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Food packaging materials and radiation processing of food: A brief review
Radiar. Phys. Chem. Vol. 34, No. 6, pp. 1005-1007, hr. J. Radiat. Appl. Instrum., Parr C Printed in Great Britain
1989
0146-5724/89 $3.00 + 0.00 Pergamon Press plc
FOOD PACKAGING MATERIALS AND RADIATION PROCESSING OF FOOD: A BRIEF REVIEW N. CHUAQUI-OFFERMANNS Radiation Applications Research Branch, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada ROE IL0 (Received 16 March 1989; accepted 8 May 1989)
Abstract-Food is usually packaged to prevent microbial contamination and spoilage. Ionizing radiation can be applied to food-packaging materials in two ways: (i) sterilization of packaging materials for aseptic packaging, and (ii) radiation processing of prepackaged food. In aseptic packaging, a sterile package is filled with a sterile product in a microbiologically controlled environment. In irradiation of prepackaged food, the food and the packaging material are irradiated simultaneously. For both applications, the radiation stability of the packaging material is a key consideration if the technology is to be used successfully. To demonstrate the radiation stability of the packaging material, it must be shown that irradiation does not significantly alter the physical and chemical properties of the material. The irradiated material must protect the food from environmental contamination while maintaining its organoleptic and toxicological properties. Single-layer plastics cannot meet the requirements of either application. Multilayered structures produced by coextrusion would likely satisfy the demands of radiation processing prepackaged food. In aseptic packaging, the package is irradiated prior to filling, making demands on toxicological safety less stringent. Therefore, multilayered structures produced by coextrusion, lamination or co-injection moulding could satisfy the requirements.
INTRODUCTION In radiation technology, ionizing radiation an energy source in various industrial
PACKAGING MATERIALS FOR RADIATION PROCESSING PREPACKAGED FOOD is used as
processes. Applications include the industrial radiation sterilization of medical disposables and packaging materials, radiation curing of surface coatings and radiation crosslinking of plastics, and are among the widely accepted applications of this technology (Markovic, 1988; Saunders, 1988). Despite more than 35 yr of study, industrial radiation processing of food is growing only slowly. There has been renewed interest in using this technology for food processing in recent years, especially after several countries removed some of their regulatory restrictions. For example, food irradiation is no longer considered an additive, and many countries have introduced regulations to control irradiation as a food process instead (Health and Welfare Canada, Health Protection Branch, 1988; U.S. Food and Drug Administration, 1986a, b). Food that is to be radiation processed is often prepackaged to prevent microbial recontamination, and so the packaging material is also exposed to radiation during processing. The renewed international interest in using ionizing radiation to process food has created a need for suitable packaging materials that can withstand radiation processing. Radiation sterilization of packaging containers for aseptic packaging of food is another application of radiation processing that requires the availability of suitable radiation-resistant materials.
