods, and extending their shelf-life. Irradiation, however, is not a cure-all process. It is not necessarily suitable for every food, and it cannot reverse spoilage that has already occurred.

Foods are treated with ionizing radiation to accomplish many different goals. This food processing technology can improve the safety of food through the reduction of pathogenic bacteria and other microorganisms and parasites that cause foodborne diseases. Irradiation also inactivates food spoilage organisms, including bacteria, molds, and yeasts. It can be effective in lengthening the shelf-life of fresh fruits and vegetables by decreasing the normal biological changes associated with growth and maturation processes, such as ripening or sprouting.

During the irradiation process, food is exposed to the ionizing energy source in such a way that a precise and specific dose is absorbed. In food irradiation, the dose is the amount of radiation absorbed by the food. It is not the same as the level of energy transmitted from the radiation source. The absorbed dose is controlled by the intensity of the radiation source and the length of time the food is exposed. The SI unit used to describe the dose, or amount of absorbed radiation, is the gray (Gy). One gray is defined as one joule of absorbed energy per kilogram.

In practice, the dose used varies according to the type of food and the desired effect. Treatment levels can be grouped into three general categories:

  • "Low" dose, up to 1 kGy, is used to delay physiological processes, such as, ripening or sprouting of fresh fruits and vegetables, and to control insects and parasites in foods.
  • "Medium" dose, 1-10 kGy, is used to reduce spoilage and pathogenic microorganisms on various foods, to improve technological properties of food, such as reduce cooking times for dehydrated vegetables, and to extend the shelf-life of many foods.
  • "High" dose, greater than 10 kGy, is used for the sterilization of meat, poultry, seafood, and other prepared foods in combination with mild heating to inactivate enzymes, and for the disinfection of certain foods or ingredients, such as spices and enzyme preparations.

The amount of radiation listed for these categories is approximate, and in some cases, a lower or higher dose may be suitable to achieve the desired effect. The foods that have current approval by the U.S. Food and Drug Administration (FDA) to be treated with ionizing radiation are provided in Table I.

Table I. Foods Permitted to be Irradiated Under FDA's Regulations

Food Purpose Dose
Fresh pork Control Trichinella spiralis 0.3 kGy min. to 1 kGy max.
Fresh foods Growth and maturation inhibition 1 kGy max.
Foods Arthropod disinfection 1 kGy max.
Dry Enzyme preparations Microbial disinfection 10 kGy max.
Dry spices/seasonings Microbial disinfection 30 kGy max.
Poultry Pathogen control 3 kGy max.
Frozen meats (NASA) Sterilization 44 kGy min.
Refrigerated meat Pathogen control 4.5 kGy max.
Frozen meat Pathogen control 7 kGy max.

Two types of radiation sources are used: machine or radionuclide. Machine sources of ionizing radiation include electron accelerators and X-ray generators. Radionuclides, radioactive materials that give off ionizing gamma-rays, include cobalt-60 and caesium-137. Irradiated food does not become radioactive. The radiation energies used in food processing cause chemical changes in the food, but not the nuclear changes that would make the food radioactive. The types of ionizing radiation used in food irradiation have been specifically chosen to ensure that there will not be an increase in radioactivity, above that which is present in the food naturally.

The energy used in this process is called ionizing radiation, because it is capable of causing the breakage of chemical bonds which leads to the formation of smaller fragments that are either electrically charged (ions) or neutral (free radicals). These ions and radicals are direct radiolysis products that react with food constituents to form stable products, or indirect radiolysis products. Whereas many foods consist mainly of water, the radiation is typically absorbed by the water forming water radiolysis products (direct radiolysis products), which then react with other constituents in food to form stable products. The major constituents of food include: lipids, proteins, and carbohydrates. Minor components of food that are also of significance are vitamins and DNA. The chemical changes in the food during treatment with ionizing radiation results in chemical modification of extremely small amounts of these components. Heat treatment, such as cooking, also causes chemical changes in the food. In fact, the chemical changes that occur in food by irradiation are several orders of magnitude less than heat treatment for a comparable effect. More than 40 years of research suggests that the chemical by-products of radiation are mostly the same as by-products of conventional cooking or other preservation methods.

Many national and international committees, organizations and regulatory agencies have reviewed the safety of irradiated foods. These include the World Health Organization (WHO), the Food and Agricultural Organization of the United Nations (FAO), the Codex Alimentarius Commission, and the U.S. FDA. These organizations have all concluded that food irradiation is safe when Good Manufacturing Practices (GMPs) and Good Irradiation Practices are used. For the evaluation of safety, three main areas of concern were addressed: potential toxicity, nutritional adequacy, and potential microbiological risk.

