Irradiation of Food and Packaging - American Chemical Society

Chapter 17

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Postirradiation Transformation of Additives in Irradiated HDPE Food Packaging Materials: Case Study of Irgafos 168 L. 1

1

Deschênes ,

2

1

D . J. Carlsson , Y. Wang , a n d

C.

1

Labrèche

Food Research and Development Centre, Agriculture & Agri-Food Canada, St-Hyacinthe, Quebec J2S 8E, Canada Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, Ontario K1A 0R6, Canada 2

This paper discusses the evolution of Irgafos 168 and its corresponding phosphate in gamma irradiated high density polyethylene used for food packaging materials. Long-term changes in the levels of the antioxidants were observed up to 6 months after irradiation. These data point out the important role of post-irradiation aging in assessing the magnitude of indirect additives and their migration from packaging materials into food. Investigation of post-irradiation aging is also relevant for a better understanding of degradation mechanisms taking place in plastics during and after irradiation process. The shelf-life of the material and of the packaged food products should be considered in the risk assessment to ensure quality and safety of irradiated food products and packaging materials.

Published 2004 American Chemical Society

In Irradiation of Food and Packaging; Komolprasert, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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278


Introduction Food packaging materials can be affected by ionizing radiation in the course of their sterilization for aseptic filling or in the course of food irradiation processes. The radiation-induced degradation in polyethylene (PE) and other polymers has been known for many years, and several reviews and papers have described in detail the effects of irradiation on their chemical structure and mechanical properties (/, 2, 3, 4). From these published data, it can be concluded that for the low doses applied upon food irradiation (usually less than 10 kGy) and packaging material sterilization (25 kGy), the mechanical and structural changes in polyethylene (PE) are of a minor extent and significance. Although there are some concerns regarding post-irradiation oxidation for prosthesis and other type of implants in the field of biomaterials (4, 5), and for food applications, these changes do not affect the mechanical performance relevant for the intended use and the shelf-life of the materials (6). The primary concern in the application of radiation processing to food packaging materials is the migration of indirect additives that can affect sensory quality of packaged products as well as food safety (7, 8, 9, 10). The salient feature in the literature, on the production of volatile compounds from irradiated packaging materials, is that most studies have been focused on measuring the levels of low molecular weight chemicals regardless of the material post-irradiation age (10, 11, 12). It was observed from electron spin resonance (ESR) spectra that long-lived radicals may be present for many years after irradiation in ultra-high molecular weight polyethylene (13, 14). A review of the literature concerning the evolution of food packaging materials after ionization treatment indicates that very few data are actually available relating to that topic and that only a few published studies looked at this aspect of irradiated food packaging materials. These few studies, however, covered only periods of a few hours or a few days (75, 16). The interest of studying the effect of irradiation on phosphite antioxidants comes from the fact that they are commonly used as additives to stabilize polyolefins (PP and PE), which are the most widely used polymers in food packaging materials. Therefore, migration rates of these additives into food should be monitored to ensure the food safety. It was previously reported that these additives and some of their degradation products were identified as migrating chemical species from irradiated food packaging materials (10, 17). The present paper reports selected results from different studies undertaken within our research group regarding the effect of time after irradiation on tris (2,4-di-ter/-butylphenyl) phosphite (Irgafos 168) levels and on the formation of its corresponding phosphate compound in high density polyethylene (HDPE) materials. The investigation on the formation of other conversion products identified in the course of these research activities will be published separately later.


279

Materials and Methods


Materials Commercial H D P E food trays ( D Y N O 528 designed for retort cooking) were obtained from Dynoplast, Norway. Tray samples, excluding the rim for sealing, were ground into powder (0.2 mm) in a Wiley mill. During grinding liquid nitrogen was added continuously to keep the mill operating at low temperature to avoid heat-induced chemical changes. The resulting H D P E powder (3 g samples) was placed in sealed vials for irradiation. Standard samples for H P L C experiments were obtained in the following way: (1) Irgafos 168 (denoted PI, Figure 1) was donated by Ciba Specialty Chemicals, (2) we prepared the corresponding phosphate (denoted P A , Figure 1) by quantitative conversion using an emulsion of hydrogen peroxide (30% in water, Aldrich) with dichrolomethane for 1 hour at room temperature. PI and P A concentrations in trays before and after irradiation were determined directly by transmission Fourier transform infrared spectroscopy (FTIR) on samples cut from trays. This method allowed in situ analyses of these phosphorous compounds, which had well characterized IR absorptions, free of interference with the HDPE's IR spectrum. Because of variations of the PI and P A content in the commercial samples between batches, the pre-irradiation concentrations were established for each sample studies by FTIR and/or liquid chromatography (see below).

rVoPI

P=0

PA

Figure 1. Structure of Irgafos 168 (PI) and its conversion to phosphate (PA)

y-Irradiation Vials filled with H D P E samples were irradiated in duplicate by a Nordion 651 PT Gamma beam pilot-scale irradiator ( C o irradiation source, dose rate of 11 kGy/h). Treatment was applied in static air at ambient temperature for absorbed doses of 1, 2, 4, 7, 10, 25, and 48 kGy. The dosimetry was carried out with MSD-Nordion ceric-cerous dosimeters GFSX-197 and F-99 types for dose range of 0-10 and 5-100 kGy, respectively. Dose variation was estimated to be 60


280 2%. The irradiated samples were stored in a dark place at ambient temperature prior to extraction.


