This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on February 15, 2012.
Irradiation is the process by which an object is exposed to radiation. Food irradiation, a nuclear technique for food preservation, is the result of decades of research conducted in various parts of the world, exploring the possibilities of using radiation as a way of alleviating the food deficiencies.
Food irradiation is an industrial process and must be carried out in an industrial irradiation facility. This consists of a large shielded chamber into which a field of ionizing energy can be introduced from a suitable source. The shielding confines the ionizing energy field, protecting nearby living things from harm. Processing consists of conveying the food packages into the chamber and leaving them there for a specified period of time, to absorb a carefully controlled dose of radiation. To complete the process the food is then conveyed out of the chamber. The food can immediately be utilized for whatever purpose it is intended.
All commercial irradiators have four primary components, a source of radiation, a method of product conveyance, “shields” to prevent exposure of personnel and the environment to radiation and safety systems. Ionizing radiation is penetrating energy and thus, products are usually irradiated after they are fully packaged. Below is a description of the four types of irradiators that are commercially available or in use today for food processing. The choice of which irradiator is most cost effective for a particular product depends on the type of product, how it is packaged, the product dose, dose uniformity requirements and, most important, logistics.
1. ELECTRON BEAM IRRIDATOR (EMPLOYING A RADIATOR CHAMBER):
The source of electron beams is an “accelerator”. Accelerators generate and accelerate electrons very fast towards the food product being irradiated. Because electrons have mass, they can only penetrate about 1.5 inches (3.8 cm) into a typical food product or about 3.5 inches (8.9 cm) if the food product is irradiated on both sides. Electrons also have an electric charge. This charge allows the stream of accelerated electrons to be scanned by magnets to track across the product. A commercial food electron beam irradiator accelerates the electrons to energy of up to 10,000,000 electron volts (10 MeV). Electron beam irradiators typically use massive concrete, steel or lead shielding. Electron Beam accelerators can be turned on and off. Safety interlocks ensure that a person cannot enter the radiation chamber where the food is being irradiated when the accelerator is “on”. Product is usually passed through the scanned “beam” on roller type conveyors.
2. GAMMA IRRADIATOR (EMPLOYING A RADIATION CHAMBER):
The source of photons in a gamma irradiator is cobalt-60. Unlike electron beams that are generated on site using electric power, cobalt-60 is produced off site in nuclear reactors and transported in special shipping containers (“casks”) to the site. Cobalt-60 is a solid radioactive metal that is contained in two welded encapsulations of stainless steel creating a “Sealed Source”.
The sealed source contains the “Radioactive” cobalt-60, but allows the photons “Radiation” to pass through the encapsulations and ultimately into the food product. Because Cobalt-60 photons have no mass, they can penetrate more than 24 inches (60 cm) of food product if irradiated on both sides. Gamma irradiators that employ a radiation chamber typically have shields made out of massive concrete or steel. Cobalt-60 continuously emits radiation and cannot be turned “off”. To allow personnel access to the chamber, the source is lowered into a storage pool of shielding water when it is not being used to irradiate product. The shielding water does not become radioactive. Safety interlocks are used to assure that a person cannot enter the chamber when the source is not in the stored position (at the bottom of the pool of water). Hanging carriers, totes and roller conveyors are typically employed to move the product through the chamber.
3. GAMMA IRRADIATOR (UNDERWATER):
Like the radiation chamber irradiator above, an underwater gamma irradiator uses cobalt-60. Unlike a radiation chamber irradiator, an underwater irradiator stores the cobalt-60 permanently at the bottom of a pool of water. Instead of raising the cobalt-60 into a shielded chamber, the product, placed in water free containers, is lowered to the bottom of the pool adjacent to the cobalt-60 to receive a dose of radiation. The water acts as the shield. The shielding water does not become radioactive. No above ground shielding or radiation chamber is present. There is no need for interlocks to prevent personnel from entering a radiation chamber when the cobalt-60 is present, because there is no radiation chamber. Typically, the product is loaded into water free containers and the containers are lowered/raised using a hoist mechanism; and
4. X-RAY IRRIDATOR (EMPLOYING A RADIATOR CHAMBER):
X-rays are photons and have similar properties to gamma rays emitted by cobalt-60. X-rays are generated by using an electron beam accelerator (above) and converting the electron beam (up to 7.5 MeV) to photons by accelerating the electrons into a high-density material such as tungsten, steel or tantalum. The sudden deceleration of the electrons generates x-rays and waste heat.
