Packaging Materials Issues

in Irradiation Processing of Foods

 

 

 

 

E-BEAM Services, Inc.

118 Melrich Road

Cranbury, NJ 08512

(609) 655-7460

 

 

 

 

Society of Plastics Engineers

Polyolefins XII International Conference

February 27 – March 1, 2000

Houston, TX

ABSTRACT

 

            Food pasteurization utilizing low doses of ionizing irradiation has been shown effective and reliable in controlling foodborne pathogens in meats, poultry, fish, and other foods. In December of 1997, the U.S. Food and Drug Administration (FDA) formally approved food irradiation of fresh and frozen red meat products at specific doses using a number of approved irradiation methods, including gamma rays, X-rays, and accelerated electrons. Foods that are to be pasteurized by the irradiation process are typically pre-packaged to prevent microbial re-contamination. Thus, the renewed interest in food irradiation processing generated as a result of the FDA's approval has created a need for suitable and affordable radiation-resistant packaging materials. Specifically, following irradiation, such materials must continue to protect food from environmental contamination while maintaining their organoleptic and toxicological properties. The purpose of this paper is to review the basic effects of ionizing radiation on polyolefins, as well as to highlight new product irradiation opportunities for manufacturers of polyolefin resins and packaging materials.

 

 

 

 

 

 

 

INTRODUCTION

 

            Foodborne illness continues to be a significant health issue in the United States today. The Centers for Disease Control (CDC) estimate that 9,000 Americans die each year from diseases caused by pathogenic bacteria such as Camphylobacter, Escherichia coli O157:H7, Listeria monocytogenes, Samonella, and Staphylococcus aureus. Additionally, the Council for Agricultural Science and Technology (CAST) and the CDC estimate that 33 to 81 million Americans suffer from foodborne diarrheal diseases each year. The United Nations Food and Agriculture Organization estimates that  up to 25% of the world’s food supply is lost every year to pests and bacteria.

            Irradiation processing (sometimes referred to as cold pasteurization) when used in concert with proper food handling and processing techniques can greatly reduce the probability that foodborne pathogens associated with poultry, meat, produce, and other types of food will reach consumers, without compromising the nutritional quality of such foods.

            Serious investigations regarding utilization of ionizing radiation for cold pasteurization were initiated in the early 1950s. However, in the 1958 Food Additive Amendment to the Federal Food, Drug, and Cosmetic Act irradiation sources were explicitly defined as food additives, rather than processes. This effectively delayed commercialization of food irradiation for several decades, since authorizing regulation prescribing safe use conditions and pre-market review, as well as acceptance by the US Food and Drug Administration (FDA) was required for each specific food use. Consequently, it was not until the mid-1980s that approvals for the first applications of irradiation for microbial control were granted (herbs, spices, seasonings, dehydrated enzymes, etc.).

            In December of 1997, the FDA amended its food additive regulations to provide for the safe use of ionizing radiation for the  treatment of fresh or frozen uncooked meats to control foodborne pathogens and extend product shelf life. The three approved sources of radiation were: (1) gamma rays (typically produced by radioisotopes of cobalt and cesium), (2) X-rays (with the maximum energy of 5 million electron volts (MeV)), and (3) beams of accelerated electrons (e-beams), with the maximum energy of 10 MeV. Previously, in 1990, the FDA made a similar amendment to allow for irradiation of fresh or frozen poultry. The Food Safety and Inspection Service (FSIS) of the US Department of Agriculture (USDA) is in the process of amending its meat and poultry inspection regulations so as to take into account the above mentioned FDA actions. 

            The purpose of this paper is to provide a brief introduction to irradiation processing and the effects ionizing radiation has on foods and polymeric packaging materials, as well as to address the various material aspects of food irradiation from the perspective of the irradiator, packaging producer, and resin supplier.

 

IRRADIATION PROCESSING

 

