Referee analysis of suspected irradiated food

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Food Control 19 (2008) 269–277

Referee analysis of suspected irradiated food

Selvarani Elahi a, Irene Straub b, Kevin Thurlow a,*, Peter Farnell a, Michael Walker a

a Laboratory of the Government Chemist (LGC), Queens Road, Teddington, Middlesex TW11 0LY, United Kingdomb Chemisches und Veterinaruntersuchungsamt (CVUA), Karlsruhe, Germany

Received 19 January 2007; received in revised form 28 March 2007; accepted 3 April 2007

Abstract

The Government Chemist has been required by statute to act in cases of dispute between traders and UK regulators for over 130years. Recently, the Laboratory of the Government Chemist (LGC) received its first case of two food samples, chilli powder, that werealleged to have been irradiated, contrary to European Law, (‘‘the referee samples’’). The confirmatory method of detection requires theuse of irradiation facilities, which LGC does not possess, so it was necessary, for the first time in the history of the Government Chemistfunction, to sub-contract the work. This paper describes the manner in which the work was organised to ensure that the evidence wasprovided by expert laboratories in such a way that it was fit for use in court. In particular, aspects of quality assurance essential to thedetection of irradiation in blends are described. Analysis was carried out using two standard techniques; photostimulated luminescenceand thermoluminescence in each of two laboratories. Both reported the referee samples as positive and correctly identified irradiation inblends of irradiated and non-irradiated chilli powder, at concentrations as low as 1% irradiated material. A second case, consisting of asample of Guarana powder, was submitted shortly after the first and was treated in a similar manner. On the basis of the result reportedby the Government Chemist, the owner of this sample accepted a formal caution from the food authority and paid the prosecution costs.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Food; Irradiation; Referee analysis; Legislation; Photostimulated luminescence; Thermoluminescence; Trading Standards; Public Analyst

1. Introduction

Twenty Acts of Parliament in the UK provide for theGovernment Chemist to analyse samples that are the sub-ject of a dispute between traders and UK regulatoryenforcement authorities. The Government Chemist isrequired to act as the national focus of technical appealin the specified areas, particularly in aspects of food andagriculture law, and, in the course of such work, encoun-ters a wide variety of food law cases. The GovernmentChemist has been required to act as a ‘‘referee’’ in foodlaw cases since the ‘‘Sale of Food and Drugs Act’’ of1875. This paper describes the first instance in which the

0956-7135/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodcont.2007.04.003

* Corresponding author. Tel.: +44 20 8943 7424; fax: +44 20 8943 2767.E-mail address: [email protected] (K. Thurlow).

laboratory has been called upon to provide evidence in adispute concerning allegedly irradiated food.

The irradiation of food is controlled by European Com-mission Directives 1999/2/EC and 1999/3/EC, enacted inEngland Wales and Scotland by the Food (Control of Irra-diation) Regulations 1990, (Statutory Instrument 1990 No.2490) as amended. These permit irradiation of some foodsfor hygiene purposes (including spices but not guaranapowder), but require a statement to this effect on the labelof the food. Similar legislation is in force in NorthernIreland as the Food Irradiation Provisions Regulations(Northern Ireland) 2000 (Statutory Rule 2000 No. 303).

1.1. The referee samples

In the UK, formal samples must be taken in accordancewith the provisions of the Food Safety Act 1990, as

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270 S. Elahi et al. / Food Control 19 (2008) 269–277

amended. This requires the sample taken to be divided intothree parts. The first part is submitted to the laboratoryacting on behalf of the local authority, the second part isgiven to the owner and the third part (referee portion) isretained in case of dispute. In the course of a sampling pro-gramme Somerset Trading Standards Department (localauthority) took formal samples and submitted the first partof each sample of chilli powder, to the Public Analyst (lab-oratory acting for the local authority) for analysis. Thefindings indicated that the samples had been irradiatedbut the owners of the samples disputed this. The localauthority and the owners agreed to send the third part (ref-eree portion) of both samples to the Government Chemist,in accordance with the provisions of the Food Safety Act1990 and the Food Safety (Sampling and Qualifications)Regulations 1990.

For most dried herbs and spices, photostimulated lumi-nescence (PSL) is used as a screening method to detect irra-diation (British Standards Institution, 2002). However, themethod states that it is necessary to confirm ‘intermediate’and ‘positive’ screening PSL results by using a furthermethod, such as BS EN 1788:2001 (British Standards Insti-tution, 2001), which uses thermoluminescence (TL). TheTL method requires the use of an irradiation source toapply a normalising dose. The Government Chemist nor-mally carries out all referee analysis ‘‘in-house’’ but inthe absence of a suitable irradiation source and direct expe-rience of the standard methods it was decided to seek exter-nal assistance. Thus two laboratories experienced in thedetection of irradiation in foods were selected to assist inthe planning procedure and subsequent analysis of thesamples. The selection criteria were:

1. experience in the techniques;2. third party accreditation to ISO/IEC/EN 17025 in both

the irradiation detection techniques to be applied; and3. good performance in collaborative trials.

