Detection of food treated with ionizing radiation

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FOOD SCIENCE

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Trends in Food Science & Technology 9 (1998) 73-82

&TECHNOLOGY Review

Detection of food treated with ionizing radiation Henry Delincee Federal Research Centre for Nutrition, Institute of Nutritional Physiology, Engesserstr. 20, D-76131 Karlsruhe, Germany (fax: + 49-7247-22820; e-mail: [email protected]).

Treatment of food with ionizing energy-'food irradiation'is finally becoming reality in many countries. The benefits include an improvement in food hygiene, spoilage reduction and extension of shelf-life. Although properly irradiated food is safe and wholesome, consumers should be able to make their own free choice between irradiated and non-irradiated food. For this purpese labelling is indispensable. in order to check compliance with existing regulations, detection of radiation treatment by analysing the food itself is highly desirable. Significant progress has been made in recent years in developing analytical detection methods utilizing changes in food originating from the radiation treatment. © 1998 Elsevier Science Ltd. All rights reserved

Recent years have experienced an enormaus interest in detection methods for irradiated food, which may reflect an increased awareness about the benefits of food irradiation. Treatment of food with ionizing radiation is increasingly being recognized as a means of reducing food-borne illnesses and associated medical and other costs [1). Similar to liquid pasteurization, irradiation. of solid food (such as spices, seafood, poultry, meat) eliminates disease-causing organisms and extends shelf-life through destruction of spoilage organisms. Irradiation of fruit and vegetables may be used for quarantine treatments instead of chemical fumigants to exclude insect pests. An ever-increasing nurober of countries has approved the irradiation of a long and growing Iist of different food items, groups or classes, ranging from spices and grains 0924·2244/98/519.00 Copyright :g 1998 Elsevier Science Ltd. All rights reserved PI/: 50924·2244(98)00002·8

to fruit and vegetables, to meats, poultry and seafood [2). In order to faci1itate international trade, control of irradiated food can be supported by analytical methods which are suitable to detect directly in the product whether or not it has been treated with radiation, i.e. using the food itself as a radiation marker. This approach has led to controversy, since it is claimed that relevant shipping documents will always accompany the irradiated product, providing information to identify the registered facility which has irradiated the food, the date(s) of treatment and Iot identification. Tagether with the records in the radiation facility, this offers a reliable documentation of the employed processing (type of radiation, dose, dose-rate, dose uniformity, applied dosimetry, nature and amount of product, packaging material, temperature of irradiation, date of treatment, etc.). However, an additional means to identify irradiated food has been particularly requested by consumer organisations. Consumers in many countries have remained sceptical about food preservation by ionizing radiation, probably mostly due to Iack of information about what happens in food upon irradiation. Some consumer groups have even grossly misinformed the public, e.g. by intentionally associating food irradiation with the issues of nuclear weapons and radioactivity, or by stating that irradiated food Iacks vitamins and nutritional value. lt should be recognized, however, that in the past decades, numerous experiments with irradiated food have been carried out, and in 1992 the W orld Health Organization stated that irradiated food produced in accordance with established good manufacturing practice can be considered safe and nutritionally adequate [1). The notion that radiationprocessed food is safe and wholesome has been endorsed by major public health institutions around the world [3, 4]. Correct and comprehensive information about food irradiation and irradiated food must reach consumers in order to enable them to reach decisions based on wellfounded reasons. In fact, it has been demonstrated that when consumers are provided with factual information about this kind of food processing, they will choose irradiated food with confidence [5). For those consumers preferring to abstain from irradiated food, the labelling of irradiated food and its enforcement by

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authoritative control is crucial. The confidence of consumers in correct labeHing can be bolstered by the availability of reliable and sensitive detection methods at the hands of food control agencies. At the international conference on 'The Acceptance, Control of, and Trade in Irradiated Food', therefore, it was recommended that governments should encourage research into detection methods [6]. Furthermore, the confusing situation of unharmonized clearances for food irradiation hampers international trade. Availability of detection methods would help countries to protect their markets and to enforce a total ban of radiation processing or to refuse imports of non-cleared items. Also, it must not be overlooked that such identification methods cannot contribute to Controlling whether correctly labelled products have been radiation processed within acceptable dose Iimits [7]. To enforce adherence to regulated minimum and/or maximum doses still remains a challenge to authorities [8]. Although some research had been carried out during the last decades, while the technology of food irradiationwas developed [9, 10], a big move occurred due to a joint F AO/IAEA world-wide programme on Analytical Detection Methods for Irradiation Treatment of Foods (ADMIT) [11] and a European programme by the Community Bureau of Reference (BCR) [12]. The final reports of these programmes offer a rich source of literature, referring to work on how to identify whether or not a food product has been treated with ionizing radiation [11, 12]. This article, therefore, mostly refers to more recent work, the restricted number of references preventing it from being exhaustive. Requirements for methods to detect irradiated food

