Advances in Food Irradiation Research - ACS Symposium Series

Chapter 20

Advances in Food Irradiation Research 1

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Karen J. L. Burg and Shalaby W. Shalaby

Downloaded by IMPERIAL COLLEGE LONDON on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch020

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Department of Bioengineering, 301 Rhodes Engineering Research Center, Clemson University, Clemson, SC 29634-0905 Center for Applied Technology, Poly-Med Inc., Pendleton, SC 29670

A brief background of the use of irradiation of food is given, limited to food for human consumption. Attention is directed to the effects of radiation on packages and organic food products as well as the resulting interactions. The main variables of interest in food irradiation include temperature, water content, packaging atmosphere, dose rate, package material, and food type. Certain food types release specific volatiles which may be used to monitor effectiveness of irradiation as well as dosage. Avenues of future research interest are highlighted. The U.S. Centers for disease control and Food and Drug Administration (FDA) estimate that between 6.5 and 33 million cases of food-borne illnesses occur each year, 9,000 resulting in death, costing the U.S. economy millions of dollars (1). The 1963 FDA ruling that bacon, irradiated as a means of preservation, was satisfactory for human consumption (2) has propelled a relatively new area of food technology research designed to address this problem. This approach greatly extended the shelf life of the food and therefore reduced the quantity lost to spoilage. No radiation processed foods were commercially available prior to 1983, and this advance instigated food irradiation research on both a national and international scale (Table I). The benefit to the consumer, in terms of reduced health care costs, and the agriculture industry, in terms of reduced product loss costs, is on the order of millions of dollars. Losses due to a food scare because of low consumption and sales can represent a substantialfinancialpenalty to the retailers and wholesalers. Internationally, large quantities of food, particularly in developing countries, are lost to insect infestation and/or spoilage. Irradiation research is particularly important with the impending Clean Air Act legislation banning such suspect ozone depleting substances as fumigant methyl bromide that are currently used to combat spoilage and infestation (1).

0097-6156y96/0620-0254$12.00/0 © 1996 American Chemical Society In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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Table I. Evolution of Food Irradiation Technology (2,3,4) Year Event 1950s "Atoms for Peace" program established by President Eisenhower 1963 FDA declares irradiated bacon fit for human consumption 1963 FDA approves disinfestation of wheat and wheat flour by irradiation 1964 FDA approves irradiation as a sprout inhibitor 1965 Office of Surgeon General of U.S. Army declares foods irradiated with doses up to 56 kGy fit for human consumption 1976 Joint Food and Agricultural Organization (FAO), International Atomic Energy Agency (IAEA), and World Health Organization (WHO) committee concludes irradiated potatoes, wheat, chicken, papaya, and strawberries unconditionally safe for human consumption 1980 Joint FAO, IAEA, and WHO committee concludes irradiated foods safe up to 10 kGy 1992 U.S. Department of Agriculture (USDA) allows irradiation of raw, packaged poultry in approved facilities

The three levels of food irradiation preservation are: (a) low radiation dosages of less than about 2 kGy (0.2 Mrad) which delay sprouting of vegetables as well as aging of fruits, (b) pasteurization or medium radiation dosages of 2 to 5 kGy (0.2 to 0.5 Mrad) which do not kill all bacteria, and (e) sterilization, with higher dosages of 20 to 45 kGy (2 to 4.5 Mrad). The most common type of radiations used to achieve these doses are high energy gamma rays which are emitted from cobalt-60 or cesium-137 sources and which have relatively high penetration depth. Another important type of commonly used high energy radiation is the beta particle which is less penetrating than the gamma ray. The beta particles are high speed electrons which can be generated by a linear accelerator or a Van de Graaff generator and therefore have the advantage of directionality. X rays are generated indirectly using an electron beam. The accelerated electron may be directed through a thin metal film of high heat resistance to produce the X ray which has slightly less penetrating capacity than the gamma rays. Both X ray and electron beam are useful in irradiating large quantities of small foods, where penetration depth is not an issue (5-6). The electron beam source is advantageous from the standpoint that the regulatory issues necessary for a radioisotope are not encountered. The capital investment for gamma and electron irradiation appear to be approximately equivalent. Generally, ionizing radiations are directed through the food where the energy of the particles disrupts the macromolecules of bacteria and microorganisms that might be present. Each growth phase of bacterial cells has a unique resistance to irradiation, where the stationary phase demonstrates the highest resistance and the logarithmic phase demonstrates the lowest (7).

