Preservation of Food with Ionizing Energy
Overview The exposure of food to ionizing energy, more commonly known as food irradiation, is a preservation technology that has generated much public debate. In this lesson you will learn important definitions relating to the technology. The types of sources of ionizing energy and irradiator layout will be introduced. You will learn about the current regulations regarding the use of ionizing energy for food preservation in Canada and in other countries. We will explore a number of issues and controversies about the use of ionizing energy in the food industry. When you have completed this lesson, you should have a sound understanding of ionizing energy as a preservation technology and be familiar with the issues that have been raised in the popular press.
Objectives: Upon completion of this lesson you will be able to:
- Understand the concept of food irradiation as a food preservation method;
- Define key terms used in conjunction with preservation of food with ionizing energy;
- Describe the principles for determining the required irradiation dose depending on the desired outcome
- Describe the principles for determining wholesomeness and safety of irradiated foods
- Learn about current regulations, and what food products are approved for irradiation in Canada versus other countries
- Articulate a personal set of values pertaining to the use of ionizing energy in food preservation
- Smith, J.S. and Pillai, S. 2004. Irradiation and Food Safety. (A scientific status summary). Food Technology, 58(11): 48-55 http://www.ift.org/food-technology/past-issues.aspx
- Division 26, Food Irradiation. Food and Drugs Act, and the Food and Drug Regulations. Ottawa. http://laws.justice.gc.ca./eng/regulations/C.R.C.,_c._870/index.html
- Guide to Food Labelling and Advertising. Section 2.14.1. Food Irradiation. Canadian Food Inspection Agency. http://www.inspection.gc.ca/english/fssa/labeti/guide/ch2ae.shtml
- Food Irradiation by the Canadian Food Inspection Agency. Nov. 2002. http://www.inspection.gc.ca/english/fssa/concen/tipcon/irrade.shtml
- Frequently Asked Questions Regarding Food Irradiation on the Health Canada website (last updated 2002/11/25) http://www.hc-sc.gc.ca/fn-an/securit/irridation/faq_food_irradiation_aliment01-eng.php
- 1 Introduction
- 2 Radiant Energy
- 3 Food Irradiation
- 4 Preservation Principle
- 5 Concerns about Food Irradiation
- 6 Factors Affecting Food Irradiation
- 7 Potential Applications of Radiant Energy
- 8 Consumer Acceptance
- 9 Regulations and Use of Ionizing Energy in Canada
- 10 Final Thoughts
Aside from the use of genetically modified organisms in foods, there is perhaps no other method of food preservation that has generated as much heated debate recently as food irradiation. Although the basic concept of food irradiation as a food preservation technology is not new (a patent for food preservation by irradiation was applied for in the United States in 1921), it is nonetheless a controversial method of food preservation.
Much of the controversy about food irradiation seems to stem from a fear of the unknown and unfamiliar - we have often seen references in media reports of food irradiation associated with radioactive fallout, accidents at nuclear power plants and concerns about nuclear war and weapons testing. This has led to further confusion in the minds of the general public about food irradiation and what the process actually involves. Upon completing this lesson, you will have a good knowledge of what food irradiation is, what its potential applications are, the physical process of food irradiation, and limitations and advantages of food irradiation as a method of food preservation.
Radiation refers to the emission and propagation of energy through matter or space by electromagnetic disturbances. These forms of energy are found within the the electromagnetic spectrum of radiation (Figure on right). This is an organized scale where we find energy ranging from radio waves, microwaves, visible light to ionizing radiation. Each of these forms of energy vary in frequency, wavelength, energy value, penetrating power, and their effects on biological systems.
The longer wavelengths of electromagnetic energy that we are familiar with include visible light, infrared and ultraviolet rays. These are characterized by having low penetrating power. Microwaves and infrared radiation are two examples of the longer wavelengths in the electromagnetic spectrum.
- Microwaves are used in food for their heating properties. The microwaves travel in straight lines and pass through air, glass, paper and plastic, but reflected by metals. They are readily absorbed by water (polar molecule), causing it to vibrate. Heat is generated by the intermolecular friction generated from the vibrating water (polar) molecules in food. Microwaves are absorbed by food up to a depth of 5 to 7.5 cm.
- Infrared energy can generate heat. They can reach temperatures above 100°C. Typical examples of infrared energy can be seen in ovens, toasters, and even those "infrared" lamps used to keep food warm.