When packaged food is radiation processed, the packaging material is also exposed to radiation, and its chemical and physical stability under irradiation becomes very important. The two major effects of ionizing radiation on polymers are cross-linking and chain scission. In general, these two competing effects occur simultaneously, and the predominating effect depends on the structure of the polymer (Bovey, 1958; King et al., 1964; Skiens, 1980; Dickson, 1988). The net effect of cross-linking reactions is to modify the mechanical properties of the material such as: (i) increasing the tensile strength; (ii) hardening; (iii) changing the solvent resistance; and (iv) decreasing the impact strength. Chain scission involves random rupturing of the molecular bonds of the material, thus leading to the formation of short-chain polymers, evolution of gases and change in the extractables. Chemicals formed as a result of the radiation treatment may interact with the food, affecting its organoleptic characteristics and/or its toxicological safety. The conditions under which the food (and packaging) are irradiated have a significant influence on the behaviour of the material (Skiens, 1980). For example, volatile compounds evolved when a lowdensity polyethylene is irradiated under vacuum consist mainly of hydrogen plus saturated and unsaturated hydrocarbons (Killoran, 1972). However, when the irradiation is done in the presence of
1005
1006
N. CHUAQUI-OFFERMANNS
oxygen, the volatiles evolved are aldehydes, ketones, carboxylic acids and saturated hydrocarbons (Azuma et al., 1983). Therefore, it is important to determine the chemical and physical stability (with respect to radiation dose, temperature and atmosphere) of the packaging materials under the expected conditions of use. Radiation treatment of prepackaged food may be classified broadly into two categories in terms of dose: (i) processes requiring doses less than 10 kGy, such as extending the shelf life of refrigerated meats, elimination of contaminant pathogens, and many others; (ii) processes requiring doses from 20 to 40 kGy for production of commerical sterility (Josephson and Peterson, 1982). The temperatures most commonly used in the process may vary from room temperature to -40°C. Recent developments in food irradiation technology involve the use of combined processes such as modified atmosphere packaging (MAP) and irradiation, or combined heat and radiation treatment, which impose more demands on the properties of the material to be used. Gamma radiation is most widely used because of its high penetrating power. Electron-beam irradiation can also be used for certain specialty prepackaged foods. In the late 1960s the U.S. Food and Drug Administration approved several materials for use in radiation prepackaged food. Unfortunately these materials were all single films that do not satisfy modern packaging needs. Some of these needs involve complex structures using two or more films with different barrier properties. Barriers to moisture, gases, aroma or flavor are frequently used in these formulations. Aroma- and flavor-barrier materials are needed to prevent extraneous odors from contaminating the product. Some foods are spiced or flavored artificially, and the packaging material should be able to retain this flavor. The most commonly used aroma-flavor barriers are nylon and ethylene viny1 alcohol (EVOH). Moisture barriers are essential in most packaging applications to keep the desired level of moisture in the product. Polypropylene, high-density polyethylene and polyvinylidene chloride are good moisture barriers. Gas barriers are essential in modified atmosphere packaging (MAP) or controlled atmosphere packaging (CAP). In these two cases, the desired atmosphere has to be kept inside the package. In many commercially available structures, nylon or polyester are used as moderate gas barriers. High-barrier structures contain either polyvinylidene chloride (PVDC) or EVOH. The studies that led to the approval of the materials listed in the U.S. Code of Federal Regulations 71, Food and Drug 179, Subpart C, neglected the effect of radiation on the additives present in the plastics. A wide range of polymers including polyvinyl chloride
(PVC), polyethylene and polypropylene contain antioxidants such as phenolic and arylphosphate compounds (Irganox 1076, 1010 and 1330, and Irgafos 168). Organotin compounds are added to PVC to protect it against thermal degradation. Studies (Allen et al., 1979; Brooks et al., 1985; Allen et al., 1985) have shown that irradiation degrades organotin compounds. Haesen et al. (1983) reported that these degradation products have a greater propensity than the stabilizers themselves to migrate into food. In recent studies Allen et al. (1987a, b, 1988) have reported that in PVC, polyethylene and polypropylene losses of up to 30% of hindered phenolic antioxidants are obtained on exposure to a radiation dose of 30 kGy. These authors have also reported that their results strongly suggest that these degradation products become covalently bound to the polymer. The confirmation of these results is essential because it would result in a significant reduction in the concerns over the migration of these extraneous substances into the food. In these studies it has also been shown that the arylphosphate stabilizer, Irgafos 168, undergoes extensive degradation due to y-irradiation. Some of the degradation products were identified in extracts of the irradiated polymers. These facts indicate that additional approvals are needed for suitable packaging materials for use in radiation processing of prepackaged food. PACKAGING
MATERIALS IN ASEPTIC OF FOODS
PACKAGING
Aseptic packaging involves sterilizing the food, the packaging and the filling environment separately. There are several types of aseptic containers that can be used in the process. Some of these are paperboard/foil structures, plastic in a single or multilayer sheet used in a thermoform/fill/seal system, plastic used in a bag-in-box system, etc. These materials can be sterilized by irradiation with ultraviolet light, gamma rays, accelerated electrons or by treatment with hydrogen peroxide in combination with low heat. The use of irradiation to sterilize packaging materials for aseptic packaging has been increasing ever since the U.S. Food and Drug Administration banned ethylene oxide as a sterilizing agent for aseptically filled pouches. U.S. Food and Drug Administration regulations allow a hydrogen peroxide sterilization system to be used only with polyethylene food-contact surfaces. Aseptic packaging has become popular because it allows the food industry to use inexpensive materials such as plastics instead of metal or glass. However, no single-layer plastic film possess all the properties required in aseptic packaging of food. Materials with oxygen- and moisture-barrier properties are often required. Multilayer constructions produced by COextrusion are commonly used.
