Packaging for Foods Treated by Ionizing Radiation

From: Packaging for Non Thermal Processing of Food
Edited by: Dr. Jung H. Han
Published: 2007 by Blackwell Publishing

Vanee Komolprasert^*

The information and conclusions presented in this book chapter do not represent new Agency policy nor do they imply an imminent change in existing policy. This chapter is chapter 6 from the above referenced citation. It is currently in press.

Introduction

Ionizing radiation is a nonthermal process utilized to achieve the preservation of food. At a maximum commercial irradiation dose of 10 kGy, irradiation does not impart heat to the food and the nutritional quality of the food is generally unaffected. The irradiation process can reduce microbial contamination on food, resulting in improved microbial safety as well as extended shelf life of the food. In the last decade, many studies have been conducted on irradiation of various foods, especially foods susceptible to foodborne outbreaks, such as meat and meat products. Nonetheless, from a commercial standpoint, foods are generally prepackaged in the final form (aka case ready) before irradiation to avoid recontamination.

Title 21 of the Code of Federal Regulations (CFR), Part 179.25 (General provisions for food irradiation), subparagraph (c), states that packaging materials subjected to irradiation incidental to the radiation treatment and processing of prepackaged foods shall comply with 21 CFR 179.45 (Packaging materials for use during the irradiation of prepackaged foods).

This list in 21 CFR 179.45 does not include many modern packaging materials presently desired by the food industry in light of the commercialization of food irradiation. This, in turn, has presented new challenges to the FDA and regulated industry. This book chapter briefly describes 1) the food additive regulations pertaining to food and packaging materials in contact with food during irradiation, 2) emerging research aimed at determining the radiolysis products (RPs) formed from new packaging materials, including polymers and additives, after exposure to irradiation, and 3) approaches to evaluating the premarket safety assessment of new packaging materials in contact with food during irradiation.

US Regulations for Irradiation of Food and Packaging in Contact with Food

Under Section 201(s) of the Food Additives Amendment to the Federal Food, Drug, and Cosmetic Act (the Act) of 1958, the definition of a food additive includes "...any source of radiation intended for such use". In connection with Section 409 of the Act, this means that a food is deemed adulterated and cannot be legally marketed if it has been intentionally irradiated, unless the irradiation is carried out in compliance with an applicable regulation under the prescribed conditions of use specified in the regulation. The use of packaging materials for irradiated food is considered a new use and is subject to premarket safety evaluation. Components of packaging materials that have been irradiated may migrate to food at different levels in comparison to unirradiated materials. This also holds true for the RPs produced as a result of irradiation of the packaging materials.  This safety concern can be considered in two scenarios, either a packaging material is irradiated before food contact or it is irradiated while in direct contact with food (FDA 1986). In either case, the packaging material used should comply with the applicable regulations, as specified in part 179.45. Thus, as dictated by Chapter 21 of the Code of Federal Regulations, Part 179.25 (denoted as 21 CFR 179.25), the irradiation of both food and packaging materials in contact with food are subject to premarket approval before introduction of the food into interstate commerce.

21 CFR 179 is the primary regulation that covers irradiation in the production, processing, and handling of food, and it is divided into subparts and sections as shown in Table 6.1. Subpart B describes radiation and radiation sources, which include gamma ray, e-beam and X-ray, as well as the general provisions for food irradiation. Subpart B also lists other radiation processes, including radiofrequency radiation, ultraviolet, and pulsed light. These radiation processes are covered elsewhere and will not be included in this chapter. Subpart C describes packaging materials for irradiated foods. The listing regulations in Part 179 are the result of approvals through the food additive petition process codified in 21 CFR 171. 

In Subpart B includes 21 CFR 179.26(b) that lists foods currently permitted to be irradiated as shown in Table 6.2. Irradiated food should be adequately labeled under the general labeling requirements in 21 CFR 179.26(c).

General Aspects of Irradiation and Food Irradiation

Ionizing radiation for the treatment of packaged food can be achieved using gamma rays (with Co-60 or Cesium-137 radioisotope), electron beams, or X-rays, as specified in 21 CFR 179.26(a). The effects of radiation on matter generally depend on the type of the radiation and energy level, as well as the composition, physical state, temperature and environment of the absorbing material, whether it is food or the packaging materials in contact with the food. Chemical changes in matter can occur via primary radiolysis effects, which occur as a result of the adsorption of the energy by the absorbing matter, and can have biological consequences in the case where the target materials include living organisms. With proper application, irradiation can be an effective means of eliminating and/or reducing the microbial load and thus the foodborne diseases they induce, thereby improving the safety of many foods as well as extending their shelf life.

