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Edward S. Josephson
The primary purpose for treating food with ionizing energy is to protect or improve human health by inactivating disease-causing organisms and by eliminating or reducing the need for chemicals that may leave toxic or carcinogenic residues when used to preserve or disinfest food and food products. The World Health Organization has identified contamination of food by disease-causing organisms as one of the world's major health hazards [1; 2].
Using ionizing radiation to preserve foods by giving them longer storage lives while maintaining their organoleptic (sensory) characteristics (e.g. taste, odour, colour, and texture) and nutritional quality can, by reducing shortages, contribute substantially to human health by alleviating hunger and malnutrition on a worldwide basis. The economic value and health benefits to humans of the food saved from spoilage would more than cover the cost for irradiation [3; 4].
The US Food and Drug Administration (FDA) has approved the use of ionizing radiation to sterilize spices and condiments which have high levels of microbial and insect contamination, to render harmless Trichinella spiralis in pork, to eliminate non-spore-forming pathogenic bacteria in poultry, to inhibit sprouting of stored potatoes, to delay ripening of fruits and vegetables, and to disinfest wheat, wheat flour, fruits, and vegetables of insects to fill the void for insect control left by the banning of ethylene dibromide, ethylene oxide, and other chemical fumigants that leave toxic or carcinogenic residues [1; 5; 6].
The health officials in 34 countries have approved irradiation for one or more of approximately 200 foods and food products, including entire irradiation-sterilized meals for consumption by hospital patients whose immune systems have been suppressed [1: 2]. These approvals cover all classes of foods except dairy products, which do not respond well organoleptically to ionizing radiation. Twenty countries, including the United States, are irradiating on a commercial scale one or more of the foods approved by their respective health authorities .
A 1985 report by the US Department of Agriculture for the US Office of Technology Assessment stated that the annual medical costs and productivity losses during illness because of human infections with salmonellae and campylobacters would be reduced between $200 million and $500 million annually if all chicken meat were irradiated, even after subtracting the cost for irradiation. According to the same report. irradiating pork and other meats for Trichinella spiralis or Toxoplasma gondii infestations would have a net annual benefit of between $100 million and $200 million . The FDA is now evaluating a petition from the Department of Agriculture to irradiate poultry to eliminate salmonellae and other pathogens, because nearly 40% of chickens sold in the United States are said to be so contaminated .
Food irradiation is versatile because of the broad spectrum of foods that can be treated in bulk, in solid frozen blocks, or packaged in sizes and shapes ranging from thin slices of meat in flexible pouches to whole hams and eviscerated poultry in large metal cans, fresh fruits and vegetables in crates, and grain in sacks. Low-dose applications (less than 1 kGy) can prevent spoilage from sprouting during storage of potatoes, onions, and other tubers and bulbs; disinfest cereals and pulses, fresh and dried fruits, dried fish and meat, and fresh pork; and delay physiological processes such as ripening in some fresh fruits and vegetables. Intermediate-dose applications (1-10 kGy) in conjunction with refrigeration can delay spoilage and eliminate pathogens in fresh and frozen seafood, poultry, eggs, meats, and some fruits (e.g. strawberries). High doses (10-71 kGy) are used to sterilize packaged meat, poultry, seafood, prepared foods, and hospital diets and to decontaminate certain food additives and ingredients such as spices and enzyme preparations, Those foods that respond well to ionizing radiation retain their quality (taste, texture, colour, nutrients) .
Because the chemical changes in food as the result of exposure to ionizing radiation are small. the likelihood of producing toxic effects in consumers is remote. The organoleptic and nutritional qualities are maintained to a degree equal to or better than foods subjected to the established methods of food processing. The changes caused by the high radiation doses needed for sterilization are minimized by irradiating these foods while frozen in the -40 °C to -20 °C temperature range with exclusion of air . Radiation sources are limited to the gamma rays from cobalt-60 and cesium-137, x-rays at energies below 5 MeV, and electrons at energies below 10 MeV to ensure the absence of measurable induced radioactivity .
Extensive toxicological studies have shown no evidence that irradiated foods contain radiation-produced carcinogens or other toxic substances. Studies have involved feeding irradiated or non-irradiated foods at 35% of the daily intake to rats, mice, and dogs over periods of two to three years, and, in the case of rats and mice, over four generations. No evidence for carcinogenesis, mutagenesis, teratogenesis. or radiolytic products of possible toxicological significance were found . Moreover, during 1976-1979 the Federation of American Societies for Experimental Biology studied the radiolytic products identified in radiation-sterilized beef and concluded they were of no toxicological significance in the quantities that would be consumed . All the radiolytic products identified also occur in unirradiated food products. No compounds unique to the food irradiation process have been found after 35 years of searching . Finally, an expert committee convened by the World Health Organization in ]980 reviewed all the available data from more than 1,200 studies and recommended that its member nations approve any food irradiated up to an average dose of 10 kGy without the need for further testing. This committee deferred making a recommendation for foods irradiated in the 10-72 kGy dose range until two large studies then in progress were completed. These studies were completed in the mid-1980s and also showed no adverse effects .