In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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IRRADIATION OF POLYMERS


Chemical Effects of Irradiation on Packaging Typically, the food is irradiated post packaging; hence, radiation effects on the food and package individually are as critical as the packaging-food-radiation interactions. Irradiation of glass containers is not aesthetically pleasing and therefore not desirable since glass acquires a brown tint upon irradiation. Canning in metal containers is an acceptable practice for food irradiation, although a tin or aluminum container may partially shield the contents from irradiation at the lower doses used for foods. Plastic packaging offers the convenient alternative of a lightweight, space minimizing storage alternative, but may suffer damage that compromises its integrity as a barrier to microbiological contaminants. Pasteurization, for example, does not eliminate all bacteria so the plastic must not be conducive to bacterial growth. Certain plastics are damaged by high energy radiation and may not effectively form a microbial resistant heat seal after damage of this kind. The irradiation process may also lead to leaching of small particles from the plastic package into the food or emission of gases, thus changing the food quality (8-12).

Irradiation Stability of Common Packaging Materials. Polystyrene (PS) and polyethylene terephthalate (PET) are quite stable when irradiated, largely due to the aromatic groups which are able to absorb and dissipate the penetrating energy. Polypropylene (PP) tends to undergo crosslinking and chain scission. Polytetrafruoroethylene (PTFE) and cellulose based products also demonstrate very poor stability. Degradation or cleavage of polymeric chains can cause loss in mechanical strength which is critical to handling and storage. Polymers may influence the odor and even taste of the sealed food due to the emission of gaseous products. Certain polymers produce gases as a result of radiation-induced chemical reactions. Any such released volatiles might migrate into the food resulting in a change as minor as food odor (13) or as major as health hazard. The polyesters, polystyrenes, and polyamides are rninimally affected by this phenomenon; however, PP packages are susceptible to irradiation damage. Low density polyethylene may release aliphatic hydrocarbons, aldehydes, ketones, and carboxylic acids, a phenomena that may be reduced by incorporating antioxidants in the polymer. Hydrogen can also be produced by the packaging material. This does not have a negative impact on the food as the gas will typically diffuse through the plastic (77). Water content of the packaged food also plays an important role in volatile production, where the higher water content causes a higher hydrogen release. Moisture in the packaging materials can intensify the radiation effects. In general, the plastics are more stable when irradiated under vacuum than in atmospheric conditions. Oxygen tends to increase main chain cleavage while inhibiting crosslinking in such polymers as PE, PP, and PS. The hydrocarbon level, however, is relatively unaffected by a change in oxygen concentration. This is attributed to the theory that the hydrocarbon formation during irradiation occurs due to short branch cleavage. Carboxylic acid generation is much greater in the case of gamma irradiation than that of electron beam when applied to polyethylene, polypropylene, and


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cellulose. A low gamma dose rate (Gy/hr) results in much lower concentrations of primary radicals and therefore a lower amount of crosslinking and a higher concentration of carboxylic acid. In contrast, higher concentrations of primary radicals tend to favor recombination, thus reducing the likelihood of oxidation (73). Lower applied radiation energies (eV) with their weak penetration forces result in lower release of carboxylic acids and corresponding smaller changes in odor.