The creating of the radiation is very similar to an electron beam irradiator (above), including the ability to be turned on and off. The shielding and product conveyance are similar to that of a chamber type gamma irradiator (above). The safety interlocks are similar to both electron beam and chamber type gamma irradiators. The advantages of x-rays over electron beams are that they have good product penetration (over 24 inches or 60 cm of food product if irradiated on both sides). The advantages of x-rays over both types of gamma irradiators are that they do not require a shielding storage pool. However, there is a substantial loss of energy during the conversion process. Thus, it suffers a severe cost disadvantage when compared to other types of irradiators for the same product volume throughout.
Various authors believe that food losses through spoilage after harvest can be estimated equivalent to the production from 12 million acres of land, or 33 million tons of grain. Another estimate is that the world food supply could be increased by 25- 30 percent, if post-harvest losses could be avoided. Such losses represent losses in soil fertility, manual labour and monetary expenditure as well as loss of product. Losses occur especially in the tropical regions where most developing countries are situated. Naturally, these losses could either be the result of the available food preservation technologies in those countries do not function efficiently in such environments or simply because they do not lend themselves to the habits of food consumption in most developing countries. People in such countries are accustomed to buying fresh food for immediate consumption at home. They would welcome any new technology, which would keep food fresh for a longer time.
Post-harvest losses in countries of the Asian region, for example, are estimated at 30 percent for grains, 20 percent to 40 percent for fruits and vegetables, and up to 50 percent for fish. In Africa, a conservative estimate shows that a minimum of 20 percent of total food production is lost after harvest. Losses of perishable items such as fruits, vegetables, and fish, for example, are even higher than 50 percent. The US National Academy of Sciences has estimated that the minimum post-harvest food losses in developing countries amounted to more than 100 million tonnes at a value surpassing US $10 billion in 1985.
Food-borne diseases continue to affect adversely the health and productivity of populations in most countries, especially in developing ones. Contamination of food – especially of animal origin – with microorganisms, particularly pathogenic nonperforming bacteria, as well as infection with parasitic helminthes and protozoa, are important public health problems and causes of human suffering and malnutrition. According to the World Health Organization (WHO), infectious and parasitic diseases represented the most frequent cause of death (35 percent) worldwide in 1990, with the majority of deaths occurring in developing countries. These diseases include malaria, diarrhea, tuberculosis, measles, pertussis, and schistosomiasis. Diarrheal disease caused about 25 percent of deaths in developing countries. It is estimated that in possibly up to 70 percent cases food is the vehicle for transmission of diarrheal diseases.
Moreover, recently 15 countries in Latin America have reported some 400 000 cases of cholera and more than 4000 deaths. The most important cause of transmission of the disease was the consumption of contaminated water and food. Elsewhere, 7 million people in the northeastern provinces of Thailand, 3 million in the Republic of Korea, and millions more in China are infected by liver fluke parasites from consumption of raw freshwater fish. The economic losses caused by these diseases in these countries are estimated to be hundreds of millions of US dollars annually.
Consequently, food safety has to be a major consideration for consumer acceptance and willingness to pay for irradiated products. For instance, according to a report, public health in the USA is burdened with foodborne diseases and 76 million cases are reported each year, which is translated to:
- 1 in 4 Americans gets a foodborne disease each year;
- 1 in 1000 Americans is hospitalized each year; and
- $6.5 billion in medical and other costs.
The figures presented in the graph (Figure 03) about illness, hospitalization, and deaths, illustrate the impact of foodborne diseases like E.coli0157, Listeria, and Salmonella.
It is reported that if 50 percent of meat and poultry were to be irradiated in the USA, the impact will be:
- 880,000 fewer cases;
- 350 fewer deaths; and
- 8,500 fewer hospitalizations.
While there appears to be a consensus about the fact that it is better to conserve what is produced than to produce more to compensate for subsequent losses, demand is also growing in both the developed and the developing countries for food which is wholesome and which has a long shelf life. There are obvious reasons for using radiation to preserve food and agricultural produce, and hence to alleviate the world’s food shortage and to produce safe foods. There is a strong possibility that in addition to practicing conservation, more food will have to be produced in order to be in the position to feed the growing global population.
Irradiation food processing, has several uses, most of which prolong the useful life of foods in the condition in which they have been obtained. Food irradiation processing induces virtually no temperature rise in the treated products, and is therefore often termed as a “cold” process. Fish, fruits, and vegetables remain fresh, and the physical state of frozen or dried commodities is unchanged. The agents causing spoilage (bacteria, insects) are reduced in numbers or eliminated from packaged food, and if the packaging materials are impermeable, the food is not re-contaminated. Irradiation of packaged food has a particular bearing when hygiene is difficult to maintain, as is often the case, for example, in tropical conditions.