Expert groups of national and international organizations as well as many regulatory agencies have generally concluded that irradiated food is safe and wholesome, and that food irradiation at commonly used dosing levels does not present any enhanced toxicological, microbiological, or nutritional hazards to the food beyond those brought about by conventional food processing techniques. These experts have agreed that irradiation of food for microbial safety should be carried out under Good Manufacturing Practices (GMPs) and Good Irradiation Practices (GIPs). Subsequently, standards on various aspects of radiation processing have been developed and internationally accepted (Farrar et al 1993).

The World Health Organization (WHO) considers ionizing radiation an important process toward ensuring food safety (Diehl 1995). It can be a useful control measure in the production of several types of raw or minimally processed foods such as poultry, meat and meat products, fish, seafood, and fruits and vegetables (Molins et al 2001). An increased interest in food irradiation for quality and microbiological safety was realized by several emerging studies on various food products, including irradiation of meat and meat products (Ahn and Lee 2004; Ahn and Nam 2004; Lacroix et al 2000, 2002, 2004; Montgomery et al 2003), processed meat and ready-to-eat foods (Sommers et al 2003, 2004a, 2004b; Jo et al 2003; Chawla and Chander 2004; Cava et al 2005), poultry products (Nam and Ahn 2003; Lacroix and Chiasson 2004; Javanmard et al 2006), chicken eggs (Pinto et al 2004), seafood (Andrews and Grodner 2004), fresh produce (Han et al 2004; Martins et al 2004; Prakash and Foley 2004; Goularte et al 2004; Chaudry et al 2004; Lacroix and Lafortune 2004), sprouts (Rajkowski and Fan 2004), and fruit juices (Fan et al 2004).  

Packaging Materials for Prepackaged Irradiated Foods

Present day food processors prefer that food be prepackaged in the final packaging form before irradiation to prevent recontamination and to facilitate prompt shipment to market after irradiation. Food could potentially become contaminated with RPs formed in the packaging materials when irradiated in contact with food. This may lead to a safety concern and, therefore, testing of packaging materials after exposure to irradiation is an integral part of the pre-market safety assessment of packaging materials irradiated in contact with food. 

Irradiation can cause changes to the packaging material that might affect its integrity and functionality as a barrier, e.g., to chemical or microbial contamination. Most food packaging materials are composed of polymers that may be susceptible to chemical changes induced by ionizing radiation that are the result of two competing reactions, cross-linking (polymerization) and chain scission (degradation). Radiation-induced cross-linking of polymers dominates under vacuum or an inert atmosphere. Chain scission dominates during irradiation of polymers in the presence of oxygen or air. Both reactions are random, are generally proportional to dose, and depend on dose rate and the oxygen content of the atmosphere in which the polymer is irradiated. The idea of cross-linking predominating under vacuum or an inert atmosphere is important because it has served as the basis for recent approvals under 21 CFR 170.39 (see below) for packaging materials irradiated in contact with food under non-oxygen atmospheres (FDA 2006a).  

The RPs formed upon irradiation of a polymer or adjuvant could migrate into food and affect odor, taste, and safety of the irradiated food (Deschênes et al 1995; Welle et al 2002; Franz and Welle 2004; Stoffers et al 2004). Radiation does not generally affect all properties of a polymer to the same degree. Therefore, the effect of radiation on the formation of RPs and the degree to which the RPs (as well as the base packaging materials) become components of an individual's daily diet under the intended conditions of use must be determined.   

Packaging materials irradiated in contact with food are subject to premarket approval by the FDA and may be used only if they comply with the regulation listings in 21 CFR 179.45 (as discussed above), are the subject of an effective food contact notification (FCN) or a Threshold of Regulation (TOR) exemption under 21 CFR 170.39 (as described in 21 CFR 179.25(c)). These three regulatory options available to all food contact substances (FCSs) are discussed in detail by Twaroski et al (2006). Table 6.3 lists the packaging materials presently authorized under 21 CFR 179.45 and includes films and homogeneous structures at various doses, most of which were initially approved several years ago for irradiation by γ-ray treatment. In response to a recent food additive petition, the listed materials under 21 CFR 179.45(b) were evaluated for being subjected to a dose not to exceed 10 kGy incidental to the use of any radiation source in the treatment of packaged food. In its safety assessment of that petition, FDA concluded that gamma, e-beam and X-ray sources are equivalent in terms of the types and levels of RPs generated in the packaging materials under the conditions at which prepackaged foods are irradiated (FDA 2001). As a result, 21 CFR 179.45(b) was recently amended to allow the listed materials to be subjected to a dose not to exceed 10 kGy incidental to the use of any radiation source in the radiation treatment of prepackaged foods. Recently, submissions for specific packaging constructions and conditions of use were exempted from the need for a regulation listing as per 21 CFR 170.39 (FDA 2006a).     

