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http://www.wisc.edu/fri/briefs/foodirrd.htm
Food Irradiation
first published March 1999
prepared by M. Ellin Doyle, Ph.D.
Food Research Institute, UW-Madison
Process of Irradiation
Effectiveness of Irradiation
Issues of Concern
Destruction of Vitamins
Formation of Lipid Oxides
Radiolytic Products in Foods
Packaging Materials
Regulatory Status of Irradiation
Summary
References
Not all pathogenic spores and viruses will be destroyed by irradiation and if food is not handled properly
after irradiation, it can become contaminated.
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The continuing saga of foodborne disease outbreaks as reported in scientific publications and the popular press has
raised consumer anxiety about foodborne illness and revitalized interest in irradiation as a method for eliminating or
reducing foodborne pathogens.In addition to the well-known problems of salmonellae on poultry products, E. coli O157:
H7 in hamburger, and, most recently, Listeria in packaged meats, there have been numerous reports of Vibrio spp. in
shellfish and of fresh fruits and vegetables contaminated with bacteria, viruses, and parasites such as Cyclospora.
Irradiation has the potential to enhance food safety for both fresh foods that will be consumed raw and for raw foods that
will be further processed. Many of the recent, well-publicized, large recalls of hamburger and other meats have raised
consumer awareness of the particular significance of food safety for the young, the old, and those with impaired immune
systems. These factors and better education of consumers about what irradiation of food involves and what it does (and
does not do) to foods appear to have increased consumer acceptance of irradiation as one means of producing safe
foods.
Decades of research in the USA and in other countries have supported the safety and wholesomeness of foods
irradiated under Good Manufacturing Practices. A number of well-respected independent organizations, including the
World Health Organization (WHO), Codex Alimentarius (21), U.S. Food and Drug Administration (2,13), American
Dietetic Association (6), Institute of Food Science & Technology (20), Institute of Food Technologists (30), and the
Council for Agricultural Science and Technology (7), have concluded that irradiation of food is safe and can be an
effective method to help eliminate foodborne contaminants. However, there remain some concerns related to the
irradiation of food which may limit its usefulness in certain situations and which cause anxiety among consumers. These
points include fate of pathogens that survive radiation; destruction of vitamins; induction of lipid oxides and off-flavors;
possible toxicity of radiolytic products in irradiated foods; and effects of irradiation on packaging materials. This paper
will briefly describe the irradiation process and its effectiveness, review issues of concern, and provide information on
the regulatory status and current use of irradiation for destroying pathogens.
Process of Irradiation
Most food irradiation facilities utilize the radioactive element cobalt-60 as the source of high energy gamma rays. These
gamma rays have sufficient energy to dislodge electrons from some food molecules, thereby converting them into ions
(electrically charged particles).Gamma rays do not have enough energy to affect the neutrons in the nuclei of these
molecules; therefore, they are not capable of inducing radioactivity in the food. Some irradiation facilities use cesium-
137 as a source of gamma rays while others use machines to produce x-rays or beams of electrons. Again, none of
these sources produce beams of high enough energy to induce the formation of radioactive isotopes in foods. All of the
energy sources do cause the production of ions and free radicals (reactive compounds with unpaired electrons),
including transient high energy oxygen radicals, which kill or damage pathogenic organisms. Since gamma rays from
cobalt-60 can penetrate several feet while electrons produced by electron beam facilities penetrate only a few inches,
cobalt-60 is usually the preferred source of radiation for food. Irradiation dosage is a function of the energy of the
radiation source and the time of exposure. Doses are usually expressed in kiloGrays (kGy); 1 Gray is equivalent to 1
joule of absorbed radiation/kg tissue or 100 rads (10). These irradiation doses can be directly related to extent of killing
of bacterial pathogens. However, D-10 values (irradiation doses required to cause a 1 log reduction of cells) will differ
for different types of foods depending on their density, antioxidant levels, moisture content, and other food components.
External factors, such as temperature and the presence or absence of oxygen, also influence the effectiveness of a
given radiation dose (10,23,30).
