Byproducts of irradiation

Previously, approximately 65 products were identified as unique compounds formed during food irradiation. Since then, all but six have been accounted for as naturally occurring or found in other food-processing situations. Free radical compounds are generated, but are not unique to food irradiation. One compound was traced to be a contaminant of a chemical used to sterilize the hooks from which carcasses were hung in slaughterhouses.

After several decades of research, including more than 1300 studies, no adverse effects have been found due to irradiation of foods. These include multigenerational feeding studies with rats, mice, dogs, and monkeys.

As with all preservation or cooking methods, some chemical changes occur in irradiated food. When high-energy particles strike matter, electrons are lost from atoms and ions are formed. Newly formed radiolytic products may then interact to create new compounds in the food that were not present before treatment, a few of which could produce off-flavors. In meat, this can be partly controlled by maintaining low product temperatures during the irradiation process. The most common chemical reaction during food irradiation is the conversion of water to hydrogen peroxide. Reactions such as these occur in all types of food preservation, and the few reactions unique to irradiation are not harmful.

The FDA concluded that "very few of these radiolytic products are unique to irradiated foods; approximately 90% of the radiolytic products...are known to be natural components of food." Some of these are fatty acids identical to those that result from the breakdown of triglycerides; amino acids that make up proteins; and compounds (hydrocarbons) commonly found in the waxy coverings of fruits such as apples, pears, and berries. Others are fatty compounds identical to those found from cooking meat by common methods such as grilling. The other 10% of radiolytic compounds are chemically very similar to natural components in food. The chemistry of irradiation is very predictable, and the products of an individual component such as proteins are not affected by the type of food or other food components present. Radiolytic products have been critically tested for toxicity and no evidence of hazards has been found.

Radiation does not impair the activity of certain nutrients, but overall nutrient retention in irradiated foods is similar to retention by other preservation methods. With regard to macronutrients, certain proteins are split or aggregated and the amino acid methionine is degraded. Also, double bonds of polyunsaturated fatty acids are broken down, producing off-flavors. Certain micronutrients are lost, such as vitamins C, E, K, B6, and riboflavin. Vitamin C (ascorbic acid) reduction has been reported, but it is attributed to a shift from ascorbic acid to dehydroascorbic acid, a change mostly insignificant from a nutritional standpoint. Tocopherol, which has vitamin E activity, appears to be very sensitive to irradiation in the presence of oxygen. Vitamin K seems relatively stable. These adverse effects of irradiation on vitamins can be reduced by excluding oxygen and light, keeping the food at a low temperature, and using the lowest dose needed to treat (process) food. However, improved processing techniques, such as excluding air from the radiation milieu or holding foods that can withstand freezing, have resulted in minimal losses and changes.

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