Food packaging materials and irradiation Once again, suitable formulations that can be radiation sterilized are needed. Much work has already been done on the radiation stability of polymeric films irradiated at sterilizing doses. Much of the resulting information is from the widespread use of radiation to sterilize medical products (Landfield, 1980; Skiens, 1980). This information is very useful to evaluate the suitability of plastic materials to be radiation sterilized for aseptic packaging of food. However, it should be emphasized that radiation stability of these materials has been assessed primarily by determining the effects of irradiation on their mechanical properties. In radiation processing of food, the considerations on the suitability of the packaging material are more stringent. A material that protects the food from contamination and spoilage may nevertheless be the source of substances migrating into the food. Specific studies about the suitability of particular multilayer plastic films in radiation sterilization of food were conducted by Killoran (Killoran et al., 1967; Killoran, 1984). None of these materials have yet been approved for use in radiation processing of food (Thayer, 1988). CONCLUSIONS
The materials already cleared for radiation processing do not satisfy all the needs of modern food packaging. In order to meet these needs, new materials must be identified and approved. Submissions for approval should contain information on the effect radiation has on extractables, volatiles, and the mechanical properties of the material. Modern analytical techniques should be used to obtain this information. Special attention should be paid to the effect of radiation on additives used as stabilizers in the formulation of plastic structures. The presence of gas-barrier films is of particular importance, because if gases are evolved due to radiation, their dissipation will depend on the composition of the barrier layer. It is possible that satisfactory packaging films could be produced by coextrusion of polymers that are already approved. This does not imply that they could be used without any further testing. However, their approval may be simpler than that of a whole new structure.
REFERENCES
Allen D. W., Brooks J. S., Richard W., Mellor M. and Williamson A. (1979) Mode of action of organotin stabilizers in PVC: a study of tin-l 19 Mossbauer spectroscopy. Chem. Indusr. 19, 663. Allen D. W., Brooks J. S., Unwin J. and McGuinness J. (1985) Gamma irradiation of food contact plastics:
R.P c
3416-I’
1007
identification of tin-containing intermediates in the degradation of organotin-stabilized PVC by gamma irradiation. Chem. Indust. 15, 524. Allen D. W., Leathard D. A., Smith C. and McGuinness J. (1987a) Effects of gamma irradiation on hindered phenol antioxidants in poly(viny1 chloride) and polyolefins. Chem. Indust. 6, 198. Allen D. W., Leathard D. A. and Smith C. (1987b) Gammairradiation of food contact plastics: the rapid destruction of an arylphosphite antioxidant in polyproylene. Chem. Indust. 24, 854. Allen D. W., Leathard D. A. and Smith C. (1988) The effects of gamma irradiation of food contact plastics on the extent of migration of hindered phenol antioxidants into fatty food simulants. Chem. Indust. 