            For over sixty years the physical and chemical changes induced by absorption of radiation sufficiently high in energy to produce ionization have been the subject of both university and industrial research. Early work dealing with chemical effects of ionizing radiation utilized the natural radioisotopes radium and radon as radiation sources. At this time, the most common commercial sources of ionizing radiation are ⁶⁰Co and ¹³⁷Cs for gamma irradiation, and electron accelerators for e-beam irradiation. When the electron beam generated by an accelerator is directed at a target consisting of a high-atomic-number metal, such as tungsten or gold, X-rays with a broad spectrum of energies are produced. The amount of energy absorbed, also known as the dose, is measured in units of kiloGrays (kGy), where 1 kGy  is equal to 1,000 Joules per kilogram, or MegaRads (MR or Mrad), where 1 MR is equal to 1,000,000 ergs per gram. With respect to food processing, irradiation applications can be categorized by dose level effects as follows: (1) low dose (up to 1 kGy): sprout inhibition of tubers, ripening delay of fruits, insect de-infestation; (2) medium dose (ca. 1-10 kGy): reduction of pathogenic and spoilage bacteria and parasites; and (3) high dose (over 10 kGy): complete sterility. Maximum doses approved for poultry and meat cold pasteurization are 3 kGy and 7 kGy respectively.  Foods currently irradiated to high doses (e.g., 44 kGy min.) include those for use by astronauts during space flight, and for consumption by hospital patients with severely compromised immune systems.

            While the ionizing radiation provided by e-beams is in the form of electrons, in the case of X-rays and gamma rays, it is provided by photons. The latter have no mass and are thus able to penetrate deeper into materials. Electrons, on the other hand, have a small mass, and are characterized by more limited penetration. Conversely, throughput efficiencies of gamma and X-rays are significantly lower than those of e-beams. For example, a typical 10 MeV, 50 kW e-beam accelerator can cold pasteurize ca. 31,500 kg of food per hour at a dose of 2 kGy. Even a low-power (1 kW)10 MeV accelerator will have a  dose-rate in excess of 450 kGy per hour. The low dose-rate characteristic of the natural radioisotope decay means that  in the case of gamma irradiation dose-rates on the order of 5-10 kGy per hour are typical. Similarly, the throughput efficiency for X-rays is limited by the fact that in addition to generating photons, heavy metal targets generate considerable heat. In fact, while X-ray target conversion efficiencies vary with the atomic number of the metal used, they are typically no higher that 5-8%. In practice this means that in order for an X-ray to process products with the same speed as a 10 MeV, 50 kW e-beam, it will need to have 625 kW of power.

            All forms of ionizing radiation interact with matter by transferring energy to the electrons present in the nuclei of target materials. These electrons may then be either released from the atoms, yielding positively charged ions and free electrons, or moved to a higher-energy atomic orbital, yielding and excited atom or molecule. These ions, electrons, and excited species are the precursors of any chemical changes observed in irradiated material. Thus, by using ionizing radiation, it is possible to synthesize, modify, crosslink, and degrade polymers. Likewise, ionizing radiation has the ability to break the chains of DNA in living organisms, such as bacteria, resulting in microbial death and rendering the space they inhabit sterile. Table 1 provides examples of established current industrial applications of irradiation processing.

 

 

EFFECT OF IONIZING RADIATION ON FOODS

 

As mentioned above, the effect of ionizing radiation on living matter is characterized by cellular destruction stemming from the disruption of the genetic material. That is, the radiation-induced cleavage of chemical bonds in the cell’s DNA results in the inability of the cell to reproduce. On the organism level, the cellular inability to reproduce results in death of the organism. The breaking of chemical bonds described above involves the formation of stable radiolytic products from the reactive ions or free radicals which are formed when  molecules absorb ionizing radiation. These radiolytic species, including glucose, formic acid, and carbon dioxide, are generally the same as those that are formed when food is treated by heat (that is, cooked). In fact in over 30 years of intensive investigation, no radiolytic products specifically unique to irradiated foods have been identified. The FDA estimates the maximum theoretical level of such products at a dose of 1 kGy to be less than 3 mg per Kg of food (3 ppm).

The overall retained nutritional quality of irradiated food depends on a number of factors, including irradiation dose, temperature, food composition, and the presence or absence of oxygen (vacuum vs. atmospheric irradiation). However, scientists believe that irradiation produces no greater nutritional loss than what occurs in other food processing methods, such as cooking or canning. Additionally, nutrient losses can be reduced by irradiating foods in an oxygen-free atmosphere, or while frozen. Table 2 presents a vitamin content comparison in 1 Kg (2.2 lbs.) of cooked chicken.

 

EFFECT OF IONIZING RADIATION ON POLYMERS

 

The effects of ionizing radiation on polymeric materials can be manifested in one of  three ways. The polymer may undergo one of the two possible reactions: those that are  molecular-weight increasing in nature, or molecular-weight reducing in nature.  Or, in the case of radiation-resistant polymers, no significant change in molecular weight will be observed. The conventional term for irradiation-induced increase in molecular weight is crosslinking. The corresponding term for irradiation-induced decrease in molecular weight is chain scissioning.