At the Belfast ADMIT Meeting the 'General Principles for the Development of Detection Methods' which are ideally required or desirable were re-confirmed [11]. Mainly, the test parameter measured should be able to identify clearly whether food has been irradiated throughout the entire storage life of the food product, without requiring a sample of the non-irradiated food for comparison from the particular batch tested. lt would be beneficial if the test could also fulfil practical

criteria, such as simplicity, low cost, high speed, applicability to a wide range of foodstuffs, and resistance to falsifying its results. In particular, the test should be capable of easy standardization and cross calibration. Standardization of detection methods

Mainly due to the international efforts described above, but also to national programmes established, e.g. in France, the UK and Germany, more than 30 interlaboratory blind trials on detection methods are now available. International cooperation has led to enormaus progress in the field of identification of irradiated food, which was unforeseen only 10 years ago. A milestone was reached at the end of 1996, when five European Standards were adopted by the European Committee for Standardization (CEN). In Table 1 the five CEN Standards [13-17] are listed, which will be converted to national standards in a nurober of European countries. It is hoped that these European Standards may be extended to world-wide standards, e.g. by the International Standards Organization (ISO). The present standards pertain to specific food items (discussed in the following sections), and are likely to be supplemented by standards for other food types as soon as appropriate validation by interlaboratory blind trials has been performed. These European standards fulfil _the requirements of the European Commission regarding harmonization of food irradiation legislation, who in the recent draft of a Council directive (proposal of 21 May 1997, document 8420/97 DENLEG 41 CODEC 296 of 5 June 1997) demand standardized methods to be used in food control if available. In addition to the now established CEN standards, several countries have adopted validated methods, e.g. in Germany the number of official methods according to §35 of the Food Act has been increased to 11 and another two methods are under preparation (in fact only three types of analytical techniques, namely thermoluminescence, gas chromatography of radiationinduced hydrocarbons and electron spin resonance spectroscopy, are being validated for various food products).

Table 1. European Standards adopted by CEN on 5 December 1996• Standard no.

Products covered

EN 1784 EN 1785

Foodstuffs-detection of irradiated Foodstuffs-detection of irradiated 2-alkyl cyclobutanones Foodstuffs-detection of irradiated Foodstuffs-detection of irradiated Foodstuffs-detection of irradiated

EN 1786 EN 1787 EN 1788 a

food containing fat. gas chromatographic analysis of hydrocarbons food containing fat, gas chromatographic/mass spectrometric analysis of food containing bone, method by ESR-spectroscopy food containing cellulose, method by ESR-spectroscopy food from which silicate minerals can be isolated, method by thermoluminescence

Standards are available at CEN (ßrussels, Belgium) or at national standardization bodies.

H. Delinu!e/Trends in Food Science & Technology 9 (1998) 73-82

Present status of detection methods

A !arge number of detection techniques are being tested on an increasing variety of food products, and newly ernerging analytical techniques are being examined for their suitability. Qualitative detection is of much greater importance than quantitative dose estimation [18], mainly due to the self-controlling nature of food irradiation: higher doses will Iead to unacceptable sensory properties (and higher costs for the radiation treatment), whereas a certain minimum dose is necessary to achieve the desired effect (e.g. in the case of chicken, Salmonella destruction). Therefore, the applied dose will only vary between narrow Iimits and needs no additional estimation. Possibly, a posteriori dose estimation may enforce Good Irradiation Practice (GIP) [19]. However, effective control at the radiation facility must have absolute priority for the enforcement of GIP. This requirement is similar to the control of hygiene measures, which have to be carried out at the production site and in food processing plants to enforce Good Manufacturing Practice (GMP). It is expected that the needs to improve food safety and the implementation of HACCP programmes will provide further impetus to the use of f_ood irradiation. Two of the proposed detection methods, namely the estimation of reduced viability of microorganisms, measuring both dead and alive microorganisms in a food product using either the direct epifluorescent filter technique combined with an aerobic plate count (DEFT/APC) or the Limulus Amoebocyte Lysate test combined with account of Gram-negative bacteria (LAL/GNB), give information about the hygiene status of the food prior and after irradiation treatment, thereby contributing to enforce both GMP and GIP. To gain an overview the progress achieved to date, discussion of the proposed detection methods is separated below into physical, chemical and biological methods; thus the results achieved can be easily compared with a previous update [18]. Some reviews about the identification of irradiated food should also be mentioned [3, 20--24]. Physical methods Table 2 summarizes the methods based on physical changes in the irradiated food and their proximity to usability for detection purposes. Electron spin resonance (ESR) spectroscopy in particular shows promise for a !arge number of food products [11, 12]. This technique detects radiation-specific radicals produced upon irradiation of food which can be quite stable in solid and dry components of the food (e.g. bones) and can be detected by ESR. The technique is non-destructive, specific and combines simplicity with rapid measurement. The availability of desktop ESR spectrometers reduces the cost of equipment, but this is still substantial and may hamper