It should be noted that the packaging materials in 21 CFR 179.45 do not generally meet today's needs as do newer materials that may be more desirable to the food industry. However, many of these newer packaging materials have not yet been evaluated by FDA. In addition to the base polymers, adjuvants such as anitoxidants and stabilizers, are also of concern with regard to RPs. Such adjuvants are prone to degradation during polymer processing and, moreover, during irradiation as they degrade preferentially over the polymer and could result in significant levels of RPs migrating into food. Therefore, the migration of both base polymers and adjuvants, as well as migration of their RPs, must be evaluated in the premarket safety assessment prior to their use.  

Evaluation of New Packaging Materials for Irradiation in Contact with Food

To demonstrate the radiation stability of a packaging material, it must be shown that irradiation does not significantly alter the physical and chemical properties of the material (Chuaqui-Offermanns 1989). Physical and chemical testing of materials is widely acceptable for radiation sterilization of medical products, in which their properties are important for the desired performance of the medical products during use. This type of testing is not sufficient for packaging materials intended to contact food during irradiation. Packaging materials and their RPs can potentially migrate into food at significant levels, although changes in the material properties of the packaging may be insignificant or undetected. Hence, the overall assessment of new packaging materials in contact with food during irradiation should include an assessment of any changes in physical and chemical properties as a result of irradiation, and a premarket safety assessment of the packaging materials and their RPs that might become components of an individual's diet under the conditions of use.

Most of the early studies that described the effects of ionizing radiation on various food packaging polymers, such as polyolefins, were conducted at fairly high dose levels compared to the levels typically used in food processing. Most of the new studies have extended irradiation to other homopolymers, as well as copolymer and multilayer structures, as well as adjuvants. Paquette (2004) surveyed the literature for the types of RPs formed and their respective concentrations in several irradiated polymers, including PS, PET, LDPE, PP, EVA, PA6 and PVC (abbreviations are listed in acronyms table at the end of this book chapter), all of which contained adjuvants, as well as concentrations of the RPs in food simulants. Paquette concluded that the formation of RPs depended on the absorbed dose, dose rate, atmosphere, temperature, time after irradiation and food simulant. The RPs from the polymers consisted of low molecular weight aldehydes, acids and olefins. The dietary exposures to most RPs formed in the materials surveyed were determined to be less than 0.5 µg/kg in the daily diet, less than the Threshold of Regulation concern level as per 21 CFR 170.39. However, the author noted that in contrast to the base polymers, the adjuvants identified in the survey were not currently listed in 21 CFR 179.45.

Polyolefins, EVOH and Polyamides

Loveridge and Milch (2004) recently demonstrated that  physical testing is necessary for determining the integrity of multilayered food pouches irradiated at a very high dose (>44 kGy). The authors observed that the irradiation significantly contributes to seal strength loss that affects the package integrity. Goulas et al (2004a) observed that gamma irradiation >30 kGy dose discolored most monolayer and multilayer commercial semi-rigid materials made of PS, PP, PET, PVC/HDPE, HDPE/PA, and HDPE. Goulas and co-worker (2003) reported that lower dose levels (5-10 kGy gamma radiation) did not significantly change the mechanical and permeation properties as well as overall migration levels of commercial multilayer films, including co-extruded PP, EVOH, LDPE, LLDPE, PA and ionomer. Chytiri et al (2006) experimentally showed that the effects of 5-60 kGy gamma radiation on the thermal, mechanical and permeation properties, as well as infrared spectra, of multilayered films containing 25-50 wt% of a recycled LDPE layer sandwiched between virgin LDPE layers, were not different from films made with 100% virgin LDPE. Regardless of the presence of recycled LDPE in the film, the authors concluded that 60-kGy irradiation dose induced mechanical change but did not affect other properties of the film.

Deschênes at al (1995) observed that a PA/PVDC/EVA multilayered barrier film produced volatile aldehydes and other hydrocarbons that caused off-odor and taints in water irradiated as low as 1 kGy. Riganakos et al (1999) detected volatile compounds generated in monolayer films of LDPE and EVA copolymer and multilayer films (PET/PE/EVOH/PE) after e-beam radiation at 5, 20, and 100 kGy doses. These volatiles were primary and secondary oxidation products including aldehydes, ketones, alcohols and carboxylic acids, all of which are known to affect the organoleptic properties and shelf-life of irradiated foods. The concentrations of these compounds increased with increased irradiation dose. Regardless of irradiation dose, the authors concluded that the infrared spectra as well as the permeability to oxygen, water and carbon dioxide of these materials were not significantly altered by irradiation even at doses as high as 100 kGy.