Each of the two types of reactions are currently being harnessed in an economically beneficial manner to add value to a wide variety of thermoplastics, elastomers, and other materials. For example, the  beneficial changes observed in crosslinked polyethylene (XLPE) include increased modulus, tensile and impact strength, hardness, deflection and service temperature, stress-crack resistance, abrasion resistance, creep and fatigue resistance, and barrier properties. On the other hand, the chain scissioning effects observed in polytetrafluoroethylene (PTFE) have been commercially exploited as an effective means to produce fine micropowders from scrap or off-spec materials.

While radiation responses of various polymers to the three types of radiation mentioned earlier are to a great extent (and with notable exceptions) similar, due to its high throughput efficiency and lack of a nuclear source requirement, e-beam processing is currently the method of choice for irradiation processing of polymers.       

PREDICTING IRRADIATION RESPONSE OF POLYMERS

 

In order to predict the behavior of carbon-chain polymers exposed to ionizing radiation, an empirical rule is used.  According to this rule, polymers containing a hydrogen atom at each carbon atom, predominantly undergo crosslinking, whereas those polymers containing quaternary carbon atoms and polymers of the -CX2-CX2- type (where X is a halogen), chain scissioning predominates. Aromatics, like polystyrene (PS) and polycarbonate (PC) are relatively resistant to EB and are thus well suited to serve as packaging materials for medical disposables which are slated to be radiation sterilized.

During irradiation, chain scissioning occurs simultaneously and competitively with crosslinking, the end result being determined by the ratio of the yields of the two reactions.  For some polymers, such as polyvinyl chloride (PVC), polypropylene (PP), and polyethylene terephthalate (PET), both directions of transformation are possible, and certain conditions exist for the predominance of each one. 

The ratio of crosslinking to scissioning depends on factors including total irradiation dose, dose rate, the presence of oxygen, stabilizers, and radical scavengers, and steric hindrances derived from structural or crystalline forces. With respect to irradiation of foods packaged in polymeric materials, the levels of volatiles, such as aldehydes, ketones, carboxylic acids, etc. generated during irradiation are considerably lower when using e-beam or X-ray technologies than when using gamma. Table 3 illustrates the comparative levels of several carboxylic acids produced during gamma and e-beam irradiation of  a LDPE film at a total dose of 20 kGy.

 

PACKAGING ISSUES

 

Because irradiation of foods in a package holds potential for migration of decomposition products directly into foods, the FDA requires that packaging  used to hold food during irradiation processing comply with regulations based on appropriate testing.  Materials that have been approved are listed in 21 CFR 179.45. These regulations have been amended only once  in recent years. For existing packaging materials approved for food use, but not specifically irradiation processing, to be legally used in cold pasteurization a separate petition or pre-market notification (PMN) is required.

Since material approvals have been added to the CFR on a petition basis, the regulation now comprises an interesting hodge-podge of packaging materials, some approved for certain types of irradiation and at certain doses only, with very few approved for irradiation processing across the board. For example, 21 CFR 179.45(b) specifies a number of packaging materials, but for use with gamma irradiation up 10 kGy only. These include glassine paper, coated cellophane, wax-coated paperboard, kraft paper, Nylon 11, as well as multi-layer PET, PVDC-VC copolymer, PS, and polyolefin films. Likewise 21 CFR 179.45(d) specifies several packaging materials, but for use with gamma or X-ray irradiation up to 60 kGy only. These include vegetable parchments and multi-layer PE, Nylon 6, PET, and PVC-VA copolymer films. Only 21 CFR 179.45(c) specifies one material – ethylene-vinyl acetate copolymers (EVA) – for use with any of the three irradiation sources approved for cold pasteurization up to a dose of 30 kGy.

To summarize the information above, there are no polymers approved for e-beam processing at doses higher than 30 kGy.  There is but a single material approved for use using all three irradiation technologies and only up to the 30 kGy dose. Likewise, the number of packaging materials suitable for high dosage applications using any irradiation source is rather limited. Finally, most of the polymers that are approved under 21 CFR 179.45 are in listed as films. That is, separate approvals would be required if these materials were to be used in rigid packaging form.

Additionally, the food processing industry is generally reluctant to use expensive materials, such as Nylons. While some news have been made with radiation-resistant grades of PP, it is not clear whether such grades contain FDA-approved stabilizer systems and are sufficiently economical to be used in the food processing industry. On the other hand, ethylene and ethylene copolymers are currently the materials of choice for general packaging in the meat and poultry industries. It is anticipated that commercial and institutional producers of ground beef products will be among the first to adopt cold pasteurization processes.