Many studies have been conducted since the late 1940s to evaluate the nutritional value of irradiated foods and to determine the safety of the process. The treatment process can be designed and controlled so that the following benefits can be realized safely without any significant reduction in the nutritional value of the food:
- Extend shelf life of food by destroying the micro-organisms that cause spoilage;
- Make food safe to eat by destroying parasites and micro-organisms that cause trichinosis and salmonella poisoning;
- Provide quarantine treatments for fruits and vegetables to ensure insect pests are not transported across borders; and
- Prolong the shelf life of foods by slowing the ripening process and inhibiting the sprouting of root vegetables like potatoes and onions.
The good news is that more than 60 countries worldwide including the USA approved the use of radiation and these countries have been using radiation to preserve food for over the past five decades. In France and the Netherlands, large quantities of seafood, vegetables, fish, and frog legs are irradiated. Other countries actively involved in food irradiation include Brazil, Mexico, Japan, Belgium and Israel. In the United States, irradiated hamburger patties are sold in every state. Papayas are irradiated in Hawaii and imported to the mainland.
In agriculture, radiation has eradicated approximately 10 species of pest insects. Food irradiation does not make the food radioactive, and it does not change the food any more than canning or freezing. The irradiation process exposes food to gamma rays from cobalt-60, a radioisotope of cobalt. Sometimes, the process uses electron beams or X-rays to produce the gamma rays.
Outbreaks of food-borne diseases have been associated with all types of foods and pathogens can be transferred to foods from different sources of contamination that arise from product handling, processing and preparation. Food irradiation acts by damaging the target organism’s DNA beyond its ability to repair. Microorganisms can no longer proliferate and continue their malignant or pathogenic activities. Spoilage-causing microorganisms cannot continue their activities. Insects do not survive, or become incapable of reproduction. Plants cannot continue their natural ripening processes.
Following an outbreak of illness traced to contaminated salad greens, the Federal Department of Agriculture (FDA) in 2008 issued a final rule approving irradiation of iceberg lettuce and spinach. The purpose was to help protect consumers from infection by such bacteria as salmonella and E. coli, the FDA said. The foods affected by the rule are loose, fresh iceberg lettuce and spinach and bagged iceberg lettuce and spinach. The FDA previously had approved irradiation of these foods to kill insects and delay spoilage. However, the doses needed for those purposes are too low to destroy most disease-causing bacteria.Here is a partial list of approved current uses of irradiated food in the USA and Canada:
a) United States:
- Wheat/Wheat Flour;
- Pork and Chicken;
- Red Meats;
- Shell Eggs;
- Pet Foods;
- Food for Space Program;
- Pending Approval:
- Ready-to-Eat Food
- Dehydrated Seasoning;
- Pending Approval:
- Prawns; and
As prolonged heating is not an appropriate treatment for all foodstuffs, food irradiation is an alternative approach for food processing and treatment. One of the significant advantages of irradiation technology is that it destroys microorganisms without significantly increasing temperature. Irradiation can be applied to fresh vegetables, fruits and frozen foods with no significant change to taste or texture. It can also be used to treat foods that have been cooked conventionally and packaged ready for distribution to consumers. Another advantage of irradiation is that it destroys spoilage causing organisms, helping to keep meat, poultry and seafood fresh for longer.
It is worth mentioning that irradiated food cost a few cents more per pound to the cost of production. However, food prices would not necessarily rise just because a product has been irradiated. In some cases, extended shelf life produces offsetting savings. A study conducted by the USDA Economic Research Service and the University of Florida found that consumers are willing to pay more for a safer food product.
Irradiated foods destined for the retail store have a label or a sign indicating that they have been irradiated. This includes the internationally recognized symbol called the “radula”. Foods that contain irradiated ingredients or foods served in restaurants do not have to be identified as being irradiated.
- Canadian Institute of Food Science and Technology;
- Food Irradiation Processing Alliance;
- IAEA – Food Irradiation Makes Progress;
- IAEA – Food Irradiation in Developing Countries: A Practical Alternative;
- Consumer Acceptance and Willingness to Pay for Irradiated Products;
- Canadian Nuclear Association – Why Food Irradiation;
- Nuclear Energy Institute – Food & Agriculture; and
- IAEA Nuclear Technology Review.