Migration of Additives. Plastics may contain low molecular weight compounds, namely oligomers and monomers, and additives such as heat and light stabilizers, antioxidants, U V absorbers, lubricants, and plasticizers. These are used for improved stability during processing to form articles with minimally compromised mechanical strength. Smaller chain molecules have much greater mobility and therefore potential to migrate into the food. The irradiation process may not only affect the polymer packaging itself, but it may also cause degradation of additives and migration of by-products into the food. Poly(vinyl chloride) (PVC), for example, is stabilized with organotin compounds such as dibutyltin bis-(isooctylthioglycollate) or dibutyltin bis-(isooctylmaleate). These stabilizers degrade largely to tin (IV) chloride upon exposure to gamma irradiation (14-15), and the degradation increases rapidly above 25 kGy (2.5 Mrad). Results indicate that the stabilizer degradation products show a far greater tendency to migrate than the intact stabilizer. Packaging Atmosphere. Packaging under modified atmospheres have been in use since the 1800s; they found international use in the 1930s with the New Zealand and Australian lamb and beef exports which were packed in a carbon dioxide enriched atmosphere (16). Modified atmosphere packaging (MAP) has been developed more recently to extend shelf life and maintain food quality by combining mixtures of carbon dioxide, oxygen, and nitrogen (17-18). The effect of the packaging atmosphere on microorganism survival is dependent on radiation dose, packaging temperature, as well as gas composition. The modified atmosphere packaging appears particularly effective in lengthening shelf life of such low acid, perishable foods as beef, poultry, seafood and dairy products. Generally the optimal irradiation package atmosphere for extending shelf life by inhibiting spoilage microorganisms appears to be a high carbon dioxide, low oxygen mixture; furthermore, there is a carbon dioxide threshold level above which an increase in carbon dioxide appears to have no additional inhibitory effect. Carbon dioxide was shown to increase the lag phase of many microorganisms while oxygen serves to inhibit growth of anaerobic pathogens. A high carbon dioxide environment, however, may be conducive to the appearance of toxins such as the microorganism toxin botulinus, produced by Clostridium botulinum found in meat. The presence of oxygen is also a major factor in the enzymatic degradation of lipids and subsequent changes in odor in chicken packaging (18). A high level of nitrogen in a package appears to have no additional inhibitory effect and, in fact, allows bacterial growth very similar to that in air. Irradiation may decrease the rate of change in headspace gas composition by destroying the microorganisms and inactivating the enzymes which cause this change. The irradiation of oxygen may form ozone which will further inhibit microorganisms.


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An increase in irradiation dose appears to be increasingly effective in a low or no oxygen environment; however, the irradiation resistance of individual strains of the same microorganisms varies.


Chemical Effects of Irradiation on Food Irradiation will not significantly alter the elemental composition of the food; however, it may alter the food components as well as the food contaminants. The absorption of the ionizing radiation causes free radical formation, the amount dependent on the material itself and the radiation dose (19). The radiation dose required for sterilization may sometimes cause a change in sensory characteristics, an "off-flavor", in certain foods (20-23). The sensory characteristics may be protected and sterilization achieved by combining a lower irradiation dose with other preservation procedures (4 23 24). The half-life offreeradicals decreases with an increase in water content while calcified tissue such as pork or chicken bone may increase the half-life. Fatty acids found in foods rich in fat, pork for example, will undergo cleavage at the ester groups, yielding hydrocarbons when irradiated, the production of which increases with the dose as well as temperature of irradiation (19). Hydrocarbon production is predictable in chicken, beef, and pork and in fact behaves as a dosimeter. Frog legs (25) and chicken (26) containing lipids are found to release hydrocarbons and aldehydes upon irradiation. Hydrogen gas can be produced by irradiation of foods; carbohydrates tend to release more gas than fats and proteins. Hydrogen gas may be releasedfromfoods such as peppers and can be detected up to 4 months after treatment (27). The irradiation of chicken meat causes the triglyceride of palmitic acid to release 2-dodecylcyclobutanone which is detectable for up to 20 days postirradiation. It was suggested that this also can be beneficially used as a marker to determine satisfactory sterilization of chicken (28). Lipids undergo irradiation in the absence of a protective environment; for instance, eggs, milk powder, orflourmay produce hydroperoxide, detectable up to she months posttreatment (29). The amount of oxidation may, however, depend more on the exposure of the food to oxygen or light as well as the storage temperature. A lower water content may make the microorganisms more resistant to irradiation (4). Spores such as B. cereus and Clostridium botulinum have been shown to be more resistant to low dose irradiation atfreezingtemperatures than at refrigeration temperatures (7). This generalization is attributed to the theory that the mobility of the hydroxyl radical is decreased at sub-freezing temperatures. This temperature dependence appears to be case specific; since, other studies have yielded contradictory results (30). Irradiation of water causes the generation of a hydroxyl radical which may interact with the amino acids found in protein-containing meats. One of the possible products formed as a result, not naturally found in protein, is 2hydroxyphenylalanine (57). This reaction depends on the dose, dose rate, and temperature of irradiation. The effect of irradiation on protein in the presence of water is lessened, possibly due to the water absorbing a portion of the energy (32). The irradiation of a dry protein cleaves primarily hydrogen in secondary and tertiary structures, which can distort the molecule and expose susceptible groups. The t