Soxhlet Extraction Powder from H D P E samples was extracted for 18h using distilled dicholoromethane solvent in Soxhlet extractors. 0.1 ml of triethylphosphite (TEP) was added into the flasks before extraction to prevent oxidation of antioxidant and radiation-induced degradation products during die work up. Glass beads were added into the flasks in order to keep uniform heating. It was verified that longer extraction time did not result in higher extraction yields. The solutions obtained were vacuum evaporated to dryness and the residues were dissolved in 5 ml of tetrahydrofuran (THF, H P L C grade, Fisher Scientific). One ml of the resultant solution was filtered with syringe filters (0.45 urn, Whatman) and filled into H P L C vials (1 ml, Kimble). H P L C experiments were conducted immediately after the sample preparation. To determine the analyte recoveries from the Sohxlet procedure, the dicholoromethane solvents were spiked with predetermined quantities o f PI and P A standards prior to extraction for blank runs (without H D P E powder). The average recoveries for both PI and P A were found to be 87% from H P L C measurements. Chromatographic Analysis Chromatographic analyses were performed with a Hewlett-Packard Series 1050 H P L C system equipped with a photodiode array (PDA) detector. A Zorbax C I 8 reverse phase column was used as a stationary phase. Mobile phase was a gradient combination of acetonitrile, water and THF. The experimental parameters were: mobile-phase flow rate: 1 ml/min; injection volume of the extracts: 20 | i l ; detector wavelengths: 260 and 280 nm. Based on H P L C chromatograms of standard samples, the retention time for Irgafos 168 (PI) and its corresponding phosphate (PA) were determined to be 24.2 and 23.6 min, respectively. For quantification, a solution with a known concentration was prepared for each standard sample in triplicate. Response factors (units of integrated peak area per microgram) were determined to be 156 for PI at 280 nm, and 118 for P A at 260 nm.

Results The efficiency of hindered phosphites as polymer stabilizers is due to their effectiveness in decomposing hydroperoxides (18). During thermal oxidation and y-irradiation, the hindered phosphite PI contained in H D P E is considered to


281 be mainly converted to its phosphate form, P A (18, 19, 20). In a previous study on degradation of Irgafos 168 in another batch of D Y N O H D P E trays, we used FTIR to measure the concentration-changes from IR absorptions of PI at 848 cm' , and P A at 965 cm" , for doses ranging from 0 to 8 kGy. The absorption assignations were confirmed by analysis of the respective pure compounds (20). Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 18, 2013 | http://pubs.acs.org Publication Date: January 2, 2004 | doi: 10.1021/bk-2004-0875.ch017

1

1

It is noteworthy that all the FTIR data were obtained within 1 h following irradiation. The data reported in Figure 2 show that the conversion rate of radiation-induced conversion of PI to P A was very significant within a dose range of 0-3 kGy. However, from 3 to 7 kGy, a slight decrease of concentration was observed for both PI and P A . Carlsson et al. (20) also reported a gradual decrease of PI and a gradual increase of P A during 12 h post-irradiation storage following the irradiation of H D P E to an intermediated dose of 1.3 kGy, where much PI still remained (Figure 2). To the best of our knowledge, this previous work reported for the first time the phosphite loss presumably resulting from long-lived radicals in the polyethylene matrix of commercial food containers. This points out the role of of long-lived radicals in the modification of the concentration profile of the antioxidants during storage of irradiated H D P E food packaging materials. In order to get an overview of this transformation during a storage time closer to reality, we have irradiated the same type of H D P E trays and then estimated the level of PI and P A over a post-irradiation period of 6 months by H P L C analysis of Soxhlet extractives.

Phosphite (848 cm-1) Phosphate (965 cm-1)

0

1

2

3

4

5

6

Dose (kGy)

7

8 Based on FT-IR measuremen

Figure 2. PI to PA conversion in irradiated HDPE trays during irradiation based on FTIR spectral evaluation. (Reproduced with permission from reference 20. Copyright 2001 Taylor & Francis.)