Adjuvants

Several recent studies focused on the effects of irradiation on hindered phenol antioxidants in packaging polymers. Deschênes et al (2004) reported that gamma radiation degraded a hindered phosphite antioxidant, Irgafos 168, in PE and PP to phosphate oxidation products. The phosphate products further degraded to other products during irradiation. Similarly, Stoffers and co-worker (2004) observed that irradiation degraded Irganox 1076 and Irgafos 168 in PE, but did not affect Irganox 1076 in PS even after irradiation at a dose up to 54 kGy. This was reported to be indicative of the stability of PS, which was also observed by Kawamura (2004). The authors also reported that adjuvant RPs and other degradation products affected the sensory properties of the irradiated LDPE, HDPE, PA6 and PA12.

Jeon et al (2007) did not detect migration of Irgafos 168 from a LLDPE pouch irradiated up to 200 kGy in food simulants (distilled water, 4% (v/v) acetic acid, and 20% aqueous ethanol). However, the authors detected decreasing migration levels of Irganox 1076 with increased irradiation dose. On the other hand, levels of the Irganox 1076 RPs (2,4-di-tert-butylphenol, 1,3-di-tert-butylbenzene, and toluene) in the simulants increased with increased irradiation dose. These common RPs were also detected by Kawamura (2004) who extensively studied the effects of 10-50 kGy gamma radiation on numerous antioxidants and UV stabilizers in commercial sheets and films of PE, PP and PS. The author concluded that polymer stability was in the order of PS>PE>PP. These results are consistent with a study of Pentimalli and co-workers (2000), which showed that PS was not affected by gamma irradiation even at high doses. Kawamura also observed that the presence of adjuvants reduced the loss of mechanical properties of the polymers and were essential during irradiation of the polymers. The authors also studied the effects of irradiation on polybutylene and some copolymers in the absence or presence of antioxidant and stabilizers, and observed the crucial role of antioxidants and stabilizers in protecting polybutylene and butadiene-containing copolymers from degradation reactions.

Plasticizers

Goulas et al (2004b) extensively studied the effects of both gamma and e-beam radiation, at doses in a range of 4-50 kGy, on migration of the plasticizers di-(2-ethylhexyl)adipate (DEHA) and acetyltributyl citrate (ATBC) from PVC and PVDC/VC copolymer films into chicken meat and olive oil. They found that 4-9 kGy radiation did not affect migration of these plasticizers into food, but that dose levels of 20-50 kGy radiation did lead to higher levels in food. Zygoura et al (2007) studied the effects of gamma radiation, in the range of 10-25 kGy, on the migration of DEHA and ATBC from a PVC film into isooctane, a solvent often used as a fatty-food simulant. The authors reported that plasticizer migration levels increased with increased gamma radiation. Unlike Irgafos 168, irradiation did not induce transformations of these two plasticizers.

Semi-Rigid PET

PET films are regulated in 21 CFR 179.45, but semi-rigid and rigid PET structures are not. The effects of irradiation on  semi-rigid and rigid PET structures may be different from the films. Komolprasert (1998) conducted a preliminary study using ten different semi-rigid PET materials (crystalline and amorphous) of various properties and characteristics, irradiated at 25 kGy gamma radiation, which are listed in Table 6.4. The non-volatile, soluble solids were extracted from these PET materials, using Soxhlet extraction with acetone, and the percent extractable results are presented in Table 6.5. These results indicated that 25 kGy gamma irradiation did not significantly increase the amount of soluble solids or PET cyclic trimer.  PET cyclic trimer was the major component that accounted for more than 50% of the total soluble solids extracted.

Komolprasert and co-workers (2001) conducted an in-depth study on the effects of a 25 kGy gamma radiation on two of the semi-rigid crystalline PET polymers listed in Table 6.4 (categories 1 and 5). The authors reported that the volatiles detected were formic acid, acetic acid, 1,3-dioxalane, and 2-methyl-1,3-dioxolane. As above, PET cyclic trimer was the major non-volatile detected in the soluble solids and the level was not affected by irradiation. The authors concluded that irradiation of PET at 25 kGy significantly increased the amount of volatiles but not the non-volatiles.     

