What Is Food Irradiation?

Food irradiation is the controlled exposure of food to ionizing radiation. The process can reduce harmful microorganisms, control insects, delay ripening or sprouting, and extend shelf life. It is used for products such as spices, fruits, vegetables, grains, meat, poultry, and seafood.

The process is sometimes compared with pasteurization because both treatments reduce microorganisms that can cause foodborne illness or spoilage. However, food irradiation does not rely primarily on heat. For this reason, it is sometimes described as cold pasteurization, although the food may warm slightly during processing.

Food irradiation is not intended to replace proper sanitation, refrigeration, cooking, or safe food handling. Instead, it provides an additional layer of protection. For example, irradiation may reduce disease-causing bacteria in meat or poultry, but the food must still be stored and prepared correctly.

How Ionizing Radiation Preserves Food

Ionizing radiation carries enough energy to remove electrons from atoms and molecules. When food is irradiated, this ionization damages cells within insects, parasites, bacteria, molds, and other organisms.

Radiation can affect microorganisms in two principal ways:

• Direct action: Radiation deposits energy directly in important biological molecules, including DNA. Damage to DNA can prevent a microorganism from reproducing.
• Indirect action: Radiation ionizes water and other molecules within a cell, producing chemically reactive fragments known as free radicals. These reactive species can damage DNA, proteins, and cell membranes.

Because most foods contain water, indirect effects are especially important. A simplified representation of the ionization of water is:

The resulting ions and electrons participate in additional reactions that can produce reactive species such as the hydroxyl radical, [latex]\mathrm{OH\boldsymbol{\cdot}}[/latex]. These species exist for only a very short time but can cause significant molecular damage before they react or disappear.

Radiation Sources and Processing Facilities

Commercial food irradiation uses three types of ionizing radiation:

• Gamma rays emitted by radioactive isotopes such as cobalt-60.
• X-rays generated electrically by directing high-energy electrons onto a metal target.
• Electron beams (e-beams) produced by particle accelerators.

Each radiation source has advantages depending on the type, size, and packaging of the food being treated. Gamma rays are highly penetrating and can treat large pallets of packaged food. X-rays also penetrate deeply but are generated electrically, eliminating the need for a radioactive source. Electron beams deliver high doses quickly but penetrate only a few centimeters, making them most suitable for relatively thin products.

Regardless of the radiation source, the goal is the same: to deliver a carefully controlled radiation dose that destroys microorganisms or insects while preserving the quality of the food.

Commercial irradiation facilities are designed with multiple layers of safety. The radioactive source remains inside thick concrete shielding during operation, and workers are never present in the irradiation room while the source is exposed. When the facility is not operating, the cobalt-60 source is automatically lowered into a deep pool of water, where the water absorbs the gamma radiation and provides effective shielding.

Facilities that use x-rays or electron beams operate differently because they do not contain radioactive materials. Instead, radiation is produced only when the accelerator is switched on, much like a medical x-ray machine.

Dose Levels and Applications

The radiation dose used for food irradiation depends on the desired outcome. A relatively small dose may be sufficient to delay fruit ripening or eliminate insects, whereas much larger doses are required to destroy disease-causing bacteria or sterilize food intended for patients with severely weakened immune systems.

In general, simpler organisms are more resistant to radiation than complex organisms. Bacteria, fungi, and insect eggs often require much larger doses than would be harmful to human cells. The goal is to deliver enough radiation to damage microorganisms without significantly affecting the appearance, texture, flavor, or nutritional value of the food.

Spices are among the foods most commonly treated with irradiation because they often contain microorganisms that survive conventional drying. Irradiation can greatly reduce bacterial contamination while preserving the flavor and aroma of the spices.

Effects on Food Quality and Nutrition

Because ionizing radiation breaks chemical bonds, food irradiation causes small chemical changes within the treated food. Most of these changes result from the production of short-lived free radicals, which react rapidly with nearby molecules. Similar chemical changes also occur during cooking, freezing, drying, and other common methods of food preservation.

The extent of these changes depends on the radiation dose and the type of food. Foods with high water content produce more free radicals than dry foods, while high doses generally produce larger chemical changes than low doses.

At the low and medium doses commonly used for food preservation, changes in taste, texture, and appearance are usually small. Many fruits, vegetables, spices, meats, and seafood retain their nutritional quality while benefiting from reduced spoilage and lower levels of harmful microorganisms.

Some foods are less suitable for irradiation. For example, leafy vegetables and certain soft fruits may develop undesirable changes in texture after treatment. Food processors therefore select irradiation only when its benefits outweigh any effects on product quality.

Nutritional Effects

Like all food preservation methods, irradiation can reduce the concentration of certain nutrients. Vitamins that are particularly sensitive to oxidation, such as vitamins A, C, E, and some B vitamins, may decrease slightly depending on the food and the radiation dose.

For most foods, however, these nutritional losses are comparable to—or smaller than—those produced by other preservation methods such as cooking, canning, or dehydration. Proteins, carbohydrates, fats, and minerals are largely unaffected by the doses used for commercial food irradiation.

Safety, Regulation, and Labeling

Extensive scientific research has shown that food irradiated under approved conditions is safe to eat. Ionizing radiation destroys microorganisms and insects by damaging their DNA, but it does not make the food itself radioactive. The radiation passes through the food during processing and does not remain behind, just as a patient does not become radioactive after receiving a diagnostic x-ray.

Although irradiation produces small quantities of chemical compounds known as radiolytic products, these compounds are also formed during other food-processing methods such as cooking, roasting, and pasteurization. Their concentrations depend on the type of food and the radiation dose, and extensive studies have found no evidence that the approved levels used for food irradiation pose a health risk.

In the United States, food irradiation is regulated by the Food and Drug Administration (FDA), which evaluates the safety and effectiveness of approved irradiation processes. The U.S. Department of Agriculture (USDA) also oversees the irradiation of meat and poultry products. International organizations, including the World Health Organization (WHO) and the Food and Agriculture Organization (FAO), support the use of food irradiation when it is performed according to established safety standards.

Food Labeling

In many countries, irradiated foods sold to consumers must be identified with labeling that indicates the product has been treated with ionizing radiation. In the United States, qualifying packaged foods must display the Radura symbol (Figure 129.3) together with wording such as "Treated with radiation" or "Treated by irradiation."

Section Summary

• Food irradiation is the controlled use of ionizing radiation to improve food safety, reduce spoilage, control insects, and extend shelf life.
• Commercial food irradiation uses gamma rays, x-rays, or electron beams. None of these methods make food radioactive because they do not produce significant nuclear reactions in the food.
• Ionizing radiation damages microorganisms through direct ionization of biological molecules and indirectly through the production of highly reactive free radicals.
• The radiation dose depends on the intended application. Low doses delay ripening and control insects, medium doses reduce harmful microorganisms, and high doses are used for specialized sterilization applications.
• At approved dose levels, food irradiation causes only small changes in taste, texture, and nutritional value. These changes are generally comparable to those produced by other food preservation methods such as cooking, freezing, or pasteurization.
• Food irradiation is regulated by national and international agencies to ensure that irradiated foods are safe for consumers and properly labeled when required.
• Food irradiation is one example of how the principles of nuclear physics are applied to improve public health by reducing foodborne illness and increasing food preservation.