282 For food irradiation purposes, doses at a lower range of 0.3 - 3 kGy are typically employed to control pests and mold in fresh fruits and vegetables. Doses at a higher range o f 25-30 kGy are employed to (1) sterilize packaging materials, (2) treat spices, herbs and dehydrated vegetables or (3) produce readyto-eat meals (17). To cover both of these ranges, approximate doses o f 1 and 25 kGy were selected to investigate the effect of post-irradiation storage time on PI and P A . From H P L C measurements, the initial levels of PI and P A in nonirradiated commercial H D P E D Y N O trays were estimated to be 407 and 551 ppm, respectively. The presence of P A in all of the non-irradiated trays probably results from oxidative degradation in the course of the extensive mechanical-shear and heating stages, producing free radicals that are involved in the compounding and the molding processes. Additional non-radiation-induced conversion of remaining PI to P A may result from slow in-storage oxidation, within the H D P E resin. AH these types of degradation of organic phosphites into phosphates in polymers are well-known phenomena (19, 21). Figure 3 shows H P L C chromatograms demonstrating the PI-to-PA conversion at various room-temperature storage times in H D P E trays irradiated to 1.1 kGy dose. These chromatograms clearly suggest a continuous conversion that is taking place over a period of 6 months.

iLmonihs. 3 months lrnonth 1 week

1 day 4

6

8

10 12

14

16

18 20

22 24

26 28

Elution time (min)

Figure 3. HPLC chromatograms of extractivesfromHDPE irradiated to 1.1 kGy: Evolution with post-irradiation storage-time at room temperature

Figure 4 shows radiation-induced changes of PI and P A concentrations in H D P E after irradiation at doses of 0, 1.1, and 25.3 kGy as a function of storage time. From these curves, it can be seen that for 0 kGy, the changes of P A and PI



283 concentrations are primarily due to the storage conditions and permeation rate of oxygen through the material. In that case, a very slow linear decrease of the PI concentration was accompanied by a slow, linear increase in the P A concentration. This is clearly shown by the corresponding time-profile of the summation of PI and P A concentrations presented in Figure 5. These concentrations were converted into units of mole/g of material to take into account the fact that PI and P A have different molecular weights, in order to obtain an absolute value of conversion. For 25.3 kGy, no PI was left in the material right after the irradiation (Figure 4). For this dose, only minor changes were observed in the level of P A over 6 months of storage, despite the presence of long-living radicals trapped in the polymer matrix due to the irradiation. Conversely, the time-profile of P A concentrations after a low irradiation dose of 1.1 kGy shows a significant increase during the first month of storage (Figure 4). Only 12 % of the initial concentration of PI was destroyed during the actual irradiation process (measurement done within less than 24 h after irradiation). But after 6 months, the PI had almost completely disappeared from the material. Both the concentration and production rate of P A in H D P E appeared to be strongly related to the availability of PI in the polymer. The time-profile for the dose of 1.1 kGy shows a significant decrease in the total level of PI and P A over this first month of storage (Figure 5), indicating the existence of additional radiation-induced degradation routes for the PI additive in H D P E following yirradiation. The subsequent conversion of PI to P A presents a rate of conversion apparently similar to that in the non-irradiated materials. This observation can be attributed to oxidative degradation reactions that are normally occurring during aging of the material, rather than being related to irradiation effects. It can be deduced from these observations that (1) P A is not affected by long-term radicals but (2) these reactive species (radiation-induced long-lived radicals) affect PI, leading to the formation of degradation products other than P A during the first month of storage. Simultaneous conversion of PI to chemical compounds other than P A from radiation-induced radicals is probably involved in the course of the irradiation process as well, explaining why the yield of conversion of PI to P A was less than 100% and was dose dependant immediately after irradiation. This hypothesis deduced from H P L C data is in accord with the data previously obtained from the FTIR measurements (20), which are presented in Figure 2. This is shown clearly in Figure 6, where the FTIR data are also presented as the summation of the PI and the P A molar concentrations. These results show that, following irradiation, conversion of PI to P A is less than quantitative and gradually decreases with dose. This important finding is in good agreement with data obtained by H P L C covering the same dose range (22).