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globular proteins tend to favor crosslinking as they are more tightly packed. The less globular proteins, such as collagen, tend to cleave and break into smaller units upon irradiation. Higher doses of irradiation destroy the primary structure of the amino acid; however, at the relatively low dose levels used in food irradiation, this is not relevant. The protein also produces fatty acids, mercaptans, and other sulfur compounds which become part of the food. Polysaccharides of carbohydrates are found in various foods, e.g. sugars and grains, and may generate such products as oligosaccharide fragments and hydroxyl radicals (53). Shrimp, for example, releases N-acetylglucosamine oligosaccharides upon irradiation. Low molecular weight carbohydrates produce gases such as hydrogen, carbon dioxide, methane, and carbon monoxide upon irradiation. Nongaseous products such as acetone, lactones, formaldehyde, and acetaldehyde may also be produced. Carbohydrate containing foods that have high water content may undergo oxidative degradation to form an array of acid derivatives. The amount of acid produced may be increased with an increase in the presence of oxygen (34). This is due to the reaction of the low molecular weight sugars with radiolytic byproducts such as hydroxy radicals. Dairy Products. Irradiation of dairy products produces such compounds as acetaldehyde and dimethyl sulfide. Butter fat may release volatiles in the form of aliphatic hydrocarbons, acids, alcohols, aldehydes, ketones, and esters. Oxygen is soluble in the fat; therefore, irradiation in the presence of oxygen has practically no added value (35). Meat and Poultry. Irradiated uncooked beef releases over 70% of its volatiles as hydrocarbons, originating from the lipids or lipoproteins. Some aromatic and sulfur groups are released from proteins. There are also many more less-volatile components released than volatile ones, including longer chain hydrocarbons and aldehydes, diol esters, and diglycerides. There is virtually no change in the amino acid content of beef as a result of irradiation (36). The solubility of collagen in beef is increased due to the cleavage of peptide chains, yielding lower molecular weight units and thus tenderizing the meat, an attribute of food irradiation. D N A molecular damage in meat, fish, and vegetables, specifically strands between bases and proteins cleaving or crosslinking, may occur as a result of irradiation (37-38). Damage may also occur to D N A bases or sugars. Grains and Vegetables. Wheat and rice have high amounts of protein which remain essentially unchanged after irradiation. The carbohydrates in the grain release peroxides, which are related to an odor change. The higher water content causes radiation induced higher maltose values and depolymerization of starch whereas low moisture content is related to radical formation (39).

Future of Food Irradiation Regulatory issues will depend on many factors including irradiation temperature, pH, packaging atmosphere, water content, microorganism, specific strain of


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microorganism, all of which warrant closer examination for additional food applications to be approved. The effects of cooking or further treatment of irradiated food have not been described in the literature. Finally, studies have shown that public knowledge of the irradiation process greatly enhances the appeal of buying such food (40); needless to say, the dissemination of information by the regulatory agencies will be necessary to develop a successful market for irradiated food. It will not be surprising to find widespread use of food irradiation for hospital patients before the end of this decade.


Literature Cited 1. Loaharanu, P. Food Technol. 1994, January, pp 104-108. 2. Urrows, G.M. Food Preservation by Irradiation; Understanding the Atom Series; U.S. Atomic Energy Commission: Oak Ridge, TN, 1968; pg 1. 3. Christensen, E.A.; Kristensen, H.; Miller, A In Principles and Practice of Disinfection, Preservation and Sterilization; Russell, A.D.; Hugo, W.B.; Ayliffe, G.A.J., Eds.; Blackwell Scientific Publications: Oxford, Great Britain, 1992, Second Edition; pp 528-543. 4. Radomyski, T.; Murano, E.A.; Olson, D.G.; Murano, P.S. J Food Prot. 1994, 57, pp 73-86. 5. Shalaby, S.W.; Williams, B.L. In Encyclopedia of Pharmaceutical Technology; Swarbrick, J.; Boylan, J.C., Eds.; Marcel Dekker, Inc.: New York, NY, 1988, Vol. 6; pg 44. 6. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pp 4-5. 7. Thayer, D.W.; Boyd, G. J Food Prot. 1994, 57, pp 758-764. 8. Agarwal, S.R.; Sreenivasan, A. J Food Technol. 1972, 8, pp 27-37. 9. Allen, D.W.; Leathard, D.A.; Smith, C. ChemInd.1988, 12, pp 399-400. 10. Lambert, A.D.; Smith, J.P.; Dodds, K.L. J Food Prot. 1991, 54, pp 94-101. 11. Pratt, G.B.; Kneeland, L.E.; Heiligman, F.; Killoran, J.J. J Food Sci. 1967, 32, pp 200-205. 12. Milz, J. In Food Product-Package Compatibility; Gray, J.I.; Harte, B.R.; Miltz, J., Eds.; Proceedings; Technomic Publishing Co., Inc.: Lancaster, PA, 1987; pp 30-43. 13. Azuma, K.; Tsunoda, H.; Hirata, T.; Ishitani, T.; Tanaka, Y. Agric Biol Chem. 1984, 48, pp 2009-2015. 14. Allen, D.W.; Brooks, J.S.; Unwin, J. Chem Ind. 1985, 15, pp 524-525. 15. Allen, D.W.; Crowson, A.; Leathard, D.A.; Smith, C. In Food Irradiation and the Chemist; Johnston, D.E.; Stevenson, M.H., Eds.; Annual Chemical Congress Special Publication No. 86; Royal Society of Chemistry: Cambridge, Great Britain, 1990; pp 124-139. 16. Williams, A.C. Jr. In Food Product-Package Compatibility; Gray, J.I.; Harte, B.R.; Miltz, J., Eds.; Proceedings; Technomic Publishing Co., Inc.: Lancaster, PA, 1987; pp 170-177. 17. Labuza, T.P.; Breene, W.M. J Food Process Preserv. 1989, 13, pp 1-69.