Food Irradiation is the application of radiation, in the form of ionizing energy, to foods. According to Health Canada, Food Irradiation means "the treatment of food with ionizing radiation" from the following sources (read the full definition in Division 26 of the Food and Drug Regulations):
- Gamma radiation from a Cobalt-60 or Cesium 137
- X-rays generated from a machine source operated at or below 5 MeV
- Electrons generated from a machine source operated at or below 10 MeV
Important Definitions pertaining to food irradiation:
- Gamma radiation is electromagnetic radiation that has very short wavelengths, similar to "short" x-rays. Isotopes such as cobalt 60 and cesium 137 emit gamma radiation as they disintegrate. The energy level of the gamma radiation emitted by these isotopes will not induce radioactivity in food materials.
- X-rays are electromagnetic radiations that are highly energetic and of short wavelength. X-rays are produced by x-ray machines that emit a beam of fast electrons which hit a metal target in a vacuum. X-ray machines emit radiation only when the machines are turned on.
- Electrons with high-energy (speed) may also be generated by an electron beam accelerator. Electron Beam Accelerators need only electricity to operate and produce no waste materials
- The Gray is a unit of energy absorbed by a food irradiated with ionizing radiation. One gray (Gy) is equivalent to the absorption of 1 joule of energy by 1 kilogram of food. One thousand grays equals one kilogray (kGy). Most of the ionizing radiation processes permitted around the world involve absorbed doses of <10 kGy.
- The Rad is another unit used to express the radiation absorbed dose (rad), where 100 rads = 1 Gy. However, the preferred unit is the kGy (above)
Atomic Energy of Canada Limited (AECL) is a Canadian federal crown corporation that is a leading agency in the development of food irradiators that use cobalt 60 as the energy source. The Canadian Nuclear Safety Commission (CNSC) (formerly the Atomic Energy Control Board or AECB) regulates the use of nuclear energy and material in Canada.
Cobalt 60 is produced in the Canadian-built Candu reactors. It is contained within the stainless steel rods that are used to control the rate of nuclear fission and as such is not extracted from spent nuclear fuel. In this context, the cobalt is housed within stainless steel rods which are transported to a facility near Ottawa where the cobalt 60 pellets within the rods are recovered and then reassembled in stainless steel rods to be used as the energy source in food irradiators, irradiators for sterilizing medical supplies (bandages, specimen containers) and devices, or for use in irradiators utilized for cancer therapy.
The figure on the right shows a typical diagram of a irradiation facility. In such a facility the food pre-packaged in boxes that are loaded into a pallet carrier where a conveyor system moves the food in the pallets. The pallets are carried into a chamber with irradiation source (eg. Cobalt 60). The pallet carriers travel through the irradiator room and around the Cobalt 60 source at a speed such that the required absorbed dose is attained. Absorbed dose depends on amount of time food is exposed to the irradiation source. Dosimeters are placed with the food to measure the dose received (absorbed) in kGy. The irradiated pallets of food exit to the unloading station which is physically separated from the loading station so that treated and untreated foods do not become intermixed.
Where are the (food) irradiation plants in Canada?
Most of the irradiation facilities in Canada process medical and personal care supplies. MDS Nordion (Laval, Quebec) and Iotron (Port Coquitlam, BC) process some dry food ingredients. The former uses gamma rays, while the latter uses electron beam technology. The Canadian Irradiation Centre (CIC) is a training centre operated as a joint venture by MDS Nordion and the Université du Québec, Institut Armand-Frappier (IAF).
Approved Applications in Canada
Though irradiation has a wide range of applications, only four food (categories) irradiated may be sold in Canada. The table below shows a list of irradiation applications and indicates those approved in Canada.
|Food||Dosage||Purpose||Approved in Canada?|
|Potatoes||0.15 kGy max.||Sprout inhibition||Yes|
|Onions||0.15 kGy max.||Sprout inhibition||Yes|
|Pork||0.3 to 1.0 kGy||Trichinella spiralis||No, approved in the US|
|Wheat, Flour, Whole Wheat Flour||0.75 kGy max.||Insect infestation control||Yes|
|Fruit and Vegetables||1.0 kGy max.||Disinfestation, ripening delay||No, approved in the US|
|Fresh Shell Eggs||3.0 kGy max.||Salmonella control||No, approved in the US|
|Dehydrated Seasoning Preparations||10.00 kGy max.||Microbial control||Yes|
|A newly approved food as of Feb 22, 2017 (study info)||Dosage?||Purpose?||Yes, as of Feb 22, 2017|
The changes induced in food by absorption of ionizing energy can arise from both direct and indirect effects. Please read the required reading Irradiation and Food Safety for details.
