Understanding Ionizing Radiations Used in Food Irradiation

Food irradiation is one of the most effective non-thermal methods for preserving food, and it all comes down to ionizing radiations. These high-energy waves or particles carry enough energy to knock electrons out of atoms and molecules, creating charged particles (ions) in the process. When directed at food, they disrupt the DNA of harmful microorganisms – bacteria, moulds, parasites – without raising the temperature of the food or leaving any chemical residues behind. Three types of ionizing radiation are approved for food processing: gamma rays, X-rays, and electron beams. Each works differently, has distinct penetration capabilities, and suits different food processing scenarios.

What makes radiation “ionizing”? 🔗

The electromagnetic spectrum includes both ionizing and non-ionizing radiation. Non-ionizing types – like radio waves, microwaves, and visible light – do not carry enough energy to remove electrons from atoms. Ionizing radiation, on the other hand, possesses sufficient energy to eject electrons from their atomic orbital shells, fundamentally changing the electrical charge of the affected atoms and molecules. This ionization process is what gives these radiations their ability to damage the genetic material of living cells.

When ionizing radiation passes through food, it interacts with the water and organic molecules present in the product. This interaction produces highly reactive free radicals and charged ions that go on to break chemical bonds – particularly in the DNA of any microorganisms present. Once the DNA is sufficiently damaged, the microbes can no longer reproduce or survive. The food itself remains safe because the energy levels used in food irradiation are far too low to make the food radioactive.

Gamma rays: the deep penetrators 🔗

Gamma rays are the most widely used form of ionizing radiation in commercial food irradiation facilities worldwide. They are electromagnetic waves – similar in nature to X-rays and visible light but with far higher energy and shorter wavelengths.

Sources of gamma rays 🔗

Gamma rays used in food irradiation come from the radioactive decay of specific isotopes, primarily Cobalt-60 (Co-60) and, to a much lesser extent, Cesium-137 (Cs-137). Cobalt-60 is a synthetic radioactive isotope produced in nuclear reactors by bombarding stable Cobalt-59 with neutrons. It has a half-life of approximately 5.27 years and emits two gamma rays with energies of 1.17 MeV and 1.33 MeV. Because its activity decays by roughly 12.5% per year, facilities must replenish their Co-60 sources annually.

Cesium-137, a byproduct of nuclear fission, emits gamma rays at a lower energy of about 0.662 MeV and has a much longer half-life of around 30 years. However, Cs-137 is water-soluble, which poses environmental contamination risks. Past leakage incidents have led to the near-complete elimination of Cs-137 in food irradiation. Today, Co-60 dominates commercial food irradiation because it is water-insoluble and therefore safer to handle and store.

How gamma irradiation facilities work 🔗

In a typical gamma irradiation plant, Co-60 is sealed inside stainless steel tubes called “pencils.” When not in use, these pencils are stored in a deep water pool (around 6-8 metres deep) that absorbs all emitted radiation. During treatment, the source is raised from the pool, and food products – already packaged – are transported on conveyor belts through the radiation field. Because gamma rays cannot be switched off (the isotope continuously emits radiation), the facility design relies on water shielding and thick concrete walls (1.5-2 metres) to protect workers and the environment.

The greatest advantage of gamma rays is their deep penetration ability. They can pass through large, dense, and even pallet-sized food shipments, making them ideal for bulk processing. This high penetration also means good dose uniformity throughout the product, which is critical for consistent food safety outcomes.

X-rays: machine-generated with excellent penetration 🔗

X-rays used in food irradiation are produced by machines rather than radioactive materials. In this process, high-energy electrons generated by an accelerator are directed at a heavy metal target (usually tungsten or tantalum). When these fast-moving electrons strike the target, their energy is converted into X-ray photons that can then be directed at food.

X-ray energy and penetration 🔗

The energy of X-rays used in food processing can reach up to 7.5 MeV in the United States, while the global standard set by the Codex Alimentarius limits X-ray energy to 5 MeV. This is notably higher than the energy of gamma rays from Co-60 (1.17-1.33 MeV), giving X-rays penetration capabilities comparable to – and sometimes better than – gamma rays.

X-rays can achieve dose rates of approximately 100 Gy per second, which is significantly faster than the typical gamma ray dose rate of about 100 Gy per minute. This makes X-ray processing considerably quicker. However, the conversion efficiency from electron beam to X-ray is relatively low – less than 10% of the electron energy is converted to X-rays, with the rest lost as heat. This energy inefficiency is the primary reason X-ray facilities have historically been more expensive to operate than gamma facilities.

Advantages of X-ray irradiation 🔗

The biggest operational advantage of X-rays is that the radiation source is machine-generated, meaning it can be turned on and off at will. There is no need to procure, transport, store, or dispose of radioactive materials. This eliminates many of the security, safety, and regulatory concerns associated with gamma sources. X-rays also offer excellent dose control and can process medium-to-large food packages with good uniformity, placing them as a strong middle ground between gamma rays and electron beams.

Electron beams: fast but limited in depth 🔗

Electron beam (e-beam) irradiation uses high-energy electrons accelerated by linear accelerators to near the speed of light. Unlike gamma rays and X-rays (which are electromagnetic photons), electron beams are streams of charged particles directed at the food product.

How electron beams work 🔗

In an e-beam facility, electrons are accelerated through an electric field and focused into a narrow beam. When these high-speed electrons strike the food, they cause ionization events similar to those produced by gamma and X-ray photons. They disrupt the DNA of microorganisms through the same mechanism – creating free radicals and breaking molecular bonds. The U.S. FDA permits electron beam energies of up to 10 MeV for food processing.