The safety of irradiated foods has been extensively studied. Research has established that the types and amounts of products generated by radiation-induced chemical reactions (radiolytic products) depend on the chemical constituents of the food and on the conditions of irradiation. Information regarding the chemical structures and the amounts of radiolytic products in particular food types, together with the information obtained from toxicological testing, forms a sound basis for evaluating the toxicological safety of an irradiated food. Scientists have repeatedly concluded that the animal feeding studies have found no toxic effects from irradiated foods. Therefore, irradiation of food does not lead to changes in the composition of the food that, from a toxicological point of view, would have an adverse effect on human health.

Research has also shown that the principles of radiation chemistry govern the extent of changes both in the nutrient levels and in the microbiological load of irradiated foods. Key factors include the specific nutrient or microorganism of interest, the food, and the conditions of irradiation. It is well known that the nutritive values of the macronutrients in the diet, such as proteins, fats and carbohydrates, are not significantly altered by irradiation. Levels of certain vitamins may be reduced, however, as a result of irradiation. The extent to which this occurs depends on the specific vitamin, the food type, and the conditions of irradiation. Not all vitamin loss is significant. The extent to which a reduction in a specific vitamin level is significant depends on the relative contribution from the food in question to the dietary intake of the vitamin. The vitamin loss in a particular food from irradiation, particularly at low doses, is often minimal, and, in many cases, is less than that from thermal processing of food. In general, the nutrient retention in irradiated foods is similar to the retention utilizing other food preservation techniques. Irradiation of food does not lead to nutrient losses to an extent that there is an adverse effect on the nutritional status of the individuals consuming these foods.

The microbiological safety of irradiated foods is one of the main reasons for the use of this technology. It is precisely the fact that radiation can destroy microorganisms that this process was first proposed as a technique for food preservation. The microbiological information available on food irradiation suggests that the number of spoilage and pathogenic microorganisms is greatly reduced for foods treated with absorbed doses of less than 10 kGy. Radiation doses greater than 10 kGy can lead to a sterilized product, as is the case with meat products prepared for the NASA space flight program. While irradiation of foods at less than 10 kGy significantly reduces the numbers of many spoilage and pathogenic bacteria, it is less effective in reducing the numbers of relatively radiation-resistant spores of certain pathogenic bacteria, such as clostridium botulinum. Therefore, for foods that are known to be occasionally contaminated with spore forming bacteria, it is important to ensure proper handling of these products to ensure the microbiological safety. Food irradiation can be an integral part of an overall Hazard Analysis Critical Control Point (HACCP) program for the processing of foods. As part of GMPs, food irradiation provides another level of safety for the consumer. One should remember that, regardless of how the food has been treated, proper storage and handling, including proper temperature controls, after processing is necessary to insure that the food will remain safe and nutritious.

The World Health Organization has concluded that irradiation has the potential to significantly reduce the risk to consumers from certain foodborne diseases, including microbiological and parasitic infestations.

Labeling and monitoring

To ensure that the consumer is informed that irradiation has been used, the FDA requires that foods that have been irradiated bear both a logo, the Radura logo, and a statement that the food has been irradiated (Figure 1). Additional labeling may be included by the manufacturer to inform the consumer why the food has been irradiated, and how to store the food to best maintain the quality of the product, provided such statements are truthful and not misleading.

In order to develop monitoring methods to ensure compliance, an extensive research effort has been undertaken to develop methods which can be used to identify foods that have been treated with ionizing radiation. As a result of this effort, a range of methods using a wide variety of chemical, physical, and biological techniques exist that can be used to identify many different food commodities that have been irradiated. Some of these methods act as screening methods, while others can provide more definitive information. None of the current methods will work equally well on all foods. Most of the methods have been developed for a particular food commodity, although many have been shown to also be applicable to other foods as well. Each method has its strengths and weaknesses, and further research will continue to refine these methods, and develop new ones.

The reader can find more information about the food irradiation process and its safety in these publications:

  • "Wholesomeness of Irradiated Food" Report of a Joint FAO/IAEA/WHO Expert Committee, Technical Report Series 659, World Health Organization, 1981.
  • "Food Irradiation: A technique for preserving and improving the safety of food" World Health Organization and Food and Agriculture Organization of the United Nations, 1988.
  • "Safety and nutritional adequacy of irradiated food" World Health Organization, 1994.
  • "Safety of Irradiated Foods" second edition, J.F. Diehl, Marcel Dekker, Inc., 1995.
  • "Detection Methods for Irradiated Foods: Current Status" Edited by C.H. McMurray, E.M. Stewart, R. Gray and J. Pearce, The Royal Society of Chemistry, 1996.