Food Irradiation Technologies: Principles, Commercial Advantages, And Limitations

By Tatiana Koutchma Ph.D., research scientist, Agriculture and Agri-Food Canada

Food irradiation has proven to be a safe and effective non-thermal process for enhancing food safety and extending shelf life of a wide variety of foods. Nearly 50 countries have approved or allow food irradiation, although the foods and doses can differ by country. Gamma irradiation technology was patented more than a century ago (in 1906) and has been one of the first non-thermal technologies thoroughly tested, validated, and adopted by medical and food industry over the past 60 years. Despite this, there is a lot of misunderstanding and concerns associated with the consumer acceptance, transport, storage, occupational hazards, and disposal of radiation sources.

Gamma rays from radioactive nuclides, energetic electrons from particle accelerators, and X-Rays emitted by high-energy electrons are suitable kinds of radiant energy because all of these three ionizing energy sources can produce similar effects in any irradiated material. High-energy electrons, gamma and X-Rays can kill or inactivate bacteria, spores, fungi, and insects by breaking molecular bonds in microbial DNA. Irradiation is also often referred to as “ionizing radiation” because it produces rays or particles strong enough to dislodge electrons from atoms and molecules, and thereby converting them to electrically charged ions. Other terms commonly used for irradiation are “cold pasteurization” and “irradiation pasteurization” or “irradiation sterilization.” The term “electronic pasteurization” has been coined for e-beams and X-Ray technologies with the added benefit that the electronic source stops radiating when switched off.

Gamma Rays
Gamma rays for food treatment are produced by radioactive isotopes of cobalt-60 or cesium-137. Gamma sources are specified in terms of their activity measured in curies (Ci). 1MCi is a moderate-sized source. The gamma rays inactivate microorganisms in the “direct hit” method and in the “indirect hit” method. In the direct hit method, the gamma ray directly collides with the genetic material of the bacterial cell, breaking the DNA. In the indirect method, the gamma ray passes very close to the DNA, ionizing a water molecule. The reactive components of the ionized water molecule react with the DNA resulting in lethal damage. In practice, both methods contribute to the inactivation of bacteria. In low-moisture environments, most notably, bacterial spores, the indirect method has little effect, which in part accounts for the higher radiation resistance of bacterial spores.

In the U.S., gamma irradiation has been widely used for decontamination of spices, seasonings, flour, shell eggs, poultry, shellfish, and more recently, for lettuce and spinach. All fruit and vegetables are approved for irradiation at specified maximum dose levels to ddelay of maturation (ripening) and disinfestation.

X-Rays And E-Beams
The advantage of X-Rays and e-beams is that they are generated from machine sources. X-Ray and e-beam sources are specified by beam-power: the range from 25 to 50 kW is typical for food applications. X-Rays are produced by reflecting a high-energy stream of electrons off a target substance — usually one of the heavy metals — into food. X-Ray irradiation is used as an alternative to methods that use radioactive materials and generated from machine sources operated at or below an energy level of 5 MeV. X-Ray irradiators are scalable and have deep penetration comparable to cobalt-60. The choice of a radiation source for a particular application depends on such practical aspects as thickness and density of the material, dose uniformity ratio (DUR), minimum dose, processing rate, and economics.

E-beam irradiation uses electrons accelerated in an electric field to a velocity close to the speed of light and are generated from machine sources operated at or below an energy level of 10 MeV. Since electrons are particulate radiation, they do not penetrate the product beyond a few centimeters, depending on product density. The product is exposed to the e-beam when it moves on the conveyor. Single or double beams are used to solve issues of packaging thickness.

A major advantage of electronic irradiation is food can be processed in its final packaging, thereby reducing, or eliminating entirely, the possibility of recontamination following this treatment. This unique operational capability makes irradiation particularly suitable for cold pasteurization of ready-to-eat foods, such as hot dogs, and other deli items, at risk of contamination with Listeria monocytogenes during post-process slicing and packaging operations. Irradiation treatment is very effective in killing microbial pathogens, such as Escherichia coli O157:H7, Listeria monocytogenes, Salmonella spp. and Vibrio spp., among others, that are significant contributors to foodborne illness.

By and large, scientific evidence from the USDA shows that the electronic pasteurization does not cause and significant difference in flavour, texture, or color of beef, poultry, or produce when irradiated at optimal levels. No loss in nutrients, vitamins, proteins, lipids, and carbohydrates occurs in food irradiated up to 10 kGy. For a comparison, according to the studies by the ARS USDA, the radiation dose of only 3.75 kGy inactivated all of the foodborne pathogens by a minimum of 5-log (99.999%), on all of the RTE meat types, which is sufficient to be labeled as pasteurized according to current regulatory requirements. Electronic pasteurization can offer solid and semi-solid foods such as meat, poultry, and fish the same benefits that thermal pasteurization has brought to milk and other liquid products.

Food irradiation by high-energy electrons with energy up to 5-10 MeV has proven to be a safe and effective process for increasing food safety and extending product shelf life. Low-energy electrons (energy of 300 keV or lower with extremely-small penetration capacity) can supplement food irradiation technologies with novel sanitary and surface treatment approach. “Soft" electrons can inactivate microorganisms on the surfaces, which can be frequently contaminated with spoilage, vegetative bacteria, and heat-resistant spores. Low-dose electrons can be used as an alternative to steam or chemicals for elimination of microorganisms on the surface of grains, dehydrated vegetables, spices, and tea leaves with little quality loss. The e-beam aseptic filling systems are emerging as alternative to chemical sterilization by hydrogen peroxide.

The pros and cons of four existing methods of irradiation are summarized in this table.

There are two business models for irradiation. One is the construction of a facility to be owned and used by the processor, while the other is to use a contracted service provided by an independent service provider. Unless a very-large quantity of product is to be irradiated continuously, most processors rely on contracted services.

Food irradiation is a powerful tool to prevent food borne illnesses. Removal of labeling requirements, use of the term pasteurization and regulatory approvals will lead to the wider acceptance of this technology. Education and skilled marketing efforts are needed to remedy this lack of awareness of the effectiveness, safety, and functional benefits that irradiation can bring to foods.

About The Author
Tatiana Koutchma is a research scientist at Agriculture and Agri-Food Canada, focusing on research related to novel processing technologies and development of new processes for industry. She is an internationally recognized expert of novel processing technologies including high pressure, ultraviolet light, and other advanced thermal and non-thermal methods.