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18. Soffer, T.; Margalith, P.; Mannheim, C.H. Int J Food Sci Technol. 1994, 29, pp 161-166. 19. Rosenthal, I. In Electromagnetic Radiations in Food Science; Yaron, B.; Thomas, G.W.; Van Vleck, L.D., Eds.; Advanced Series in Agricultural Sciences; Springer-Verlag: New York, NY, 1992, Vol. 19; pp 55-59. 20. Urrows, G.M.; Food Preservation by Irradiation; Understanding the Atom Series; U.S. Atomic Energy Commission: Oak Ridge, TN, 1968; pp 15-16. 21. Silverman, G.J. In Disinfection, Sterilization, and Preservation; Block, S.S., Ed.; Lea & Febiger: Philadelphia, PA, 1983; pp 98-99. 22. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pp 126-127. 23. Thakur, B.R.; Singh, R.K. Trends Food Sci Technol. 1995, 6, pp 7-11. 24. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pp 257-263. 25. Morehouse, K.M.; Ku, Y.; Albrecht, H.L.; Yang, G.C. Radiat Phys Chem. 1991, 38, pp 61-68. 26. Meier, W.; Burgin, R.; Frohlich, D. Radiat Phys Chem. 1990, 35, pp 332-336. 27. Dohmaru, T.; Furuka, M.; Katayama, T.; Toratani, H.; Takeda, A. Radiat Res. 1989, 120, pp 552-555. 28. Boyd, D.R.; Crone, A.V.J.; Hamilton, J.T.G.; Hand, M.V.; Stevenson, M.H.; Stevenson, P.J. J Agric Food Chem. 1991, 39, pp 789-792. 29. Katusin-Razem, B.; Mihaljevic, B.; Razem, D. Nature. 1990, 345, pg 584. 30. Monk, J.D.; Clavero, R.S.; Beuchat, L.R.; Doyle, M.P.; Brackett, R.E. J Food Prot. 1994, 57, pp 969-974. 31. Karam, L.R.; Simic, M.G. Anal Chem. 1988, 60, pp 1117A-1119A. 32. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pg 48. 33. Den Drijver, L.; HoLzapfel, C.W.; van der Linde, H.J. J Agric Food Chem. 1986, 34, pp 758-762. 34. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pp 38-39. 35. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pg 74. 36. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pg 67. 37. Grootveld, M.; Jain, R.; Claxson, A.W.D.; Naughton, D.; Blake, D.R. Trends Food Sci Technol. 1990, 1, pp 7-14. 38. Moseley, B.E.B. In Food Irradiation and the Chemist; Johnston, D.E.; Stevenson, M.H., Eds.; Annual Chemical Congress Special Publication No. 86; Royal Society of Chemistry: Cambridge, Great Britain, 1990; pp 97-108. 39. Urbain, W.M. Food Irradiation; Food Science and Technology Series; Academic Press, Inc.: Orlando, FL, 1986; pp 74-78. 40. Pohlman, A.J.; Wood, O.B.; Mason, A.C. Food Technol. 1994, 48, pp 46-49. RECEIVED August 10, 1995


Advances in Food Irradiation Research - ACS Symposium Series

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