In addition, in order to comply with the statutoryrequirement for the work to be carried out under the direc-tion of the Government Chemist, it was necessary that theprocedures were directly witnessed and evaluated by amember of his staff.

2. Methods

2.1. Photostimulated luminescence (PSL)

This European method is based on work carried out atthe Scottish Universities Research and Reactor Centre(SURRC). PSL has the advantage that it does not destroythe sample, although repeated measurements will cause thesignals to decrease. The basis of the method is optical stim-ulation of mineral debris, typically silicates, bioinorganicmaterial such as calcite (from shells or exoskeletons) orhydroxyapatite (from bones or teeth) that can be foundin most foods. Irradiation of food causes any such minerals

in it to store energy in charge carriers which, when stimu-lated with optical energy, release the energy trapped inthe charge carriers as luminescence. The amount of lightdetected during photostimulation, in photon count rate(PSL intensity), is compared with two thresholds, whichhave been established from collaborative trials. Irradiatedmaterials usually produce a strong signal above the upperthreshold (5000 counts/60 s, ‘positive’ result), whereasnon-irradiated materials usually produce a weaker signal,below the lower threshold (700 counts/60 s ‘negative’result). An ‘intermediate’ (P700 counts/60 s < 5000counts/60 s) or ‘positive’ result shows that further testsshould be carried out, using a different validated methodsuch as thermoluminescence.

Intermediate or in conclusive PSL signals can occur, forexample when a low dose has been applied to the food, ifthe sample has a low mineral content, if there is a mixtureof irradiated and non-irradiated minerals, or if the sampleexhibits low sensitivity. PSL is normally used only as ascreening method so it was essential to apply both PSL(screening) and TL (confirmation) techniques to the refereesamples.

2.2. Thermoluminescence

Thermoluminescence detection of irradiation is basedon the principle that light energy can be released fromthe trapped charge carriers present in the silicate mineralcontaminants of irradiated food when the sample is heated.In practice, silicate minerals are first isolated from the foodmaterial and then placed on stainless steel discs. The discswith the silicate minerals on them are heated incrementally(at 5 �C/s). The intensity of the emitted light is continu-ously measured as a function of temperature to producea glow curve, known as ‘Glow 1’. After Glow 1 has beenmeasured, the discs with the minerals/bioinorganic mate-rial on them are irradiated with a defined dose, usingcobalt-60 gamma rays (60Co), or other rays like roentgen-or b-rays.. After irradiation, the TL is measured again,producing a second glow curve, Glow 2. The ratio of inte-grated TL intensities of Glow 1 to Glow 2 over a statedtemperature interval (glow ratio) is evaluated. Irradiatedsamples exhibit a different glow ratio from samples thathave not been irradiated, and the shape and position ofthe peaks provides further information about the irradia-tion status of the sample. Generally, irradiated food hasa higher glow ratio and its Glow 1 maximum occurs at sig-nificantly different temperatures from food that has notbeen irradiated. TL glow ratios from irradiated samplesare typically greater than 0.1, whereas those from non-irra-diated samples are usually below 0.1. In addition to the TLglow ratio, interpretation of the shape of the glow curves isneeded to decide whether the sample has been irradiated ornot. Usually, Glow 1 curves of irradiated foodstuffs exhibita maximum between 150 �C and 250 �C, whereas low-levelnatural radioactivity causes TL signals from deep traps attemperatures above 300 �C. In cases where only part of

Table 1Preparation of blended control samples

Blendconcentration

Weight of non-irradiated chillipowder (g)

Tolerance(±g)

Weight of10 kGy chillipowder (g)

Tolerance(±g)

1% blend 495 5.00 5.00 0.0510% blend 180 2.00 20.00 0.2050% blend 100 1.00 100.00 1.00

S. Elahi et al. / Food Control 19 (2008) 269–277 271

the food has been irradiated, e.g. spice blends with one ormore irradiated ingredients, the TL glow ratio can bebelow 0.1 but the shape and position of the peak in Glow1 will clearly indicate irradiation treatment. Collaborativetrials have shown that using the shape and position ofthe glow curves, in addition to the TL-ratios, is reliableeven for detection of low-level irradiation.

Herbs and spices are amongst the products most com-monly irradiated in commercial irradiation plants. Theyare also products with diverse sources and a complex multi-national supply and distribution chain. They are frequentlysubject to mixing, either during processing of different pro-duction batches, or as a result of blending aimed at produc-ing consistent qualities of flavour. For these reasonsirradiated herbs and spices can find their way, in dilutemixtures, into the food supply chain, presenting a differentrange of analytical problems from those associated with thepure irradiated product. A previous MAFF project, carriedout by SURRC (Carmichael & Sanderson, 1999), exam-ined the impact of blending on the detection of irradiatedherbs and spices. The survey concluded that standardmethods were able to detect a significant portion of irradi-ated blends at concentrations above 1–10%. Below theseconcentrations, there was a significant probability of non-detection, particularly for low sensitivity components.

2.3. Analytical quality assurance and sample preparation

Quality assurance was carried out by analysis of pre-pared samples of known irradiation status, includingblends of irradiated and non-irradiated spices and blanksamples.