Microorganisms may be killed by a direct effect of the ionizing energy upon genetic material within the microbial cells that leads to death of the microorganism. As mentioned in the required reading (Irradiation and Food Safety) "the damage occurring from ionizing radiation can be random and extensive, making DNA repair near impossible". In some cases, even relatively small changes in the DNA can destroy bacterial cells, and the disruption of genetic material in living cells by irradiation also enables destruction of insects, inactivation of parasites, delaying of ripening, and prevention of sprouting.
The basis of food preservation by treatment with ionizing energy is the ability of the absorbed quanta of energy to dislodge electrons from molecules with the concomitant creation of free radicals without inducing radioactivity in the food.
When ionizing energy from sources permitted for use in food irradiation is absorbed by food and collides with a molecule or atom, an ion-pair is produced if the energy from the collision is sufficient to dislodge an electron from an atomic orbit. This phenomenon can lead to breaking up of one or more bonds between atoms in the molecule, leading to new molecular fragments possessing unshared electrons (free radicals). Because of the unshared electron, free radicals are extremely reactive, and tend to react with other free radicals or other molecules with unshared electrons.
One out of every six billion chemical bonds in bacteria or food molecules are broken by irradiation as indicated on right. The formation of ion pairs and free radicals, the reaction of free radicals with one another or other molecules, and the chemical and physical phenomena that occur as a consequence of these events, form the mechanisms for the inactivation of microorganisms, enzymes and alterations of food constituents during food irradiation.
Irradiation also lead to the formation of hydrogen peroxide (H2O2), an antimicrobial agent that kills bacteria, yeasts and moulds in foods.
Concerns about Food Irradiation
Presence of Free Radicals
It is true that free radicals are produced in foods during irradiation. However, free radical formation is not unique to foods which have been irradiated with ionizing energy. For example, oxidative reactions in foods containing unsaturated fats also involve free radical formation, and free radicals are also formed during the course of the Maillard browning reactions. Free radicals are also produced within our bodies and other living tissues during normal metabolism. Mechanisms (chemical and enzymatic) for inactivation of free radicals exist within the human body and other living tissues.
Irradiation using approved sources provides enough energy to knock an electron from the outer orbit (that is why it is termed "ionizing radiation" or "irradiation"); however, it does not have sufficient energy to penetrate the nucleus and eject neutrons, which would be required to induce radioactivity. Therefore food will NOT become radioactive by irradiation conducted using approved energy sources.
To become radioactive, food would need to be exposed to a minimum of 15 MeV of energy. The energy output of Cobalt 60, Cesium 137, and e-beam accelerators is carefully regulated. Currently, the maximum energy outputs allowed are 5 or 10 MeV, which are too low to induce radioactivity in food.
You may be interested to know that all foods are naturally radioactive, although of course at a very low level. This low background level of radioactivity arises from the naturally occurring isotopes in elements such as carbon, phosphorus, potassium and sulfur.
Another concern expressed about food irradiation is the possible formation of unique radiolytic products. However, in fact the molecular changes in foods treated with ionizing energy are not usually unique or distinct from those found in non irradiated foods such as those treated by thermal processing. The few unique radiolytic products that have been found are at such trace levels, that they are not considered to be of any significance, and toxicological studies have not found evidence of any harmful effects.
Other changes can occur in foods during irradiation with ionizing energy via indirect effects. These changes can involve the radiolysis of water molecules to produce reactive hydroxyl radicals, or reactions in foods of peroxides and peroxide free radicals with fats, leading to lipid oxidation (rancidity). Some vitamins are also sensitive to radiation. The extent of effects on both macronutrients and micronutrients is of course dependent on the dose of irradiation. Page 51 of Irradiation and Food Safety describes the effects of food irradiation on macro- and micro- nutrients. It also describes some of the sensory changes that have been perceived and their implications with the consumer.
Factors Affecting Food Irradiation
The factors that need to be considered and controlled during food irradiation include:
- Safety and wholesomeness of the foods
- Resistance of food to irradiation
- Resistance of microorganisms to ionizing energy
- Resistance of enzymes to ionizing energy
Safety and Wholesomeness of Irradiated Foods
In Canada, the Health Products and Food Branch of Health Canada considers issues about safety and wholesomeness of irradiated foods, in addition to the safety and wholesomeness of foods preserved by other food preservation methods. The Canadian Food Inspection Agency considers aspects related to labelling of irradiated foods. The issues of safety and wholesomeness of irradiated foods revolve around criteria of the following four principles:
- Radiological safety: ensuring that foods do not become radioactive during irradiation
- Toxicological safety: ensuring that production of toxic and possibly carcinogenic substances does not occur
- Microbiological safety: ensuring the efficacy of the radiation process with respect to the ability of the prescribed absorbed dose to kill disease-causing microorganisms that could be in the food
- Nutritional adequacy: ensuring that undue losses of nutrients do not occur as a consequence of treatment of food with ionizing energy
The conclusions drawn by the Canadian and international regulatory agencies about food irradiation are that foods treated such that the absorbed dose is below 10 kGy do not contain toxicants at undesirable levels. That is, the irradiated foods which have absorbed a dose of less than 10 kGy are wholesome and safe for long-term consumption.