Effectiveness of Irradiation
Sensitivity to irradiation varies among microbial species and is affected by the components of foods and temperature
during irradiation and subsequent storage (12,14,16,28). Bacteria and parasites are much more sensitive to irradiation
when tested in laboratory media than in real foods. Therefore, it is important to test likely contaminants in relevant foods
under realistic, expected storage conditions and also under conditions of temperature abuse to determine the
effectiveness of different irradiation doses. Under some conditions (high bacterial load, inadequate radiation dose, high
concentrations of antioxidants), damaged cells may be able to repair themselves and grow to detectable and dangerous
levels (8,25,26,31,32,33). Although it has been suggested that irradiated meats and fish be vacuum packaged or stored
under anaerobic conditions to prevent revival of injured cells, there has been concern that Clostridium botulinum would
thrive under these conditions.
Irradiation readily kills most non-spore-forming bacteria and parasites in foods. Published data on D values range from
0.022 kGy for Vibrio parahaemolyticus in freshwater fish homogenate at 24°C to 0.78 kGy for Salmonella stanley in
ground beef at 18-20°C. In general, Salmonella and Listeria are more resistant to irradiation than E. coli and
Staphylococcus. Yersinia, Vibrio, Arcobacter, Aeromonas, and Campylobacter are the most sensitive species.
Pathogenic protozoa (such as Toxoplasma and Cyclospora) and parasitic worms (Trichinella, tapeworms, liver flukes)
are killed by radiation doses of <1 kGy. However, enteric viruses, spores of Clostridium spp., Bacillus spp., and molds,
and microbial toxins from molds, Staphylococcus aureus, and Clostridium botulinum are extremely resistant to irradiation
and cannot be effectively eliminated at approved doses of irradiation considered reasonable for most foods (£10 kGy)
(28). Irradiation doses approved for foods normally range from <1 up to 10 kGy. Larger doses (up to 30 kGy) have been
approved for dried herbs, spices, and dehydrated vegetables, and up to 44 kGy are used to sterilize packaged meats
for astronauts (12,13, 14, 21,27).
The safety of irradiated raw pork was found to depend on the size of the initial population of a pathogen, such as
Yersinia. A dose of 1 kGy was sufficient to eliminate relatively low, naturally occurring levels of Yersinia. However, with
higher initial cell concentrations (106 cfu/g), even 6 kGy was insufficient to kill all the Yersinia and injured cells
recovered and grew during storage at 2-4°C (31). Experiments comparing the effects of irradiation on Yersinia in raw
pork and in processed pork products revealed that bacteria in ham and salami were killed more easily by irradiation than
those in raw ground pork (22). It is likely that the salt, spices and other additives in preserved meats enhance the killing
by irradiation or inhibit recovery of radiation-injured bacteria.
Fresh foods containing the unicellular parasites Cyclospora and Cryptosporidium have become a food safety concern in
the USA in the past several years. In tests to estimate the radiation sensitivity of these parasites, a related species,
Toxoplasma gondii, with a better characterized life cycle and well-established tests for infectivity, was inoculated onto, or
injected into, raspberries and then exposed to irradiation. When unsporulated oocysts (the resting stage) were
irradiated at 0.4-0.8 kGy, they were able to sporulate but were not infective to mice. In most cases, already sporulated
oocysts on and in raspberries were unable to infect mice after a dose of 0.4 kGy (11). These results indicate that low-
dose irradiation may be an effective method for decontamination of fresh fruits and vegetables.
So far, experimenters have not observed the development of radiation resistance in microbes that survive irradiation of
foods. In fact, a number of experiments demonstrated that these survivors are weakened and more susceptible to high
or low temperatures and to increases in salt concentrations (19,25,33). Nevertheless, microbes are extremely adaptable
and the possible evolution of greater resistance to radiation and to other environmental stresses in survivors of
irradiation should be monitored.
Issues of Concern
Along with damage to molecules in foodborne pathogens, irradiation also causes some chemical changes in the
molecules of foods. Cooking and thermal processing of foods also cause chemical changes and in many cases these
are similar to reactions occurring with irradiation (10,23). With both processes, we have concerns about destruction of
micronutrients such as vitamins; oxidation of lipids; changes in protein and carbohydrate molecules leading to the
formation of heat- or irradiation-related compounds; and effects on packaging materials.