Komolprasert and co-workers (2003a) continued their study using two of the semi-rigid amorphous PET copolymers listed in Table 6.4(categories 9 and 10) irradiated using both gamma and e-beam radiation at 5, 25 and 50 kGy. The authors reported detection of the same volatiles as in the previous study, but acetaldehyde, 1,3-dioxolane and 2-methyl-1,3-dioxolane were quantifiable. Levels of these volatiles increased with increased radiation dose. Besides PET cyclic trimer, terephthalic acid, bis-(2-hydroxyethyl) terephthalate, dimethyl terephthalate, mono-hydroxy ethylene terephthalate, and tetramer are additional non-volatiles that are generally known to be present in the soluble fraction from PET. Levels of these non-volatiles did not significantly increase after irradiation at all doses. The authors also conducted migration studies on these polymers using 10% aqueous ethanol and 100% heptane solvents under various conditions of use (i.e., FDA migration testing protocols). Migration levels of these non-volatiles were less than 5 ppb under Conditions of Use E (room temperature filled and stored) and F (refrigeration storage), with levels higher than 5 ppb under condition of Use D (hot filled or pasteurized below 66^oC). Based on the overall results obtained, the authors concluded that gamma and e-beam radiation did not generate any new, non-volatile RPs and radiation effects on these PET materials are similar, regardless of the irradiation dose.

Nylon 6I/6T

Nylon 6I/6T, a polyamide of 1,6-hexamethylene, terephthalic acid and isophthalic acid, is also among the new packaging materials of interest to industry for use in contact with food during irradiation. Nylon 6I/6T of certain compositions is listed for certain uses in 21 CFR 177.1500(a)(12), but it is not authorized for contact with food during irradiation. Komolprasert et al (2002) and McNeal et al (2004) studied the effects of gamma irradiation on Nylon 6I/6T powder (amorphous, 1.2 g/cc) at doses up to 50 kGy. The authors reported that irradiation generated volatiles, i.e., n-butanal, acetic acid, methyl-cyclopentene-1-one, methyl ethyl ketone and n-pentanal at levels as high as 2 ppm, 30 ppm, 2 ppm, 2 ppm and 72 ppm, respectively, in the irradiated powder samples. The authors evaluated non-volatiles by determining the percent soluble solids extracted from the powder specimens before and after 5-50 kGy irradiation, using 10% and 50% ethanol solutions at 40^oC for 10 days. As shown in Table 6.6, irradiation did not significantly increase the percent soluble solids from the powders.

Fig. 6.1 contains the representative HPLC chromatograms of the 10% ethanol extractables for 50 kGy irradiated and non-irradiated Nylon 6I/6T powder. The chromatograms are not significantly different, suggesting that irradiation had no effect on the formation of non-volatiles present in the ethanol extractable solids. The largest peak, with a parent ion m/z = 738 (verified by LC/MS), was consistent with a Nylon 6I/6T oligomer for n=3. The overall results indicated that irradiation significantly increased the concentrations of volatiles, but had little or no measurable effect on the extractable non-volatiles.

Figure 6.1. Representative Liquid Chromatograms of Extractable Solids from Non-irradiated (NIR) and Irradiated Powdered PA Test Specimens (IR).  Extraction Conditions: 10% ethanol/water maintained at 40^oC for 10 days. HPLC-PDA Analysis at 210 nm.
m/z =738 Represents the Parent Ion consistent with an Oligomer of n=3.

Ethylene-vinyl alcohol copolymer (EVOH)

EVOH copolymers listed in 21 CFR 177.1360 are widely used as a non-food contact barrier layer in food packaging. They  significantly improve the gas and vapor barrier properties of multilaminate food packages. EVOH copolymers are not currently approved for use in contact with food during irradiation. Komolprasert et al (2003b) and McNeal et al (2004) studied the effects of 5-50 kGy e-beam irradiation on an EVOH copolymer powder (38 mol-% ethylene, 1.17 g/cc) manufactured with and without α-methyl styrene dimer as an inhibitor.

For volatiles, the authors reported that e-beam radiation produced many low molecular weight aliphatic hydrocarbons and oxidation products in EVOH powder without inhibitor, while fewer polymer breakdown products were detected in EVOH powder with inhibitor. For EVOH powder containing inhibitor, the major volatiles were breakdown products of the inhibitor, including alkyl aromatics such as tert-butyl benzene and isobutyl benzene, and oxygenated aromatics such as acetophenone and 2-phenyl isopropanol. The results, contained in Table 6.7, indicate that e-beam irradiation also produced propanal, methyl ethyl ketone, 2-butanol, tert-butyl benzene, and 2-methylpropylbenzene in measurable amounts. Volatiles detected in both non-irradiated and irradiated test specimens were acetic acid, cumene, α-methyl styrene, acetophenone, and 2-phenyl isopropanol. The authors concluded that the concentrations of most volatile substances in the EVOH powder samples increased with the irradiation dose, but the increase in concentration was nonlinear.

