Irradiation of foods can increase food safety by helping to prevent food-borne illness, controlling insect infestation, and extending product shelf-life. Irradiated foods are safe, wholesome, and nutritious. Irradiation is endorsed by regulatory agencies and health organizations, and has gained approval for a variety of foods worldwide. However, consumer education is needed for the successful marketing of irradiated foods. This article describes food irrradation from a US industry perspective.
by Stefan Ehling, Shannon Cole, and Jeffrey Barach
Principles of food irradiation
Food irradiation is the process in which food products are exposed to a controlled amount of radiant energy to kill harmful bacteria, control insects and parasites, reduce spoilage, extend shelf-life, and inhibit ripening and sprouting.
The dose of irradiation is measured in units known as the Gray (Gy) and its multiple, the kiloGray (kGy). This is a measure of the amount of energy transferred to the food, a microbe or other substance being irradiated (1 Gy = 1 Joule/kg). The amounts of radiation needed to destroy various microorganisms are listed in Table 1. Lethality due to irradiation depends on several factors such as the density of the food, the temperature, the level of moisture and the atmosphere in the food package. Synergy between irradiation and other sanitising agents, such as chlorination, has been observed.
Three different irradiation technologies are used for foods.
Gamma rays use the radiation given off by a radioactive substance, such as Cobalt-60 or Caesium-137, and can penetrate foods to a depth of approximately 50 cm. This technology has been used routinely for more than 30 years to sterilize medical, dental and household products, and it is also used for the radiation treatment of certain cancers.
Electron beams are streams of high energy electrons, propelled out of an electron gun and can penetrate food only to a depth of about three centimeters. Two opposing beams can treat food that is twice as thick. Electron beam medical sterilizers have been in use for at least 15 years.
X-ray irradiation is the newest technology, and is also being commercially developed. Like gamma rays, X-rays can pass through thick foods, and require heavy shielding for safety [1].
Benefits of food irradiation
Pathogen reduction. Irradiation within approved doses (typically 1-10 kGy), has been shown to generally destroy at least 99.9% of common food-borne organisms, including pathogens such as Salmonella spp., Campylobacter jejuni, Escherichia coli 0157:H7, and Listeria monocytogenes (Lm). Irradiation has been shown to effectively reduce viable E. coli cells internalized in lettuce, against which surface treatments are less effective. It is also effective against Vibrio species associated with seafood and against parasites such as Toxoplasma gondii and Trichinella spiralis.
The effect of irradiation at typical doses is comparable to that of heat pasteurization, so irradiated food must still be properly refrigerated and cooked prior to consumption. Irradiation is an effective critical control point in a Hazard Analysis and Critical Control Points (HACCP) system.
Spoilage reduction. Low doses of radiation (up to 1 kGy) can prolong the shelf-life of many fruits and vegetables by reducing the number of spoilage bacteria and mould, and by inhibiting sprouting and maturation. Irradiating strawberries extends their refrigerated shelf-life to up to three weeks without decay or shrinkage, versus three to five days for untreated berries. An extended shelf-life may offset the slightly increased cost of production of irradiated foods. Reducing the normal, spoilage microflora of meats through irradiation does not enhance the growth of pathogens such as Escherichia coli 0157:H7 or Salmonella spp.
Irradiation is not particularly effective against pre-formed microbial toxins and mycotoxins. However, irradiation is effective in preventing the production of patulin during storage of apple juice. Actual mycotoxin production in fruits, grains, beans, and peanuts during storage decreased with increasing radiation dose. Although irradiation at 6-10 kGy had no effect on deoxynivalenol (DON) levels in raw barley, DON decreased by 60-100% in finished malts prepared from treated barley.
Insect control. Irradiation of spices, herbs, and dry vegetable seasonings is an alternative to the use of chemicals or fumigants, such as ethylene oxide and methyl bromide, that can impact the environment [2].
Allergen modification. Irradiation reduces the allergenicity of crab protein, may increase (or decrease) that of shrimp protein in a dose-dependent manner, increases the allergenicity of wheat gliadin, yet has no effect on nut proteins.
Consumer acceptance of irradiated foods
Studies in the US have shown that, in general, Caucasians, males, and persons with a college education are more likely to purchase irradiated foods than are Hispanics, minorities, and those with lower education. Individuals who consume more beef have less concern about the use of irradiation. Acceptance of irradiated foods is greatest in certain upscale markets. Results of a 1994 survey conducted in the Atlanta metro area indicate that 72% of consumers were aware of irradiation and 45% would buy irradiated food. Consumers express greater concern about other food-related hazards such as pesticide residues and bacterial contamination than they do about irradiation. Consumer concern about irradiation can be significantly decreased through unbiased, fact-based educational efforts and through endorsement by health professionals. Consumers are willing to pay financial premiums for a safer product, depending on their level of awareness and the provision of sufficient background information.