Nutritional changes reflect the chemical changes in food caused by irradiation and hear an almost linear relationship to dose. At low doses (below I kGy), the loss in nutrients is insignificant . In the mediumdose range (1-10 kGy), there are no significant nutritional losses at doses up to 3 kGy [1; 6]. At higher radiation doses in the medium-dose range, it may be desirable to irradiate at low temperatures and/or with the exclusion of air . At high doses ( 10-70 kGy), it is necessary to irradiate moist foods at low temperatures with exclusion of air to maintain nutritional and organoleptic quality equal to or better than that of the same foods sterilized by heat .
The overall digestibility of fats is unaffected even at high doses, indicating that the changes associated with absorption of the ionizing radiation do not seriously affect digestion or absorption. The availability from high-dose irradiation is not adversely affected [1; 2].
Chemical analyses and animal-feeding studies have shown that the nutritional value of protein is not significantly affected even by high doses of radiation. No significant changes were observed in essential and non-essential amino acids, protein efficiency ratio, true digestibility, biological value, or net protein utilization .
The main effects of ionizing radiation on carbohydrates are those of hydrolysis and oxidative degradation. Complex carbohydrates are converted into simpler compounds. Although ionizing radiation may cause changes in the physical and chemical properties of high-carbohydrate foods, such changes are not nutritionally significant. Some foods with extremely high sucrose content, however, become unpalatable and lose nutritional quality in response to high doses of ionizing radiation [1; 2; 6].
Vitamins in food are less vulnerable to ionizing radiation than in pure solutions because of the protective characteristics of food constituents [ 1; 2]. Riboflavin, niacin, and vitamin D are stable to irradiation. Vitamins A, E, K, and thiamin are more easily destroyed and may have to be protected, particularly against medium- and high-dose irradiation, by using low temperatures and excluding oxygen. Little is known about the effects of irradiation on folic acid. Reports conflict on the effects of radiation on vitamin C in fruits and vegetables. Conversion to the biologically active dehydroascorbic acid may account for these discrepancies. Tests on high-dose-irradiated beef and chicken for antithiamine and antivitamin B6 gave negative results .
There is no evidence to indicate that free radicals formed in irradiated food are toxic or carcinogenic or behave differently from free radicals formed by baking, frying, or exposing food to sunlight. Although free radicals may persist for several months in dry foods, they disappear in contact with moisture (e.g. saliva). A three-year multigeneration feeding study on rats and mice with diets containing 35% and 80% milk powder that had received 45 kGy of radiation and had a high content of long-lived free radicals did not show any carcinogenic or chronic effect .
The labeling of irradiated foods has been a matter of discussion, the purpose of the label being to inform the consumer and not to stigmatize a food. The Codex Alimentarius Commission (CAC), established jointly by the World Health Organization and the Food and Agriculture Organization of the United Nations to facilitate international trade in all foods whether or not irradiated, is attempting to obtain uniform agreement on a label for irradiated foods (wording and logo) among its approximately 130 member nations. Because the United States is a member of the CAC, the FDA has adopted as an interim measure a requirement that the internationally agreed upon logo with the words "Irradiated" or "Radiated" should be used until a decision is made on a universally agreed upon label [7-10]. The FDA position for any labeling requirement is that it "must not imply that the labeling is intended as a warning" and that "there is no more need for a warning for an irradiated ingredient than with a typical non-irradiated ingredient."
The radiation process requires a source of ionizing radiation in a chamber inside a labyrinth with sufficient shielding to protect people working in the facility or in the surrounding area. A conveyor system operated from a control panel outside the labyrinth carries the food into a chamber for irradiation and then out again after the irradiation is completed. Every conceivable safeguard is automatically activated to warn and physically prevent anyone from entering the labyrinth when it is in the operating mode. Closed-circuit television permits surveillance of the irradiation chamber and labyrinth at all times by persons operating the control panel.
Food-irradiation facilities differ from those required in power plants or weapons production as they contain no uranium or other fissionable material and no source of neutrons to produce fission. There are no hot fluids or gases that could generate an explosion, no radioactive gases, liquids, or solids that could be accidentally disseminated in the surrounding environment, and no known ways in which the sources could be used to produce nuclear weapons. The Canadian government, which has a virtual worldwide monopoly on cobalt-60 production, will accept its return after it has decayed beyond usefulness. Cesium-137, a byproduct of nuclear weapons manufacture, will probably not be used in any significant amount in commercial food irradiation because of its relative scarcity and high processing cost compared with cobalt-60 and machine sources of radiation. The safety of the radioactive sources during transport and handling is assured by double incapsulation in stainless steel jackets and by adhering to federal and state regulations. They have been used commercially and for sterilization of medical and surgical items for 30 years without any mishaps in transportation. Transport of machine sources poses no special hazard because they operate only when plugged into a source of electricity and switched on.
It can be concluded that the effect of irradiation on food is nutritionally insignificant for most applications. The effects of food irradiation are no more deleterious to health or nutritional quality than other food processing methods now being used commercially, and the advantages are many.
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