12, 399. Azuma K., Hirata T., Tsunoda H., Ishitani T. and Tanaka Y. (1983) Identification of volatiles from low density polyethylene film irradiated with electron beam. Agric. Biol. Chem. 47 (4), 855. Bovey F. A. (1958) The Effects of Ionizing Radiation on Natural and Synthetic High Polymers. Interscience, New York. Brooks J., Allen D. and Unwin J. (1985) A tin-119M Mossbauer study of the degradation of organotinstabilized PVC by gamma irradiation. Polym. Deg. Stab. 10 (1) 79. Dickson L. W. (1988) A preliminary assessment on the effects of radiation on polymer properties. Atomic Energy of Canada report, AECL-9556. Haesen G., Depaus R., van Tilbeurgh H., Le goff B. and Lox F. (1983) The effect of gamma-irradiation on the migration behaviour of organotin additives in PVC. J. Indust. Irradiat. Technol. i (3), 259. Health Protection Branch (1988) Information Letter No. 746. Josephson E. S. and Peterson M. S. (1982) Preservation of Food bv Ionizing Radiation. Vol. l-3. CRC Press. Boca Raton,‘Florida.Killoran J. J. (1972) Chemical and physical changes in food packaging materials exposed to ionizing radiation. Radial. Res. Rev. 3, 369. Killoran J. J. (1984) Packaging materials for use during the ionizing irradiation sterilization of prepackaged chicken products. Raltech Scientific Services, Inc. Report, ERRCARS-40-82-84. Killoran J. J., Breyer J. D. and Wierbicki E. (1967) Development of flexible containers for irradiated food. Fd Technol. 21 (87), 73. King R. W., Broadway N. J., Mayer R. A. and Palinchak S. (1964). Polymers. In Effect of Radiation on Materials and Components. Chapt. 3, p. 84. Reinhold, New York. Landfield H. (1980), The effects of radiation on various materials and the qualification tests required for their use in medical devices. Radial. Phys. Chem. 15, 39. Markovic V. (1988) Radiation Technology. IAEA/RCA Seminar, Jakarta. Saunders C. B. (1988) Radiation processing in the plastics industry: current commerical applications. Atomic Energy of Canada Report, AECL-9569. Skiens W. E. (1980) Sterilizing radiation effects on selected polymers. Radiat. Phys. Chem. 15, 47. Thayer D. W. (1988) Chemical changes in food packaging resulting from ionizing radiation. Am. Chem. Sot. Symp. Ser. 365, 181. U.S. Food and Drug Administration (1986a) Federal Register 51 (75), 13376. U.S. Food and Drug Administration (1986b) Code of Federal Regulations, Title 21, Subpart C, 179.45, 356.
1989
0146-5724/89 $3.00 + 0.00 Pergamon Press plc
FOOD PACKAGING MATERIALS AND RADIATION PROCESSING OF FOOD: A BRIEF REVIEW N. CHUAQUI-OFFERMANNS Radiation Applications Research Branch, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada ROE IL0 (Received 16 March 1989; accepted 8 May 1989)
Abstract-Food is usually packaged to prevent microbial contamination and spoilage. Ionizing radiation can be applied to food-packaging materials in two ways: (i) sterilization of packaging materials for aseptic packaging, and (ii) radiation processing of prepackaged food. In aseptic packaging, a sterile package is filled with a sterile product in a microbiologically controlled environment. In irradiation of prepackaged food, the food and the packaging material are irradiated simultaneously. For both applications, the radiation stability of the packaging material is a key consideration if the technology is to be used successfully. To demonstrate the radiation stability of the packaging material, it must be shown that irradiation does not significantly alter the physical and chemical properties of the material. The irradiated material must protect the food from environmental contamination while maintaining its organoleptic and toxicological properties. Single-layer plastics cannot meet the requirements of either application. Multilayered structures produced by coextrusion would likely satisfy the demands of radiation processing prepackaged food. In aseptic packaging, the package is irradiated prior to filling, making demands on toxicological safety less stringent. Therefore, multilayered structures produced by coextrusion, lamination or co-injection moulding could satisfy the requirements.