Destruction of Vitamins
One undesirable side effect of irradiation is the destruction of some vitamins in foods (24,30). Early experiments testing
vitamin survival in water or some solution (rather than in foods) suggested that the loss of certain vitamins during
irradiation might be significant. However, as with pathogenic bacteria, vitamins are less sensitive to irradiation when
present in the complex matrices of foods. Recent experiments with chicken breast meat demonstrated that some
reduction in thiamin levels occurred during irradiation but these losses were less if irradiation was done at lower
temperatures (18). It should be remembered that vitamins are also destroyed during cooking and other thermal
preservation processes (23). Several researchers have concluded that vitamin losses due to irradiation would not be
significant to those consuming a typical American or European diet (30). An analysis of the possible effects of irradiation
on vitamin levels in the Argentine diet (29) came to a similar conclusion. However, normal levels of vitamin D and folacin
are low in the Argentine diet and the authors caution that folacin concentrations, in particular, should be monitored.
Formation of Lipid Oxides
Another undesirable effect of irradiation is the formation of lipid oxides by the reaction of membrane lipids and other
lipids in foods with oxygen radicals, produced by gamma rays (5,10). These oxides may impart off-odors and tastes to
foods and may contribute to lipid-related diseases. For this reason some foods, such as fatty fish and meat and some
dairy products, are not considered good candidates for irradiation (12). Formation of these oxides can be decreased by
reducing oxygen and temperature levels during irradiation. Other approaches which have met with some success
include the addition of carnosine, an antioxidant, to ground turkey (32) and the addition of vitamin E, another
antioxidant, to feed for turkeys and chickens (3,4,15). The increased vitamin E levels in muscles of the poultry
decreased formation of lipid oxides during subsequent irradiation of the meat. All of these procedures also affected the
destruction of pathogens, usually requiring a greater irradiation dose to be effective.
Radiolytic Products in Foods
Although irradiation does not make food radioactive, it does induce some chemical changes in foods leading to
production of small amounts of so-called radiolytic products. These new or increased amounts of certain chemical
compounds found in irradiated foods are not unique but are similar to compounds formed in foods during cooking
(9,10). In fact, much larger amounts of some of these compounds are formed during ordinary cooking. Numerous
experiments with laboratory animals and some trials with humans have demonstrated that no adverse health effects
occur when irradiated foods containing these compounds are consumed.
Radiolytic compounds formed from carbohydrates and proteins in foods are largely a product of reactions with hydroxyl
radicals (powerful oxidizing agents) and hydrated electrons (powerful reducing agents), both of which are generated
from water molecules by gamma rays. Free radicals reacting with proteins may cause breaks in the protein chains or
changes in the secondary or tertiary structure of proteins. These changes would be lethal to a living organism
(foodborne bacteria) but do not affect the nutritional quality of the food. Reaction of hydroxyl radicals with starch
produces formic acid, some aldehydes and ketones, and different sugars which contain one less carbon. Radiolytic
products formed from fats are not usually a result of reactions with disrupted water molecules. Rather, gamma rays
interact directly with lipid molecules to form cation radicals or excited lipid molecules. These products may then generate
lipid oxides (which result in off odors and tastes) and small amounts of fatty acids, aldehydes, esters, ketones, and other
compounds. Again, it should be mentioned that these chemical changes in foods are small and these compounds are
also produced by cooking or thermal processing.
Packaging Materials
Since post-irradiation contamination can be minimized by irradiating foods in their final packaging, the effects of
irradiation on packaging materials and the migration of components such as plasticizers into foods must be considered
(10). Experiments conducted with food-grade PVC (polyvinyl chloride) film exposed to high doses of electron beam
irradiation (20-50 kGy) demonstrated that increased amounts of dioctyl adipate, a plasticizer, migrated into olive oil after
the higher dose of irradiation (17). These radiation doses are in excess of those usually recommended for foods.
However, this experiment points out a potential problem for irradiation of foods in plastic containers. Any such plastics
must be tested for effects of irradiation on the migration of components of the plastic into the types of foods which would
be stored in these containers. Irradiation can also affect the structure and stability of some plastics, thereby rendering
them unsuitable for exposure to irradiation.