For non-volatiles, the authors observed that the percent extractable soluble solids from EVOH powder was greater with 50% ethanol than with 10% ethanol (Tables 6.8 and 6.9). For EVOH powder with inhibitor, e-beam irradiation at 5-50 kGy doses did not significantly increase the levels of extractable solids soluble in 10% and 50% ethanol (Table 6.8). On the other hand, for EVOH powder without inhibitor, e-beam radiation significantly increased the levels of extractable soluble solids in both 10% and 50% ethanol (Table 6.9). The authors concluded that EVOH powder containing α-methyl styrene dimer was chemically more stable than EVOH without when exposed to e-beam radiation up to 50 kGy.

An adsorbent pad

Another packaging material that is integral to prepackaged food is the absorbent pad. The adsorbent pad is widely used for refrigerated, uncooked meat, poultry, pork, and seafood products. Komolprasert (1999) conducted a preliminary study (unpublished) to determine the effects of 7 kGy gamma irradiation on an adsorbent pad. In the study, the absorbent pad was comprised of two white-pigmented LDPE layers, one of which was perforated, which were then sealed on all four sides to contain a cellulose pad. The identity of additives and adhesives used in the LDPE layers of the adsorbent pad were unavailable. Test pads were exposed to 7 kGy gamma-irradiation at room temperature, analyzed for volatile and semi-volatiles using HS/GC/MS, and subsequently extracted for non-volatiles with 10% ethanol and 2-propanol at 40^oC for 10 days.

The initial HS/GC/MS results showed that the irradiation generated 2.3-3.14 ppm of 1,3-di-tert-butyl benzene, 0.78-1.40 ppm of nonanal, and 0.24-0.41 ppm of cyclopentanone based on the adsorbent pad weight. 1,3-Di-tert-butylbenzene was a degradation product of hindered phenolic and arylphosphite antioxidants (Marque et al 1998; Krzymein et al 2001). Other volatiles detected at lower levels by GC/MS were acetophenone, 2-butoxyethanol, 1-cyclopentylethanone, and benzaldehyde.  

The solvent extraction results showed that the soluble extractable solids using 10% ethanol increased from 0.28% before irradiation to 0.43 % after irradiation, and the solids in 2-propanol increased slightly from 0.84% before irradiation to 0.91% after irradiation. Qualitative HPLC/PDA analysis of the soluble solids showed that irradiation did not generate any new substances, rather the residues consisted of BHT, Irganox 1010, Irganox 1076, and 2,4-di-tert-butylphenol (largest peak), a by-product from the degradation of phenolic antioxidants like BHT, Irganox 1010 and Irgafos 168 (Marque et al 1998; Carlsson et al 2001; Krzymein et al 2001), which are commonly used in LDPE (Ehret-Henry et al 1992).

Additional HPLC analyses were performed to determine degradation products in the cellulose pad. The results showed that irradiation did not generate significant amounts of glucose and cellobiose, as judged by their migration levels in 10% ethanol (< 10 ppb) after a storage at 40^oC for 10 days. Irradiation could produce hydrolyzed cellulose material (Saeman et al 1952), which may lead to progressive degradation of cellulose and production of the low molecular weight carbohydrates during a longer storage time. It was observed that after a storage at 22 ± 2^oC for 16 weeks, the concentrations of glucose and cellobiose in 10% aqueous ethanol increased to 26 ± 11 ppb and 92 ± 36 ppb, respectively. These high concentrations, however, were postulated as unlikely resulting from irradiation; rather they were due to the long storage time which was determined to be unrealistic for refrigerated, uncooked meat packaged with the adsorbent pad.

Colorants for polymers

The effects of irradiation on several antioxidants and stabilizers have been studied, but colorants for polymers have not yet been studied in a systematic manner. Komolprasert and co-workers (2006) studied the effects of 10-20 kGy gamma radiation on two colorants present at levels of 0.1-1 wt.-% in PS. The colorants were Chromophtal Yellow 2RLTS (2,3,4,5-tetrachloro-6-cyanobenzoic acid, aka Yellow 110) and Irgalite Blue GBP (copper (II) phthalocyanine blue, aka Copper II blue). Chromophtal Yellow is an organic pigment (85-90 wt.-%) with 5-15 wt.-% hydrogenated rosin. Irgalite Blue is an organometallic pigment (90-99 wt.-%) with 1-5 wt.-% polymerized rosin and 1-5 wt.-% of a copper phthalocyanine derivative. Both colorants are non-volatile, water insoluble, relatively heat stable and are regulated under 21 CFR 178.3297 (Colorants for polymers).  Neither colorant is regulated for use as a component of packaging materials irradiated in contact with food.

