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 (ebeams), 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 deinfestation; (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 Xrays 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 lowpower (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 radiationinduced 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 molecularweight increasing in nature, or molecular-weight reducing in nature. Or, in the case of radiationresistant 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
Net
Effect
Irradiation
Target
Dose Range
(kGy)
Food
Cold pasteurization
0.3-60
Medical disposable items
Sterilization
10-60
Cellulose/Pulp
Depolymerization
5-50
Coatings
Curing
30-160
Polyolefin foams
Crosslinking
40-80
Heat-shrinkable materials
Memory Imparted
75-250
Rubber
Vulcanization
80-400
Fluoropolymers
Degradation
500-1500
Gemstones
Coloration
10,000+
TABLE 2: NUTRITIONAL CONTENT OF COOKED CHICKEN
Vitamin
Non-Irradiated
Sample
Irradiated
Sample
Vitamin A (IU)
2200
2450
Vitamin E (mg)
3.30
2.15
Thiamin (mg)
0.58
0.42
Riboflavin (mg)
2.10
2.25
Niacin (mg)
58.0
55.5
Vitamin B6 (mg)
1.22
1.35
Vitamin B12 (mg)
21
28
TABLE 3: CARBOXYLIC ACID GENERATION IN IRRADIATED LDPE FILM
Carboxylic Acid
E-Beam (mg/g)
Gamma (mg/g)
n-Valeric
0.60
0.86
n-Butyric
1.46
2.07
Propionic
7.11
10.25
Acetic
9.38
17.09