E-beam processing is the fastest of the three irradiation methods. Its dose rates are significantly higher than both gamma and X-ray methods, allowing large volumes of food to be treated in seconds rather than minutes or hours. This speed translates to higher throughput and greater processing efficiency.

The penetration limitation 🔗

The main drawback of electron beams is their limited penetration depth. At 10 MeV with double-sided treatment, e-beams can only penetrate food to a maximum depth of about 8 cm. This makes them less effective for bulk or dense food products that require deeper treatment. E-beams are best suited for surface or near-surface applications – treating thin products, grain, ground spices, packaged fruits and vegetables, and eliminating surface contamination on prepared meals.

Despite this limitation, e-beam technology has several practical advantages. Like X-rays, it is entirely machine-generated, so there are no radioactive materials involved. The system can be switched on and off, requires no isotope replenishment, and can even be integrated directly into production lines for in-line treatment.

How ionizing radiation preserves food 🔗

Regardless of whether the source is gamma rays, X-rays, or electron beams, the preservation mechanism is essentially the same. When ionizing radiation enters a food product, it transfers energy to the molecules it encounters. This energy transfer causes ionization – the removal of electrons from atoms – and the production of highly reactive free radicals, particularly from water molecules in the food.

These free radicals and ions then interact with the DNA of any microorganisms present, causing strand breaks and other damage that prevent the microbes from reproducing. The U.S. FDA notes that these interactions produce chemical changes primarily in the microbial cells, while the food itself undergoes minimal alteration. The radiation dose is carefully calibrated – measured in units of kiloGray (kGy) – to achieve the desired effect, whether that is disinfestation (low dose, below 1 kGy), pasteurization (medium dose, 1-10 kGy), or sterilization (high dose, above 10 kGy).

Why it is called a “cold process” 🔗

One of the most important characteristics of food irradiation is that it does not significantly raise the temperature of the food being treated. This is why it is often referred to as “cold pasteurization”. Conventional thermal processing methods – such as canning or pasteurization – rely on heat, which can alter the texture, flavour, colour, and nutritional content of food. Ionizing radiation achieves microbial control without these heat-induced changes, preserving the freshness and sensory qualities of the product.

This makes food irradiation particularly valuable for products that are sensitive to heat, such as fresh fruits, vegetables, spices, and raw meat. The treated food looks, tastes, and feels essentially the same as untreated food – consumers generally cannot distinguish irradiated food from non-irradiated food by sight, smell, taste, or texture.

A cleaner alternative to chemical fumigants 🔗

Before food irradiation became widely available, chemical fumigants like methyl bromide and ethylene oxide were commonly used to control pests and pathogens in food products. These chemicals raised significant concerns: they could leave harmful residues on food, required careful handling to protect workers, and contributed to environmental problems such as ozone depletion.

Ionizing radiation offers a clean alternative that avoids the procurement, storage, and disposal issues associated with chemical treatments. It leaves no chemical residues, produces no harmful byproducts, and can be applied to pre-packaged food – meaning the packaging protects the food from recontamination after treatment. More than 60 countries have approved food irradiation, and international bodies including the WHO, FAO, IAEA, and the U.S. FDA have all endorsed its safety after decades of rigorous evaluation.

Quick comparison: gamma rays vs. X-rays vs. electron beams 🔗

Safety and regulatory framework 🔗

A common concern about food irradiation is whether the treated food becomes radioactive. The answer is definitively no. The energy levels permitted for food irradiation – up to 10 MeV for electron beams, 7.5 MeV for X-rays (in the U.S.), and the inherent energies of Co-60 and Cs-137 – are all far too low to induce radioactivity in the food product. As the U.S. EPA explains, the process is comparable to how medical X-rays pass through your body without making you radioactive.

All irradiated foods sold to consumers must display the Radura symbol – an internationally recognized green logo – along with a statement such as “Treated with Radiation” or “Treated by Irradiation.” Regulatory agencies including the FDA, USDA, WHO, and IAEA have set strict dose limits and safety standards that irradiation facilities must follow. The Codex Alimentarius has established a general maximum dose of 10 kGy for most food products, ensuring that treatment remains within safe and effective limits.

Current applications and the road ahead 🔗

Today, food irradiation is used across a wide range of products: spices and dried herbs (which frequently carry high microbial loads), fresh and frozen meat and poultry, tropical fruits for phytosanitary treatment, shellfish, and even seeds for sprouting. In the United States, the list of FDA-approved foods for irradiation continues to expand.

The industry trend is increasingly moving toward machine-generated sources – electron beams and X-rays – and away from isotope-based gamma irradiation. This shift is driven by concerns over the safety and security of radioactive materials, limited global production capacity for Co-60, and advances in accelerator technology that are making e-beam and X-ray systems more cost-effective and versatile. The IAEA has been actively coordinating research to develop practical tools and establish treatment parameters for broader adoption of these machine-source technologies.

As food supply chains grow longer and more complex, and as consumer demand for chemical-free preservation increases, ionizing radiation is set to play an even larger role in ensuring global food safety.

What do you think? Given that ionizing radiation preserves food without heat, chemicals, or residues, why do you think consumer acceptance of irradiated food remains relatively low in many countries? And could the shift from radioactive isotopes to machine-generated radiation sources like electron beams and X-rays help change public perception?

References

1. https://www.fda.gov/food/irradiation-food-packaging/overview-irradiation-food-and-packaging
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC12637162/
3. https://www.epa.gov/radtown/food-irradiation
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC7404640/
5. https://www.iaea.org/publications/15188/development-of-electron-beam-and-x-ray-applications-for-food-irradiation
6. https://www.cdc.gov/radiation-health/food-irradiation/index.html
7. https://www.foodengineeringmag.com/articles/102388-understanding-e-beam-effectiveness-in-food-safety-applications
8. https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/irradiation-and-food-safety-faq