Yet another issue is that packaging materials approved for irradiation of poultry (to a max. dose of 3 kGy) do not fair well at higher irradiation doses which are required to fully cold pasteurize beef (up to 7 kGy). For example, vinegar odors have been noted in irradiated EVA packaging materials used in radiation-sterilized medical devices. Likewise, standard fresh meat overwrap PVC (which, it must be noted,  is not currently approved as an irradiation packaging material) has been demonstrated to have taint-transfer problems at a dose of just under 4 kGy.

Thus, there appear to be a number of new opportunities for manufacturers of PE and copolymer resins to develop and obtain FDA approval for irradiation-suitable packaging constructions, both in film and rigid form,  based on these resins. The approvals petitioned for should include all three approved irradiation technologies, since it is likely that throughput economics will direct certain foods to be preferentially processed by specific irradiation methods (e.g., hamburger patties by e-beam, whole chickens by gamma, etc.).

 

OPPORTUNITIES

 

With respect to the regulatory aspects, major activities already underway include efforts to add X-ray and e-beam processes to 21 CFR 179.45(b) and e-beam process to 21 CFR 179.45(d), which would effectively allow the majority of packaging materials currently approved for one or two irradiation methods to be used with any of the available technologies to doses up to 60 kGy. Additional efforts are also pushing for those materials currently approved for use in film form to be similarly approved for use in rigid form.

However, another major avenue for addressing the packaging issues in irradiation processing of foods is development (followed by subsequent regulatory approval) of new polymeric systems, including both resins and additives, which would widen the packaging options available to food processors, without significantly changing the package manufacturers’ current production methods. It is in this area where substantial new opportunities exist for the polyolefin producers. Any such new materials must be radiation-resistant to, at the very least, total doses slightly higher than the maximum doses approved by the FDA. These materials must be translucent, sealable, pinhole-resistant, and possess appropriate vapor, moisture, aroma and flavor barrier properties. Likewise, the effects of irradiation on polymer additives must also be considered, so as to prevent both loss of additives such as anti-oxidants, thermal stabilizers, etc. and their migration into foods.

 

CONCLUSION

 

            Despite decades of study and lack of significant forward progress, industrial radiation processing of foods in the U.S. has experienced a recent surge of renewed interest, spurred in part by FDA’s approval of meat and poultry cold pasteurization in the 1990’s, and in part by highly publicized relatively recent outbreaks of E. coli and Listeria.  Thus, the FDA actions have created a need for new suitable and affordable radiation-resistant packaging materials in both film and rigid forms. Due to several economic factors, as well as the fact that ethylene and ethylene copolymers are currently the materials of choice for general packaging in the meat and poultry industries, it appears that the polyolefin industry is faced with a unique  opportunity to take the lead in development of such novel radiation-stable packaging materials.

 

BIBLIOGRAPHY

 

Bly, J.H.; Electron Beam Processing. Yardley, PA: International Information Associates (1988).

Chuaqui-Offermanns, N.; Radiation Physics and Chemistry, 34:1005-1007 (1989).

Ellis, J.R.; Irradiation of Meat and Poultry Products and the Need for New Packaging. in Proceedings of Polyolefins XI Conference, Houston, TX: Society of Plastics Engineers (1999).

Food and Drug Administration; Irradiation in the Production, Processing, and handling of Food. 21 CFR Ch. I 179.21-179.45, Washington, DC: FDA-HHS (1999).

Ivanov, V.S.; Radiation Chemistry of Polymers. Utrecht, The Netherlands: VSP BV (1992).

Minbiole, P.R.; Radiation Physics and Chemistry, 46:421-428 (1995).

Murano, E.A, ed.; Food Irradiation: A Sourcebook. Ames, IA: Iowa State University Press (1995).

Olson, D.G.; Food Technology, 52:56-62 (1998).

Pauli, G.H.; U.S. Regulatory Requirements for Irradiating Foods. Washington, DC: FDA Office of Premarket Approval (1999).

Redlinger, P. and D. Nelson; Food Irradiation: What Is It? NCSE Publication 437. Ames, IA: University of Iowa Cooperative Extension Service (1997).

Thayer, et. al; Radiation Pasteurization of Food. Issue Paper  No. 7. Ames, IA: Council for Agricultural Science and Technology (1996).

 

APPENDIX

 

Table 1: Selected Irradiation Processing Applications

 

Table 2: Nutritional Content of Cooked Chicken

 

Table 3: Carboxylic Acid Generation in Irradiated LDPE Film