- In 1997, a Study Group was convened by the World Health Organization(WHO), the Food and Agriculture Organization (FAO), and the International Atomic Energy Agency (IAEA), to evaluate wholesomeness of food irradiated with doses above 10 kGy. In a report published in 1999, "The Study Group concluded that food irradiated to any dose appropriate to achieve the intended technological objective is both safe to consume and nutritionally adequate ... Accordingly, irradiated foods are deemed wholesome throughout the technologically useful dose range from below 10 kGy to envisioned doses above 10 kGy" (WHO Technical Report Series 890). Applying the concept of "substantial equivalence", even high-dose irradiated foods are considered to be as safe as foods sterilized by conventional thermal processing, such as canning of low-acid foods.
Resistance of Foods to Ionizing Energy
Not all foods are amenable to preservation by treatment with ionizing energy. The same can be said for thermal processing, freezing and dehydration as methods of food preservation. The quality of some foods may be adversely affected by irradiation, depending on the dose, temperature and conditions during irradiation. For example, colour, flavour or textural changes may result after exposure of food components to ionizing energy. Lipids or fats are particularly susceptible to oxidative reactions triggered by the radiolytic reactions and presence of free radicals. Losses of some vitamins may also occur; vitamins A, C, E and B1 (thiamine) are the most sensitive, particularly at higher doses and in foods packaged in air.
As mentioned above, these changes may be minimized by irradiating foods in the frozen state, in a vacuum, and/or with the addition of radical scavengers such as ascorbic acid. Additional strategies include applying the lowest effective irradiation dose and choosing appropriate packaging in terms of moisture and oxygen barriers.
Resistance of Microorganisms to Ionizing Energy
As in the case of thermal processing (pasteurization, commercial sterilization), microorganisms vary in their resistance to the killing effects of ionizing energy. Analogous to thermal processing where Clostridium botulinum is the most heat resistant pathogen, C. botulinum spores are the most radiation resistant forms of pathogenic bacteria.
- You will recall that we discussed the decimal reduction time (D-value) in Lesson 6 in conjunction with thermal processing of foods. Similarly, it is possible to determine the dose of ionizing energy necessary to effect a 90% destruction of the particular microorganism in question. When we calculated the D-value in the lesson on thermal processing, we referred to the length of time at a constant temperature required to create a 90% decrease in the population of the microorganisms or spores in question. That time at a constant temperature could be converted to an absorbed dose of thermal energy.
With food irradiation, we calculate the absorbed dose of ionizing energy that produces a 90% decrease in the microbial population (D10 values). To achieve an appropriate margin of safety, a 5D or a 12D radiation treatment would have to be applied to acid and low acid foods, respectively. Although the source of the energy and the mechanisms by which microorganisms and spores are killed are different, the same concept (decimal reduction value) is applied during determination of the efficacy of thermal processing and preservation of food with ionizing energy.
|Take a look at D10-values of some important foodborne pathogens, listed in Table 1 of the IFT Scientific Status Summary on "Irradiation and Food Safety" by Smith and Pillai:
The D10-values for E. coli 0157:H7 (in ground beef patties, at 5oC) and for Salmonella spp. (in in turkey breast meat at 5oC) are 0.27-0.38 and 0.71 kGy, respectively.
Is it true that irradiation can mask food spoilage?
Irradiation cannot be effectively used to mask or cover up food spoilage since the microorganisms can be easily killed but the spoilage odours, off-flavours and colour changes caused by the spoilage microorganisms can not be changed or eliminated by ionizing radiation. Thus, claims that ionizing radiation can be used to mask signs of poor quality in food are untrue.
What about microbiological safety?
There has been concern about the creation of "superbugs" or mutants that are more dangerous but this is not a significant issue at the doses of ionizing radiation used in food processing.