Nutrional value of irradiated foods
Food treated by irradiation is generally just as nutritious as, or better than, the same food treated by conventional, familiar processes such as cooking, drying, or freezing. Certain vitamins (e.g. thiamine, ascorbic acid, retinol, and α-tocopherol), can be reduced by irradiation, as well as by other food processing methods or by simple storage. The use of vacuum packaging and irradiation at cold temperatures can reduce this loss of retinol and α-tocopherol upon irradiation. To determine the impact of irradiation treatment, the significance of any loss of vitamins must be evaluated relative to the role of the irradiated food as a source of those vitamins, and take account of the variability of the levels of those vitamins in the food being irradiated [3],[4].
Irradiation at doses up to 2 kGy has negligible effect on the anthocyanin content of pomegranate juice. Irradiation at doses up to 3 kGy causes no significant loss of flavonoids in strawberries and has negligible effect on the saturated, unsaturated, and trans-fatty acid content of beef or olive oil. Irradiation does have an effect on the proportions of individual conjugated linoleic (CLA) acid isomers in lamb meat.
In certain cases irradiation can improve the nutritional quality of the food. For example, irradiation has been shown to increase the total phenolic content of lettuce, and the isoflavone and total phenolic content of soybeans [5],[6],[7].
Sensory quality of irradiated foods
Irradiation of many foods according to a validated protocol does not significantly change taste, texture or appearance. Some foods treated by irradiation may taste slightly different, just as pasteurized milk tastes slightly different from
unpasteurized milk.
Poultry and pork can be sensitive to flavour and colour (pinking) changes; however in at least one instance, consumers have shown preference for the pink colour, possibly because the irradiated product appeared “fresher”. Irradiation flavour in meat is most probably caused by sulphur compounds. Exclusion of oxygen, reducing the temperature during irradiation, the use of antioxidants, as well as subsequent cooking and/or storage of meat all help to reduce the potential off-flavours generated through irradiation.
Certain foods (e.g. eggs, milk, dairy products, and some fruits and vegetables) are however considered unsuitable for irradiation due to the strong off-flavours they develop [8].
Safety of irradiated foods
Several extensive reviews of toxicological data by regulatory and health organizations, including the US Food and Drug Administration (FDA), Health Canada, the Codex Alimentarius Commission, and the European Commission’s Scientific Committee on Food have determined that food irradiated at doses below 10 kGy is safe [9]. Food irradiation is endorsed by national and international food and public health organizations, such as the American Medical Association, the American Dietetic Association, the American Council on Diet and Health, the US Public Health Service, the Mayo Clinic, the Center for Disease Control and Prevention, and the World Health Organization (WHO). A comprehensive review of scientific studies on the effects of irradiation on food was released by the WHO in 1999 [8].
Irradiation can produce changes in food, similar to those caused by cooking, but in smaller amounts.
Alkylcyclobutanones (ACBs) were considered unique radiolytic products until their natural occurrence in (non-irradiated) cashew nuts and nutmeg was demonstrated. There is no compelling evidence that ACBs are genotoxic or mutagenic when consumed in a normal diet [10].
Radiolytically produced benzene (a known carcinogen at high doses) is present in irradiated beef at much lower levels than is found naturally in a variety of common foods, such as eggs or dairy products. Scientific studies using laboratory animals have conclusively demonstrated that irradiated foods do not cause cancer. Astronauts have consumed high dose-irradiated foods (pathogen-free) for years without incident.
Irradiation of fresh fruits and vegetables and apple cider generates small amounts of furan (a proven carcinogen at high doses in rodents), i.e. less than the amounts formed by heat-processing or even refrigerated storage.
Irradiation may cause the oxidation of cholesterol in meat to cholesterol oxides, which are believed to have various adverse health effects. The formation of these by-products can be greatly reduced by adding antioxidants to the diet of food animals, and by excluding oxygen during storage [5].
Studies have failed to bring forth evidence for the mutagenicity of irradiated foods using an Ames test or in feeding trials with test animals. Irradiated foods do not become radioactive.
Irradiation facilities are subject to strict federal and state regulations. In the US, facilities using radioactive sources are regulated by the Nuclear Regulatory Commission (NRC). Electron beam and X-ray sources are monitored by the section of the FDA that regulates medical X-ray devices. While several accidents resulting in radiation exposure have occurred worldwide over the past 30 years, all of these were the consequence of bypassing safety systems and control procedures. Furthermore, in North America, there has not been a single accident resulting in the escape of radioactive material in the environment in over four decades [11].
Identification and detection of irradiated foods
Several methods are available for the detection of irradiated foods. Electron spin resonance (ESR) measures the concentration of free radicals in irradiated matter (e.g. foods containing bones, shells, or other particles). Thermoluminescence (TL) measures light emission by excited molecules upon heating material and is suitable for foods containing minerals. Chemical methods, such as gas and liquid chromatography, measure volatile radiolytic products such as alkanes, alkenes and 2-alkylcyclobutanones, or non-volatile compounds such as 6-ketocholesterol and o-tyrosine (especially in fatty foods). A number of DNA-based methods and biological screening approaches are also available [1].