Although adipate plasticizers are not potent toxins, they have been shown to have some deleterious effects in laboratory
animals. Since plastics are so widely used, there is a potential for exposure to these plasticizers from many sources.
Whether the accumulated dose from these different exposures can be harmful to humans is as yet uncertain but should
be considered.
Regulatory Status of Irradiation
In the USA, irradiation has been approved by the FDA (13,14,27) for the purpose of microbial disinfestation of the
following:
Product to a limit of:
dry or dehydrated enzymes <10 kGy
spices, herbs, dehydrated vegetables <30 kGy
fresh or frozen uncooked poultry <3.0 kGy
pork carcasses and meat (Trichinella) <1.0 kGy
packaged meat for NASA flights <44 kGy
fresh or frozen red meat <4.5 kGy (fresh)
<7 kGy (frozen
Irradiation of red meat was approved by the FDA in December 1997, and the recommended procedures for irradiating
meat were published by the USDA in the Federal Register on February 24, 1999 (14). Following a 60-day period for
comments, the final regulations will be published and then commercial irradiation of meat could commence.
A number of individual European countries have regulations in place permitting (or in some cases prohibiting) irradiation
of foods under specified conditions. The European Community is at this time working to establish a common set of
guidelines. According to data from the International Atomic Energy Agency (IAEA, 21), as of 1997, a total of 35 countries
worldwide had approved irradiation of certain types of foods under specified conditions.
Summary
Acceptance of irradiation as a tool for food preservation is increasing but it should be emphasized that Good
Manufacturing Practices in all aspects of food production are still essential in order to produce safe food. Not all
pathogenic spores and viruses will be destroyed by irradiation and if food is not handled properly after
irradiation, it can become contaminated. However, irradiation is a safe and effective means of destroying many
foodborne pathogens and it should be useful in contributing to a safe food supply.
References
1. WHO decides — Food irradiation safe at any level. Public Health 1998; 113(1):6.
2. FDA approves irradiation of meat for pathogen control. J. Am. Vet. Med. Assoc. 1998; 212(2):165.
3. Ahn DU, Sell JL, Jeffery M, Jo C, Chen X, Wu C, and Lee JI. Dietary vitamin E affects lipid oxidation and total volatiles
of irradiated raw turkey meat. J. Food Sci. 1997; 62(5):954-958.
4. Ahn DU, Sell JL, Jo C, Chen X, Wu C, and Lee JI. Effects of dietary vitamin E supplementation on lipid oxidation and
volatiles content of irradiated, cooked turkey meat patties with different packaging. Poultry Sci. 1998; 77(6):912-920.
5. Ahn DU, Olson DG, Lee JI, Jo C, Wu C, and Chen X. Packaging and irradiation effects on lipid oxidation and volatiles
in pork patties. J. Food Sci. 1998; 63(1):15-19.
6. American Dietetic Association. Position of the American Dietetic Association: Food Irradiation. www.eatright.org/airradi.
html
7. Council for Agricultural Science and Technology. Radiation Pasteurization of Food. www.cast-science.org/past_ip.htm
8. Collins CI, Murano EA, and Wesley IV. Survival of Arcobacter butzleri and Campylobacter jejuni after irradiation
treatment in vacuum-packaged ground pork. J. Food Prot. 1996; 59(11):1164-1166.
9. Delincee H. Detection of food treated with ionizing radiation. Trends Food Sci. Technol. 1998; 9(2):73-82.
10. Diehl JF. 1995. Safety of Irradiated Foods. New York: Marcel Dekker, Inc.
11. Dubey JP, Thayer DW, Speer CA, and Shen SK. Effect of gamma irradiation on unsporulated and sporulated
Toxoplasma gondii oocysts. Int. J. Parasitol. 1998; 28(3):369-375.
12. Farkas J. Irradiation as a method for decontaminating food. Int. J. Food Microbiol. 1998; 44:189-204.
13. Food and Drug Administration. Irradiation in the production, processing and handling of food. Federal Register 62
(232), Dec. 3, 1997.