Resistance of Enzymes to Ionizing Energy
The majority of food enzymes are more resistant to ionizing energy than spores of C. botulinum. The term DE (D-enzyme) is used to determine the radiation dose that produces a 90% reduction of enzyme activity. The DE values are of the order of 5 Mrad. Four DE values (5 x 4 = 20x106 rad or 200 kGy, 100 rad is equivalent to 1 Gray of absorbed ionizing energy and 1000 Gray equals 1 KGy) would produce nearly total enzyme destruction; however, 200 KGy would also destroy many food constituents!
From this calculation, you will have noted that enzymes cannot be easily inactivated by treatment with ionizing radiation. Ionizing energy could never be used for blanching vegetables. One of the concerns expressed by groups opposed to food irradiation is that enzymes in food are destroyed by exposure to ionizing energy. Clearly, from the example above, that is not the case especially if you consider the maximum dose permitted in Division 26 of the Food Regulations of Canada is 10 kGy.
If vegetables were to be preserved with ionizing energy, they would first have to be blanched with heat followed by treatment with ionizing energy to inactivate the microorganisms of concern.
After the issues of safety and wholesomeness have been satisfied, economic factors must be considered in evaluating the feasibility of an application of food irradiation. Food irradiation may be economically viable if it results in substantial increases in storage life and therefore marketing time and decreases in post-harvest or catching losses. This may be the case in terms of radicidation (discussed below) treatments of fresh fish or some fresh fruits. In cases where the process does not offer advantages (such as nutrition retention, technological advantages, economic advantages) it would not be economically viable.
Potential Applications of Radiant Energy
The following examples illustrate various potential applications of ionizing energy in food preservation:
- Much of the food consumed by American and Russian astronauts has been preserved with ionizing energy.
- Irradiation has proven effectiveness in the elimination of Salmonella from cut-up, packaged chicken. The chicken has received good consumer acceptance in test market trials in the United States.
- Use of low dose radiation was approved in the United States, in December 1997, for general use in irradiation of ground beef to eliminate the bacteria (Escherichia coli O157:H7) that cause "hamburger disease".
- A similar application is under consideration by Health Canada to allow irradiation of ground beef, poultry, shrimp and prawns, and mangoes.
The ultimate factor which will determine the economic viability of preserving foods with ionizing energy is consumer acceptance. In North America, the jury is still out as to whether radiation preserved foods would be accepted by consumers. Results of recent consumer surveys, described under Consumer acceptance in the Scientific Status Summary by Smith and Pillai, suggest acceptability rates ranging from 45% to over 90%. However, there are lingering concerns expressed by anti-irradiation groups, including the use of food irradiation to overcome poor sanitation practices and environmental concerns related to irradiation facilities.
Regulations and Use of Ionizing Energy in Canada
As you noticed in the required reading, Irradiation and Food Safety, in the U.S. food irradiation is regulated as a food additive. This is another good example of the differences that exist in the food additive definition between Canada and the U.S. (reviewed in Lesson 4).
In Canada, the use of ionizing energy for irradiation of food is considered as a process and is regulated under Division 26 of The Food and Drug Act and Regulations. Note that the sources and energy levels of ionizing energy that would be permitted for use in Canada are clearly defined (Section B.26.001). Specified types of information would have to be submitted to the Health Products and Food Branch of Health Canada when an application is made to treat a specific food commodity with ionizing energy (B.26.004). Specific records would have to be maintained by a food processor employing ionizing energy (B.26.005).
The various options suggested for labelling of irradiated foods are presented in Section 2.14.1 of the Guide to Food Labelling and Advertising from the CFIA. Note that in addition to the mandatory basic labelling (Lesson 4), food treated with ionizing radiation MUST also include:
- A statement indicating that the food has been treated by irradiation, or treated with radiation, or irradiated.
- The radura symbol is also used to indicate that a food has been irradiated (see below).
- If an irradiated food is used as an ingredient of another food, it must be declared as irradiated in the ingredients listing only if it constitutes 10% or more of the final food
|Visit the Fact Sheeton Food Irradiation by the Canadian Food Inspection Agency. There is also a useful link at the end of the fact sheet to Frequently Asked Questions Regarding Food Irradiation on the Health Canada website.
- What are the risks and benefits of the application of ionizing radiation or irradiation as a food preservation technology?
- What is your personal stand on irradiated foods?
FNH 200 Course content on this wiki page and associated lesson pages was originally authored by Drs. Brent Skura, Andrea Liceaga, and Eunice Li-Chan. Ongoing edits and updates are contributed by past and current instructors including Drs. Andrea Liceaga, Azita Madadi-Noei, Nooshin Alizadeh-Pasdar, and Judy Chan.