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ONLINE EXCLUSIVE: Toxicological Safety of Irradiated Foods
Irradiation has already been approved by a number of countries
by Christopher H. Sommers, Henry Delincee, J. Scott Smith, and Eric Marchioni
Editor’s Note: This article is excerpted from a chapter in “Food Irradiation Research and Technology,” which was edited by Christopher H. Sommers, PhD, and Xuetong Fan, PhD. The book was published in 2006 by Wiley-Blackwell, which also publishes Food Quality magazine. To download the full article and references, click here.
Generation of cancers in animals requires the mutation or deletion of oncogenes or tumor suppressor genes, resulting in a loss of heterozygosity at those allele locations. Mutation (point mutations or frame-shift mutations) and deletion of genes can be induced by exposure of cells to genotoxic chemicals or can occur naturally as part of the cellular DNA repair and replication process.
Many consumers are simply unaware that foods contain carcinogens, either natural or artificial, and cause cancer. A very small subset of naturally occurring carcinogens in foods include compounds such as benzene and formaldehyde. A number of studies have confirmed the mutagenicity of cooked meats and their fats, and the formation of nitrosamines as part of the meat curing and cooking process.
Tumor promoters present in cooked meat and poultry include oxidization products of fats and oils, heme, and cholesterol. Alcohol is known to induce the formation of tumors in the gastrointestinal tract of rodents. It was recently found that high-temperature frying and baking of starch-containing foods results in the formation of acrylamide, a suspected human carcinogen. Furan, a carcinogen in animals, is formed in foods as a result of thermal processing.
Compounds used in the pickling, salting, and smoking processes are associated with gastrointestinal cancers in humans. Discussions pertaining to food irradiation, therefore, have to be placed in context with the risks associated with consumption of irradiated foods versus foods processed using technologies and additives that are known to cause cancer in animals and humans.
Food irradiation is perhaps the single most studied food processing technology for toxicological safety in the history of food preservation. Studies pertaining to the safety and nutritional adequacy of irradiated foods date back to the 1950s and were frequently associated with the use of radiation to sterilize foods.
Hundreds of short-term and long-term safety studies led to the approval of one or more foods for irradiation by presently more than 60 countries. These studies are thoroughly reviewed in “The Safety and Nutritional Adequacy of Irradiated Foods,” published by the World Health Organization in 1994.
In the United States, the Food and Drug Administration reviewed the available studies for the quality of experimental design, rigor, and statistical validity before approving irradiation of a variety of food products including grain, fruits and vegetables, spices and dried herbs, meat and poultry, and eggs for human consumption.
The vast majority of the studies failed to find adverse effects associated with consumption of or exposure to irradiated foods. Not surprisingly, a small number of studies produced equivocal results pertaining to the safety of irradiated foods. However, in-depth review of those studies determined that they were deficient in experimental design, used insufficient numbers of test subjects for proper statistical analysis, or suffered from experimenter error.
The preferred method for assessing the toxicological safety of irradiated foods has been long-term feeding studies in animals, often for multiple generations. Toxicologists prefer to use animals for these types of evaluations, as opposed to using people or their children, for obvious reasons. Swallow reported that animals used for toxicological research, fed diets of radiation-sterilized foods for 40 generations, suffered no ill effects from consumption of irradiated foods.
Thayer and others reported that rodents fed diets of radiation-sterilized chicken meat (45–68 kGy) did not suffer an increased risk of cancer or birth defects. The same study also failed to find adverse effects associated with long-term consumption of irradiated meat in beagle dogs. De Knecht-van Eekelen and others conducted single- and multiple-generation feeding studies in rats without finding adverse effects due to consumption of the irradiated chicken diet. Poling and others reported no evidence of changes in survival, histopathology, or reproduction in three generations of rats fed radiation-sterilized ground beef. Feeding studies in animals have been very consistent in the lack of adverse effects associated with long-term consumption of irradiated foods.
Cancer in animals and humans has been associated with many factors, including excessive consumption of fried, smoked, and barbecued meats and fish, pickled foods, and alcohol. Carcinogens such as formaldehyde, furan, acrylamide, nitrosamines, and benzene are naturally occurring in many foods, or formed as a result of thermal processing. Tumor promoters present (at milligram and gram quantities) in meat include lipids and oxidized lipids, hemes, and cholesterol.
Particular attention has been drawn to a special class of cyclic compounds being formed on irradiation of lipids. A wealth of radiolytic products are formed on irradiation of, for example, triglycerides, among them fatty acids, hydrocarbons, aldehydes, ketones, esters, and dimeric and polymeric components, but to date one class of components, the 2 alkylcyclobutanones (2-ACBs), is of particular interest.
This new class of cyclic components was reported more than 30 years ago to be formed on irradiation of pure saturated triglycerides containing C6, C8, C10, C12, C14, C16, and C18 fatty acids with a high dose (60 kGy under vacuum). These compounds were identified as the 2 alkylcyclobutanones of the same carbon number as the precursor fatty acid. It has been proposed that these compounds may result from cleavage of the acyloxy bond via the formation of a six-membered ring intermediate.
Because levels of 2-ACBs are present in sufficient (albeit µg) quantities to be considered an indirect food additive, assessment of their toxicological potential should be a priority in the science of food irradiation. It should also be recommended that any toxicological risk assessment pertaining to the 2-ACBs should be in the context of the total human diet and the potential benefit of food irradiation in reducing illnesses, hospitalizations, and deaths associated with foodborne illness.
Paracelsus, the fifteenth-century philosopher and scientist, observed that all substances are poisons; it is only a matter of dose. Although it is almost impossible to prove the absolute safety of any food or food processing technology, it is difficult to conceive—considering the toxicological database—that radiation-pasteurized foods, including meat and poultry, pose a significant risk to human health when consumed as part of a healthful, well-balanced diet. This is true especially when compared to other, “more established” food processing and preservation methodologies that have been directly associated with the formation of cancers in animals and humans.