INTRODUCTION In radiation technology, ionizing radiation an energy source in various industrial
PACKAGING MATERIALS FOR RADIATION PROCESSING PREPACKAGED FOOD is used as
processes. Applications include the industrial radiation sterilization of medical disposables and packaging materials, radiation curing of surface coatings and radiation crosslinking of plastics, and are among the widely accepted applications of this technology (Markovic, 1988; Saunders, 1988). Despite more than 35 yr of study, industrial radiation processing of food is growing only slowly. There has been renewed interest in using this technology for food processing in recent years, especially after several countries removed some of their regulatory restrictions. For example, food irradiation is no longer considered an additive, and many countries have introduced regulations to control irradiation as a food process instead (Health and Welfare Canada, Health Protection Branch, 1988; U.S. Food and Drug Administration, 1986a, b). Food that is to be radiation processed is often prepackaged to prevent microbial recontamination, and so the packaging material is also exposed to radiation during processing. The renewed international interest in using ionizing radiation to process food has created a need for suitable packaging materials that can withstand radiation processing. Radiation sterilization of packaging containers for aseptic packaging of food is another application of radiation processing that requires the availability of suitable radiation-resistant materials.
When packaged food is radiation processed, the packaging material is also exposed to radiation, and its chemical and physical stability under irradiation becomes very important. The two major effects of ionizing radiation on polymers are cross-linking and chain scission. In general, these two competing effects occur simultaneously, and the predominating effect depends on the structure of the polymer (Bovey, 1958; King et al., 1964; Skiens, 1980; Dickson, 1988). The net effect of cross-linking reactions is to modify the mechanical properties of the material such as: (i) increasing the tensile strength; (ii) hardening; (iii) changing the solvent resistance; and (iv) decreasing the impact strength. Chain scission involves random rupturing of the molecular bonds of the material, thus leading to the formation of short-chain polymers, evolution of gases and change in the extractables. Chemicals formed as a result of the radiation treatment may interact with the food, affecting its organoleptic characteristics and/or its toxicological safety. The conditions under which the food (and packaging) are irradiated have a significant influence on the behaviour of the material (Skiens, 1980). For example, volatile compounds evolved when a lowdensity polyethylene is irradiated under vacuum consist mainly of hydrogen plus saturated and unsaturated hydrocarbons (Killoran, 1972). However, when the irradiation is done in the presence of
1005
1006
N. CHUAQUI-OFFERMANNS
oxygen, the volatiles evolved are aldehydes, ketones, carboxylic acids and saturated hydrocarbons (Azuma et al., 1983). Therefore, it is important to determine the chemical and physical stability (with respect to radiation dose, temperature and atmosphere) of the packaging materials under the expected conditions of use. Radiation treatment of prepackaged food may be classified broadly into two categories in terms of dose: (i) processes requiring doses less than 10 kGy, such as extending the shelf life of refrigerated meats, elimination of contaminant pathogens, and many others; (ii) processes requiring doses from 20 to 40 kGy for production of commerical sterility (Josephson and Peterson, 1982). The temperatures most commonly used in the process may vary from room temperature to -40°C. Recent developments in food irradiation technology involve the use of combined processes such as modified atmosphere packaging (MAP) and irradiation, or combined heat and radiation treatment, which impose more demands on the properties of the material to be used. Gamma radiation is most widely used because of its high penetrating power. Electron-beam irradiation can also be used for certain specialty prepackaged foods. In the late 1960s the U.S. Food and Drug Administration approved several materials for use in radiation prepackaged food. Unfortunately these materials were all single films that do not satisfy modern packaging needs. Some of these needs involve complex structures using two or more films with different barrier properties. Barriers to moisture, gases, aroma or flavor are frequently used in these formulations. Aroma- and flavor-barrier materials are needed to prevent extraneous odors from contaminating the product. Some foods are spiced or flavored artificially, and the packaging material should be able to retain this flavor. The most commonly used aroma-flavor barriers are nylon and ethylene viny1 alcohol (EVOH). Moisture barriers are essential in most packaging applications to keep the desired level of moisture in the product. Polypropylene, high-density polyethylene and polyvinylidene chloride are good moisture barriers. Gas barriers are essential in modified atmosphere packaging (MAP) or controlled atmosphere packaging (CAP). In these two cases, the desired atmosphere has to be kept inside the package. In many commercially available structures, nylon or polyester are used as moderate gas barriers. High-barrier structures contain either polyvinylidene chloride (PVDC) or EVOH. The studies that led to the approval of the materials listed in the U.S. Code of Federal Regulations 71, Food and Drug 179, Subpart C, neglected the effect of radiation on the additives present in the plastics. A wide range of polymers including polyvinyl chloride