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its application. Some reviews about the use of ESR methodology to detect irradiated food appeared recently [25-28]. The CEN standard EN 1786 for food cantairring bone [15] has been validated by interlaboratory tests (see references in EN 1786) with meat bone (beef, chicken) and fish bones (trout). lt is expected that the procedure can be extended to all meat and fish species containing bone. Even for ready-prepared food like chicken burgers, which contain some amount of mechanically recovered poultry meat, and therefore also Submillimeter bone fragments, ESR can be applied to detect radiation treatment of the deboned meat [11]. Another CEN standard, EN 1787, relies on the formation of cellulose radicals upon irradiation [16]. If the content of crystalline cellulose in the food is adequate and maisture content is low enough, typical 'cellulosic' radicals can be detected by ESR. The standard EN 1787 has been validated in interlaboratory tests (see references in EN 1787) with pistachio nut shells and paprika powder, and shows promise for extension to berries (fresh or frozen). A Iimitation of the ESR cellulose method is that positive identification of the cellulose radicals is evidence of irradiation, "but absence of the signal does not constitute evidence that the sample is unirradiated. For instance in paprika powder maisture will Iead to decay of the signal, and its stability may be shorter than the shelf-life of paprika [27, 29]. If dose Ievels are lower than 1 kGy, as is the case for the irradiation of some fruit and vegetables, difficulties may arise in detecting the cellulosic radical, as has been discussed in an interlaboratory trial for strawberries [30]. A further promising application of ESR spectroscopy is the analysis of foods containing crystalline sugar. Although for some dehydrated fruits a multicomponent ESR spectrum with a total signal width corresponding to a magnetic field strength of "'7-10mT allows unequivocal identification of irradiated samples (e.g mangos, papayas, figs, raisins), some other fruits seem 'inactive' and do not give expected spectra [31]. For unirradiated samples, both single and multicomponent spectra have been observed, but the signal width was less than 7 mT. More information about the behaviour of various dehydrated fruits upon irradiation seems desirable. Detection of radicals in the exoskeleton of crustaceae was expected to be Straightforward as in the case of bone, but the chemistry of the cuticle is rather complex, leading to different ESR signals for various species and even to different spectra for identical species (from various geographical regions) [28, 32-34]. Therefore, application of ESR to detect the radiation treatment of crustaceae has to be thoroughly validated, most likely on a species-by-species basis. A collaborative trial in Germany with brown shrimp (Crangon crangon) and Norway Iobster (Nephrops norvegicus) yielded good

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Table 2. Physical methods for the detection of irradiated food and their proximity to usability Status

Foods studied

Changes in physical properlies Electrical impedance Viscosity of Suspensions Thermal analysis (e.g. ice nucleation) Near-infrared spectroscopy (NIR) Nuclear magnetic resonance (NMR)

B D A A ?

Potatoes Pepper Fish, prawns, egg white Spices

F F E E E D D D D

Food containing bone (poultry, meat, fish, frog legs) Food containing cellulose (pistachio nut shells, paprika powder) Food containing cellulose (strawberries) Food containing crystalline sugar (dried mango, dried fig) Some crustaceae (brown shrimp, Norway lobster) Food containing cellulose (pepper) Food containing crystalline sugar (raisins, dried papaya) Same crustaceae (pink shrimp, crevette, Norway lobster) Egg shells Food containing bone fragments (mechanically recovered meat) Shellfish Food containing cellulose (grapes, various berries -chilled or frozen- french prunes, some spices) Dehydrated mushrooms, macaroni, snails, gelatin, crustaceae, barley, seeds of fruits (figs, dates)

Detection of free radicals• Electron spin resonance (ESR)

c

c

B, C

B Luminescence: Chemiluminescence

Thermoluminescence

D B A F

E D D D D Photo-stimulated luminescence

c

Same spices, herbs and dehydrate~ vegetables Frozen chicken, wheat flour Shellfish, crustaceae, poultry bones Food from which silicate minerals_