Irradiation and food packaging
Irradiated packaging materials in contact with food are subject to pre-market approval by the FDA and may be used only if they comply with existing regulations. Approved packaging materials do not generally meet today’s needs as do newer materials that may be more desirable to the food industry. However, many of these newer packaging materials have not yet been evaluated by FDA. Testing and approving a wider array of packaging materials will be critical for the successful commercialisation of irradiated foods.
Most food packaging materials are composed of polymers that may be susceptible to chemical changes induced by radiation. In addition to the base polymers, additives such as antioxidants, stabilizers, plasticizers, and colorants may degrade preferentially more than the polymer itself and could result in radiolytic products that may migrate into food.
The dietary exposures to most radiolytic products formed in several irradiated polymers were determined to be <0.5 µg/kg in the daily diet, i.e. less than the threshold of regulation concern level [12].
The migration of both base polymers and additives, as well as migration of their radiolytic products must be evaluated in the pre-market safety assessment of a packaging material in contact with food during irradiation.
Regulatory status of food irradiation
Food irradiation has already gained approval in more than 37 countries worldwide.
In the United States, the Food and Drug Administration (FDA) has primary regulatory responsibility for ensuring the safe use of irradiation on all foods. At the same time, the United Stated Department of Agriculture (USDA)’s Food Safety and Inspection Service (FSIS) is responsible for the lawful processing of meat, poultry, and some egg products. To date, irradiation has been approved by the FDA (and the USDA, where applicable) for a variety of foods [Table 2] [13,14].
While much work has been done already, it is important to prioritise future studies and products that need to be evaluated by their implication in outbreaks and/or volume of consumption.
A common concern often stated by those who oppose food irradiation is that it is an “easy” alternative to proper food processing plant sanitation and
cleanliness practices [4].
Labelling of irradiated foods
The FDA currently requires that labels on irradiated foods contain an identifying symbol called a “radura” (Fig.1) together with the words “treated with radiation” or “treated by irradiation”. A number of labelling statements about the purpose of radiation processing have also been authorized such as “Irradiated for food safety” and “Treated with irradiation for food safety”. Foods that contain irradiated spices or foods served in restaurants do not have to be identified as being irradiated [13].
On the part of the USDA, irradiated beef can be listed in the ingredients statement either as “Irradiated beef,” or “Beef, treated by irradiation.” Irradiated, single ingredient meat or poultry can be labelled “all,” “pure”, or “100%”. An irradiated product can not be labelled as “natural” since irradiation is considered to be more than minimal processing. Nor can irradiated products be labelled “certified Organic by (a certifying entity)” [14].
References
1. Molins R in Food Irradiation: Principles and Applications. 2001. John Wiley & Sons, Inc, New York, NY.
2. International Consultative Group on Food Irradiation (1999). Enhancing Food Safety through Irradiation. ICGFI, Vienna, Austria.
3. WHO. Safety and Nutritional Adequacy of Irradiated Food. (1994) WHO, Geneva, Switzerland.
4. Smith JS & Pillai S. Food Technol. 2004; 58(11): 48.
5. O’Bryan CA et al Crit Rev Food Sci. Nutr 2008; 48(5): 442.
6. Polovka M & Suhaj M. Food Rev Intl 2010; 26(2): 138.
7. Arvanitoyannis IS et al P. Crit Rev Food Sci Nutr 2009; 49(5): 427.
8. WHO (1999). High-dose irradiation: Wholesomeness of food irradiated with doses above 10 kGy. Report of a joint FAO/IAEA/WHO study group. WHO technical report series 890. WHO, Geneva, Switzerland.
9. CAC. Codex General Standard for Irradiated Foods. CODEX STAN 106-1983, joint FAO/WHO Food Standards Programme, (1983) United Nations FAO/WHO, Rome, Italy.
10. Health Canada. Evaluation of the significance of 2-dodecylcyclobutanone and other alkylcyclobutanones. (2003) Available: www.hc-sc.gc.ca/fn-an/securit/irridation/cyclobutanone-eng.php. Accessed August 19, 2010.
11. Diehl JF (1995) in Safety of Irradiated Foods. Marcel Dekker, Inc., New York, NY.
12. Paquette K. Irradiation of Prepackaged Food: Evaluation of the Food and Drug Administration’s Regulation of the Packaging Materials. In Irradiation of Food and Packaging: Recent Developments, edited by Komolprasert V and Morehouse K pp. 182-202. ACS Symposium Series 875: (2004) Oxford Press.
13. FDA U.S. Regulatory Requirements for Irradiating Foods. 2009 Available: www.fda.gov/Food/FoodIngredientsPackaging/IrradiatedFoodPackaging/ucm110730.htm. Accessed August 19, 2010.
14. USDA. Labeling and Consumer Protection: Irradiations Questions and Answers. (2001) Available: www.fsis.usda.gov/oppde/larc/Policies/IrradiationQA.htm. Accessed August 19, 2010.
The authors
Stefan Ehling, Shannon Cole and
Jeffrey Barach
Grocery Manufacturers Association
1350 I Street, NW, Suite 300
Washington, D.C., 20005, USA
Tel. +1-202-639-5900
e-mail: www.gmaonline.org