Qualitatively, the results indicated that gamma radiation did not affect the infrared spectra of the pure colorants and the PS test specimens. Qualitative HS/GC/MS analyses identified many volatiles in the colorants, with more volatiles detected from the pure yellow colorant than the pure blue colorant, regardless of irradiation dose. Volatiles found in pure colorants were not found in PS test specimens containing colorants. Irradiation did not generate new compounds in PS containing either colorant at concentrations of up to 1 wt.-%. The amounts of PS solids migrating into 10% and 50% ethanol maintained at 40^oC for 10 days were in a range of 0.0035 - 0.013 wt.-%. Total polymer dissolution of the irradiated PS test samples and HPLC/PDA analyses of the extracts showed typical residuals expected from PS, e.g., phenol, phenyl ethanol, benzaldehyde, acetophenone, and styrene (Buchalla et al 2002). Irradiation increased the concentrations of these residues, with the exception of styrene (unchanged, ca. 200 ppm). Based on GPC/PDA analysis, the extracts also contained oligomers with molecular weights <500 Dalton. The overall results suggested that both yellow and blue colorants are relatively stable to irradiation. 

Approaches to the Safety Assessment of New Packaging Materials Irradiated in Contact with Foods

Any new packaging material not yet listed in 21 CFR 179.45 or the subject of an effective FCN or TOR exemption is subject to a pre-market safety assessment by FDA prior to irradiation in contact with food. As discussed in detail by Twaroski et al (2006), the safety information required in a new submission to the Agency includes chemistry, toxicology and environmental components. With regard to the chemistry and toxicology data, the safety assessment focuses on the likely consumer exposure and available toxicology information on those substances (i.e., packaging materials and their RPs) that become components of food from the intended use. With regard to the chemistry data, this includes the identity and amounts of migrants, as well as migration or other data to allow the calculation of dietary exposures for the packaging materials and their RPs under the intended conditions of use. FDA recommends that all data and information be generated in accordance with the available guidance documents (http://www.cfsan.fda.gov/~dms/guidance.html).

The first step to an appropriate testing protocol might involve an irradiation experiment designed to simulate the actual application conditions for determining the effects of irradiation on the packaging materials. After irradiation, the test materials are analyzed using methods and techniques that may require various analytical instruments. Because the effects of ionizing radiation on packaging material are random, the identities of potential RPs are generally unknown, but, in many cases, may be deduced from the structure of the polymer or adjuvant. However, any analytical methods used in the analysis for RPs in an irradiated test specimen should give some consideration to identification and quantification of an unknown migrant. GC/MS is commonly used for identification of volatile RPs, which can be achieved using a well established chemical library database. On the other hand, LC/MS would be preferred for identification of non-volatile RPs. However, LC/MS is not as widely available and its use is limited due to lack of chemical databases for identifying the unknowns as well as other technical difficulties, e.g. mobile phase solvents that might affect the unknown analytes. Other analytical methods and techniques such as solvent extraction and GPC may be used as well. Once the identities and levels of packaging materials and their RPs are known, exposure estimates can be determined.

Identification of RPs can depend on the test parameters, such as the analytical techniques employed or sample preparation procedures. Komolprasert and co-workers (2001) reported that headspace analyses detected formic and acetic acids in 25 kGy gamma irradiated PET test specimens, but these acids were not detected by a thermal desorption technique. They reported that these acids possibly underwent further reactions with ethylene glycol during thermal desorption to form 1,3-dioxolane and 2-methyl-1,3-dioxolane. Therefore, these two dioxolanes were detected at higher concentrations using thermal desorption than by the headspace analyses. The authors also reported that acetaldehyde was detected, but was not quantifiable, and postulated that the GC column could also dictate the separation and detection of compounds, and affect the identification of the unknown RPs.

In addition, different forms of test specimens can give different results. Komolprasert and co-workers (2003a) observed that the amount of volatiles detected in ground PET test specimens were greater than those detected in PET sheet specimens. For PET sheets and powders of identical mass, the larger surface area of ground PET samples contributed to faster desorption of volatiles, resulting in larger concentrations of volatiles detected. This suggests that results obtained from the powder irradiation are a worse case.   

Alternatives to an Experimental Approach

An irradiation experiment is expensive and time consuming, considered that several experiments may be required in addition to the need for migration studies on irradiated materials. Hence, alternative approaches to the safety assessment are desirable. One approach is through the use of migration modeling instead of the assumption of 100% migration to food. Migration modeling based on the principles of diffusion has been accepted for determining the suitability of a recycled plastic for food contact (FDA 2006b). Therefore, a modeling approach might be considered for evaluating the safe use of packaging materials in contact with food during irradiation. 