14. Food Safety and Inspection Service. Irradiation of meat and meat products. Federal Register 64(36), Feb. 24, 1999.
www.fsis.usda.gov/oa/fr/99-4401.htm
15. Galvin K, Morrissey PA, and Buckley DJ. Effect of dietary alpha-tocopherol supplementation and gamma-irradiation
on alpha-tocopherol retention and lipid oxidation in cooked minced chicken. Food Chem. 1998 Jun; 62(2):185-190.
16. Gamage SD, Faith NG, Luchansky JB, Buege DR, and Ingham SC. Inhibition of microbial growth in chub-packed
ground beef by refrigeration (2°C) and medium-dose (2.2 to 2.4 kGy) irradiation. Int. J. Food Microbiol. 1997; 37(2-3):
175-182.
17. Goulas AE, Riganakos KA, Ehlermann DAE, Demertzis PG, and Kontominas MG. Effect of high-dose electron beam
irradiation on the migration of DOA and ATBC plasticizers from food-grade PVC and PVDC/PVC films, respectively, into
olive oil. J. Food Prot. 1998; 61(6):720-724.
18. Graham WD, Stevenson MH, and Stewart EM. Effect of irradiation dose and irradiation temperature on the thiamin
content of raw and cooked chicken breast meat. J. Sci. Food Agric. 78(4):559-564, 1998.
19. Gursel B, and Gurakan GC. Effects of gamma irradiation on the survival of Listeria monocytogenes and on its
growth at refrigeration temperature in poultry and red meat. Poultry Sci. 1997; 76(12):1661-1664.
20. Institute of Food Science & Technology. The use of irradiation for food quality and safety. www.easynet.co.
uk/ifst/hottop11.htm
21. International Atomic Energy Agency. Commercial Activities on Food Irradiation. www.iaea.
org/icgfi/documents/commeact.htm
22. Kamat AS, Khare S, Doctor T, and Nair PM. Control of Yersinia enterocolitica in raw pork and pork products by
gamma-irradiation. Int. J. Food Microbiol. 1997; 36(1):69-76.
23. Lagunas-Solar MC. Radiation processing of foods: an overview of scientific principles and current status. J. Food
Prot. 1995; 58(2):186-192.
24. Lakritz L, Fox JB, and Thayer DW. Thiamin, riboflavin, and alpha-tocopherol content of exotic meats and loss due to
gamma radiation. J. Food Prot. 1998; 61(12):1681-1683.
25. Lucht L, Blank G, and Borsa J. Recovery of Escherichia coli from potentially lethal radiation damage —
characterization of a recovery phenomenon. J. Food Safety 1997; 17(4):261-271.
26. Lucht L, Blank G, and Borsa J. Recovery of foodborne microorganisms from potentially lethal radiation damage. J.
Food Prot. 1998; 61(5):586-590.
27. Mates TJ. Food irradiation — commercial applications. J. Assoc. Food Drug Off. 1998; 62(4):41-45.
28. Monk JD, Beuchat LR, and Doyle MP. Irradiation inactivation of food-borne microorganisms. J. Food Prot. 1995; 58
(2):197-208.
29. Narvaiz P, and Ladomery LG. Estimation of the effect of food irradiation on total dietary vitamin availability as
compared with dietary allowances — study for Argentina. J. Sci. Food Agric. 1998; 76(2):250-256.
30. Olson DG. Irradiation of food. Food Technol. 1998; 52(1):56-62.
31. Shenoy K, Murano EA, and Olson DG. Survival of heat-shocked Yersinia enterocolitica after irradiation in ground
pork. Int. J. Food Microbiol. 1998; 39(1-2):133-137.
32. Stecchini ML, Deltorre M, Sarais I, Fuochi PG, Tubaro F, and Ursini F. Carnosine increases irradiation resistance of
Aeromonas hydrophila in minced turkey meat. J. Food Sci. 1998; 63(1):147-149; 63(4):744.
33. Thayer DW, Boyd G, Kim A, Fox JB, and Farrell HM. Fate of gamma-irradiated Listeria monocytogenes during
refrigerated storage on raw or cooked turkey breast meat. J. Food Prot. 1998; 61(8):979-987.
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