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-*-PI,0kGy Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 18, 2013 | http://pubs.acs.org Publication Date: January 2, 2004 | doi: 10.1021/bk-2004-0875.ch017

-*-PA,OkGy - & - P I , 1.1 kGy - • - P A , U kGy

-X-PI, 25.3 kGy - • - P A , 25.3 kGy

Figure 4. Time-dependence of concentration changes post-irradiation induced in PI and PA in HDPE trays DYNO 528 (from HPLC measurements)

-PI+PA,OkGy -PI+PA,UkGy -PI+PA,25.3kGy

50

100

150

200

Time (days) Figure 5. Summation of PI and PA concentrations as a function of storage time (from HPLC measurements)



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0,8 H

1

1

1

0

2

4

6

8

Dose (kGy) Figure 6. Summation of PI and PA molar concentrations as a function of absorbed dose (calculated from FTIR data obtained less than one hour after irradiation)

From the combined evidence from Figures 4, 5, and 6, we can conclude that the primary PI-to-PA radiation-induced conversion is effected by free radicals and takes place within one month after irradiation. This conclusion is valid for a PI initial level of 407 ppm that has been studied. The results also strongly suggest that subsequent conversion of PI to P A is mostly due to oxidative degradation processes related to normal aging of the material with only minor effect related to radiation-produced, long-lived free radicals.

Discussion H D P E extraction followed by H P L C analysis for quantification of Irgafos 168 and its corresponding phosphate gave the results that are in good agreement with those obtained from the direct FTIR measurement of these additives in the polymer. As expected, irradiation process induced significant effects on the antioxidant additive. It was observed that the radiation-induced oxidation continues during storage. This can lead to embrittlement of the polymer as already reported for polypropylene articles such as syringes that were radiationsterilized for medical applications (23). As previously mentioned, from the literature, the hindered phosphite PI contained in H D P E is believed to be mainly converted to its phosphate form, P A , during thermal oxidation and ^-irradiation. Our data presented here indicate that several, radiation-induced, degradation routes for PI are probably taking place simultaneously. On the other hand, it should be kept in mind that the



286 phosphate itself may be degraded in the course of the radiation process; as phosphates are recognized to have a high capture cross-section for the fast electrons, which result from irradiation of the polymer as reported by Halmann (24). However, Stevenson and Stein (25) mentioned that phosphates do not act as stabilizers in the decomposition bf hydroperoxides. From the data generated in the course of the present study, there are indications that the primary degradation of P A takes place during irradiation. This degradation involves species other than the long-lived radicals causing the degradation of the phosphite during post-irradiation storage, or it reflects the difference in the reaction rate of these antioxidants with the remaining radicals. Both PI radiation-induced conversion to chemical species other than P A and P A radiation-induced degradation can explain why the stoichiometric conversion of PI to P A was less than 100% immediately after irradiation and during the postirradiation storage of the material. The formation of low molecular weight fragments resulting from degradation of the excited species in irradiated PP, L D P E , and H D P E stabilized with hindered phosphite antioxidants was reported by several research groups (26, 27, 28). The identified compounds included 2,4-di-ter/-butyl-phenol and l,3-di-/erf-butyl benzene (the later being considered as a radiation-specific conversion product), resulting from the radiation degradation of the organic phosphites and their phosphate conversion products. These investigations greatly contributed to a better understanding of the transformation of Irgafos 168 in irradiated polyolefins. However, data taking into account the highly significant long-term conversion and fragmentation processes are still lacking in the literature. A n ongoing study in our laboratory is focused on measuring the production of 2,4-di-tert-butyl-phenol and 1,3-di-tert-butyl benzene in irradiated H D P E containing Irgafos 168 at a wide range of irradiation doses and postirradiation storage periods in order to gain a better understanding of the conversion rate profile of PI to P A in the course of y-irradiation as well as during the subsequent long-term storage. Last but not the least, we should keep in mind that the migration of small organic fragments from the packaging material into the food is typically faster than that of their organic phosphite and phosphate parents. Hence, it is of great importance to properly study their evolution, as well as the dose and storagetime dependence of their inventory in the packaging, for reliability in assessing the safety of foods packaged therein.

Conclusions

•

From both FTIR and H P L C evidences, conversion rate of the radiationinduced phosphite-to-phosphate antioxidants in H D P E was found to be dose and time dependent.


287 • •


•

•

•

In-storage conversion takes place primarily within 1 month following irradiation for an initial phosphite level of407 ppm. Radiation-induced phosphite-to-phosphate degradation was found to be accompanied by alternative degradation routes, leading to transformation products other than the corresponding phosphate. It would be appropriate to identify all the radiation-induced conversion products of the phosphite, to study their efficiency in stabilizing the polymers of the packaging, and to assess their potential impact on the quality and safety of the packaged food. The organic phosphate degradation was found to take place primarily in the course of irradiation, be highly dose dependant, and be insignificantly affected by post-irradiation storage time under the selected experimental conditions. In phosphite-stabilized food packaging materials processed by radiation, with or without food content, it is essential to assess the radiation-induced loss of die stabilizer at all dose ranges employed. It is even more essential to assess the radiation-affected inventory of the stabilizer and all its degradation products in the packaging material, at all the dose ranges employed, to ensure the sustained quality of the food packaged therein.

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Irradiation of Food and Packaging - American Chemical Society

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