Simulation modeling of food irradiation is the subject of recent studies. Brescia and co-workers (2003) recently studied the e-beam processing of an apple, and observed that the energy (dose) distribution within the apple depended on the entrance region of the electrons, the path the electrons follow as a result of scattering due to irregular (curvature) shape, and the maximum penetration distance into the apple. They reported that e-beam would not deposit uniform energy unless the apple was tilted 30^o against and towards the irradiation source. The results were realized through a simulation modeling using Monte Carlo N-Particle (MCNP) software. This software is a general-purpose Monte Carlo N-Particle code that can be used for neutron, photon, electron, or coupled neutron/photon/electron transport. Their simulation results were recently validated by Kim and co-workers (2006) who used Monte Carlo simulation to calculate dose in e-beam irradiation of complex foods, using an apple as a model. They reported that measured and calculated dose distribution values were similar. Cleland and co-workers (2005) also applied the Monte Carlo simulation to determine energy distribution in industrial X-ray food processing. They reported that the experimental measurements and the simulation results were verified for suitability and accuracy of the method between 3 and 8 MeV.    

The Monte Carlo simulation may be a useful tool for predicting the energy distribution of an ionizing radiation source as well as the degree of its effects on packaging materials irradiated in contact with food. The dose distribution data obtained by the simulation might provide dose distribution to predict the degree of degradation in the irradiated packaging material. Theoretically, dose or energy required to degrade a polymer depends on the polymer structure and its chemical moieties or functional groups. Sadler (2004) recently applied ion chemistry to predict the amount of ionized species that may be formed in a polymer at a dose of 10 kGy. The calculations were based on the ion-pair energy of the polymer bond. Sadler concluded that this approach may be used to evaluate the levels of RPs in food. Earlier, Kothapali and Sadler (2003) irradiated EVOH samples with gamma rays at 3-10 kGy dose and analyzed the irradiated samples versus the controls. GC/MS analysis and subsequent migration testing (95% aqueous ethanol, 40^oC for 10 days) suggested that irradiation did not produce any new RPs. Although acetic acid levels were found to increase after irradiation, the authors noted that acetic acid is affirmed as generally recognized as safe (GRAS) for certain uses in food under 21 CFR 184.1005 (Acetic acid) and is of no safety concern. Based on the overall results, they concluded that the amounts of RPs migrating from EVOH would not result in a DC to exceed 0.5 ppb. This type of analysis may well prove to be generally useful after studies are conducted.

In addition to dietary exposure to packaging materials and their RPs, FDA's pre-market safety assessment includes an evaluation of the toxicological information on these substances.  As discussed above, the identities of RPs are generally unknown, but may be deduced from the structure of the polymer or adjuvant. This, in turn, might allow like RPs, such as low molecular weight carboxylic acids, to be grouped and evaluated as a structural class rather than individually. Bailey et al (2005) recently reported on the use of structure-activity analysis (SAR) in the food contact notification (FCN) program to determine the toxicity of components of food packaging materials according to their structural similarities with many industrial chemicals that have been analyzed for toxicological concern. SAR has been shown to be a useful tool in the FCN program and has potential to be useful in the safety assessment of unknown RPs from the irradiation of packaging materials in contact with food.       

Conclusions

Improved microbiological safety of food may be attained by ionizing radiation. Many packaging materials are authorized for use in the manufacture of food packaging, but only a few are authorized for use in contact with food during irradiation. FDA's safety assessment for food contact articles, including packaging materials irradiated in contact with food, hinges on likely consumer exposure to and available toxicological information on the packaging materials and their RPs produced as a result of radiation. Any testing protocols used in evaluating packaging materials incidental to irradiation should account for the possibly RPs that are generally unknown but may, in part, be deduced from the structure of the material.

Monte Carlo simulation has been shown to be a useful tool in modeling the irradiation of food and may have potential for predicting the effects of irradiation on packaging materials. FDA recently began applying the use of SAR in the safety assessment of components of packaging materials. Given that the identity of RPs are generally unknown, but may often be grouped into classes of substances, SAR may be useful. The combination of Monte Carlo simulation modeling for predicting the types of RPs as well as their amounts, in conjunction with SAR for toxicity, may prove to be useful in evaluating the safety of the packaging materials irradiated in contact with food.  

References

^*Dr. Vanee Komolprasert is a Consumer Safety Officer in the Division of Food Contact Substance Notification and Review; Office of Food Additive Safety; Center for Food Safety and Applied Nutrition; US Food and Drug Administration; 5100 Paint Branch Parkway; College Park, MD 20740.