(PVC), polyethylene and polypropylene contain antioxidants such as phenolic and arylphosphate compounds (Irganox 1076, 1010 and 1330, and Irgafos 168). Organotin compounds are added to PVC to protect it against thermal degradation. Studies (Allen et al., 1979; Brooks et al., 1985; Allen et al., 1985) have shown that irradiation degrades organotin compounds. Haesen et al. (1983) reported that these degradation products have a greater propensity than the stabilizers themselves to migrate into food. In recent studies Allen et al. (1987a, b, 1988) have reported that in PVC, polyethylene and polypropylene losses of up to 30% of hindered phenolic antioxidants are obtained on exposure to a radiation dose of 30 kGy. These authors have also reported that their results strongly suggest that these degradation products become covalently bound to the polymer. The confirmation of these results is essential because it would result in a significant reduction in the concerns over the migration of these extraneous substances into the food. In these studies it has also been shown that the arylphosphate stabilizer, Irgafos 168, undergoes extensive degradation due to y-irradiation. Some of the degradation products were identified in extracts of the irradiated polymers. These facts indicate that additional approvals are needed for suitable packaging materials for use in radiation processing of prepackaged food. PACKAGING
MATERIALS IN ASEPTIC OF FOODS
PACKAGING
Aseptic packaging involves sterilizing the food, the packaging and the filling environment separately. There are several types of aseptic containers that can be used in the process. Some of these are paperboard/foil structures, plastic in a single or multilayer sheet used in a thermoform/fill/seal system, plastic used in a bag-in-box system, etc. These materials can be sterilized by irradiation with ultraviolet light, gamma rays, accelerated electrons or by treatment with hydrogen peroxide in combination with low heat. The use of irradiation to sterilize packaging materials for aseptic packaging has been increasing ever since the U.S. Food and Drug Administration banned ethylene oxide as a sterilizing agent for aseptically filled pouches. U.S. Food and Drug Administration regulations allow a hydrogen peroxide sterilization system to be used only with polyethylene food-contact surfaces. Aseptic packaging has become popular because it allows the food industry to use inexpensive materials such as plastics instead of metal or glass. However, no single-layer plastic film possess all the properties required in aseptic packaging of food. Materials with oxygen- and moisture-barrier properties are often required. Multilayer constructions produced by COextrusion are commonly used.
Food packaging materials and irradiation Once again, suitable formulations that can be radiation sterilized are needed. Much work has already been done on the radiation stability of polymeric films irradiated at sterilizing doses. Much of the resulting information is from the widespread use of radiation to sterilize medical products (Landfield, 1980; Skiens, 1980). This information is very useful to evaluate the suitability of plastic materials to be radiation sterilized for aseptic packaging of food. However, it should be emphasized that radiation stability of these materials has been assessed primarily by determining the effects of irradiation on their mechanical properties. In radiation processing of food, the considerations on the suitability of the packaging material are more stringent. A material that protects the food from contamination and spoilage may nevertheless be the source of substances migrating into the food. Specific studies about the suitability of particular multilayer plastic films in radiation sterilization of food were conducted by Killoran (Killoran et al., 1967; Killoran, 1984). None of these materials have yet been approved for use in radiation processing of food (Thayer, 1988). CONCLUSIONS
The materials already cleared for radiation processing do not satisfy all the needs of modern food packaging. In order to meet these needs, new materials must be identified and approved. Submissions for approval should contain information on the effect radiation has on extractables, volatiles, and the mechanical properties of the material. Modern analytical techniques should be used to obtain this information. Special attention should be paid to the effect of radiation on additives used as stabilizers in the formulation of plastic structures. The presence of gas-barrier films is of particular importance, because if gases are evolved due to radiation, their dissipation will depend on the composition of the barrier layer. It is possible that satisfactory packaging films could be produced by coextrusion of polymers that are already approved. This does not imply that they could be used without any further testing. However, their approval may be simpler than that of a whole new structure.
REFERENCES
Allen D. W., Brooks J. S., Richard W., Mellor M. and Williamson A. (1979) Mode of action of organotin stabilizers in PVC: a study of tin-l 19 Mossbauer spectroscopy. Chem. Indusr. 19, 663. Allen D. W., Brooks J. S., Unwin J. and McGuinness J. (1985) Gamma irradiation of food contact plastics:
R.P c
3416-I’
1007
identification of tin-containing intermediates in the degradation of organotin-stabilized PVC by gamma irradiation. Chem. Indust. 15, 524. Allen D. W., Leathard D. A., Smith C. and McGuinness J. (1987a) Effects of gamma irradiation on hindered phenol antioxidants in poly(viny1 chloride) and polyolefins. Chem. Indust. 6, 198. Allen D. W., Leathard D. A. and Smith C. (1987b) Gammairradiation of food contact plastics: the rapid destruction of an arylphosphite antioxidant in polyproylene. Chem. Indust. 24, 854. Allen D. W., Leathard D. A. and Smith C. (1988) The effects of gamma irradiation of food contact plastics on the extent of migration of hindered phenol antioxidants into fatty food simulants. Chem. Indust. 12, 399. Azuma K., Hirata T., Tsunoda H., Ishitani T. and Tanaka Y. (1983) Identification of volatiles from low density polyethylene film irradiated with electron beam. Agric. Biol. Chem. 47 (4), 855. Bovey F. A. (1958) The Effects of Ionizing Radiation on Natural and Synthetic High Polymers. Interscience, New York. Brooks J., Allen D. and Unwin J. (1985) A tin-119M Mossbauer study of the degradation of organotinstabilized PVC by gamma irradiation. Polym. Deg. Stab. 10 (1) 79. Dickson L. W. (1988) A preliminary assessment on the effects of radiation on polymer properties. Atomic Energy of Canada report, AECL-9556. Haesen G., Depaus R., van Tilbeurgh H., Le goff B. and Lox F. (1983) The effect of gamma-irradiation on the migration behaviour of organotin additives in PVC. J. Indust. Irradiat. Technol. i (3), 259. Health Protection Branch (1988) Information Letter No. 746. Josephson E. S. and Peterson M. S. (1982) Preservation of Food bv Ionizing Radiation. Vol. l-3. CRC Press. Boca Raton,‘Florida.Killoran J. J. (1972) Chemical and physical changes in food packaging materials exposed to ionizing radiation. Radial. Res. Rev. 3, 369. Killoran J. J. (1984) Packaging materials for use during the ionizing irradiation sterilization of prepackaged chicken products. Raltech Scientific Services, Inc. Report, ERRCARS-40-82-84. Killoran J. J., Breyer J. D. and Wierbicki E. (1967) Development of flexible containers for irradiated food. Fd Technol. 21 (87), 73. King R. W., Broadway N. J., Mayer R. A. and Palinchak S. (1964). Polymers. In Effect of Radiation on Materials and Components. Chapt. 3, p. 84. Reinhold, New York. Landfield H. (1980), The effects of radiation on various materials and the qualification tests required for their use in medical devices. Radial. Phys. Chem. 15, 39. Markovic V. (1988) Radiation Technology. IAEA/RCA Seminar, Jakarta. Saunders C. B. (1988) Radiation processing in the plastics industry: current commerical applications. Atomic Energy of Canada Report, AECL-9569. Skiens W. E. (1980) Sterilizing radiation effects on selected polymers. Radiat. Phys. Chem. 15, 47. Thayer D. W. (1988) Chemical changes in food packaging resulting from ionizing radiation. Am. Chem. Sot. Symp. Ser. 365, 181. U.S. Food and Drug Administration (1986a) Federal Register 51 (75), 13376. U.S. Food and Drug Administration (1986b) Code of Federal Regulations, Title 21, Subpart C, 179.45, 356.