Chilli powder was obtained from British Pepper andSpice Co. Ltd., and preliminary TL analysis was carriedout by SURRC, to confirm that it had not been irradiated.

Wholly irradiated control samples were prepared undersupervision by Government Chemist staff, at the NationalPhysical Laboratory (NPL) by irradiation of the non-irradiated chilli powder in polystyrene pots packed insideplastic bags. A 60Co gamma ray source was used to applydoses of 3, 5 and 10 kilograys (kGy), each with 5% uncer-tainty. Dosimetry was carried out to confirm the accuracyand precision of the applied dose. Care was taken to avoidcross-contamination.

Blends of irradiated and non-irradiated material werethen prepared by Government Chemist staff according tocomposition and dose shown in Table 1. A known weightof non-irradiated chilli powder was weighed accurately intoa clean plastic bowl, followed by the appropriate weight ofirradiated chilli powder. The powder was mixed using aclean plastic spoon. The bowl was then sealed in a plasticbag and a conventional cake mixer attachment insertedthrough a small hole in the plastic bag. The contents weremixed for exactly 2 min. Owing to the pungency and poten-tial airborne dispersal of the chilli powder this operationwas carried out in a fume cupboard and the analyst worea suitable dust mask throughout. In order to maintain a

consistent approach, the blank and Referee samples werealso individually homogenised using the mixing proceduredescribed above. This mixing procedure was reported ashaving produced homogeneous blends in the MAFF pro-ject on blended herbs and spices (Carmichael & Sanderson,1999). All samples were stored in amber pots inside doubleplastic bags, and kept in the dark. PSL analysis of the con-trol samples, to confirm homogeneity, was carried out byan independent Public Analyst laboratory, Lincolne, Sut-ton and Wood Limited. All samples and control materialswere coded in a non-sequential order, as shown in Table 2,before submission to the two selected expert laboratories:SURRC in Scotland and the Chemisches und Veterinarun-tersuchungsamt (CVUA) in Karlsruhe, Germany.

2.4. Analysis

Samples and controls were submitted ‘‘blind’’ exceptthat the laboratories were informed that the control sam-ples supplied consisted of some which had been whollyirradiated and some which were blends of irradiated andnon-irradiated material. The concentrations but not theidentity of the blended control samples was also revealed.PSL analysis of the control samples, to assess homogeneity,had shown that the 1% blends gave equivocal PSL results,so it was decided that any results from these blends wouldnot be used in evidence. All the other QC samples wereconsidered to be homogenous based on the PSL results.The laboratories were asked to analyse and interpret theresults in accordance with the specified European Stan-dards for the PSL and TL methods, which, for the latter,includes an assessment of TL-ratios and ‘Glow 1 PeakShapes’. Additionally, the laboratories were advised thatit was necessary for a member of the Government Chem-ist’s staff to witness the procedures as required by theUK regulations. CVUA, Karlsruhe and SURRC carriedout PSL and TL analysis on the referee and control sam-ples (wholly irradiated and blends) as outlined in Table 2.

2.5. Results

Results are presented in Table 2. Analysis was carriedout in duplicate for all sample pots submitted. One potwas submitted for all samples except the blended controlsample at 1% concentration. The MAFF blending project(Carmichael & Sanderson, 1999) had reported that there

Table 2Summary of chilli results from SURRC and CVUA, Karlsruhe

Sample description: chillipowder

Code SURRC PSLresult

SURRC TLa conclusion CVUA KA PSLresult

CVUA KA TLa conclusion

Wholly irradiated at 5 kGy 1 Positive Positive Positive Irradiation has taken placePositive Positive

Referee sample – D3007363 2 Positive Positive Positive Irradiation has taken placePositive Positive

50% blend 3 Positive Positive Positive Irradiation has taken placePositive Positive

1% blend 4 Intermediate Evidence of an irradiatedcomponent

Negative Irradiation has taken place

Intermediate NegativeNon-irradiated 5 Negative Negative Negative No irradiation has taken

placeNegative Negative



10% blend 8 Intermediate Positive Positive Irradiation has taken placeIntermediate Positive

1% blend 9 Intermediate Evidence of an intermediatecomponent

Intermediate Irradiation has taken place

Intermediate IntermediateReferee sample – D3009631 10 Positive Positive Positive Irradiation has taken place

Positive Positive1% blend 11 Intermediate Positive Intermediate Irradiation has taken place

Negative IntermediatePositiveNegativeIntermediateIntermediateIntermediateIntermediate

a The laboratories used different phrases for describing the TL results.


is a significant probability of not detecting irradiation inblends at concentrations of <10%. In order to increasethe probability of detecting irradiation in the 1% blendedcontrol sample, three pots of this sample were submittedto each laboratory, requesting duplicate analysis of eachpot, which produced six results per laboratory for this sam-ple. Additionally, SURRC reported repeat TL results on ablended sample that initially gave conflicting PSL results.This confirmed that it had been irradiated, and also con-firmed the expectation that blended samples might giveinconclusive results by PSL, owing to homogeneity issues.The two laboratories achieved excellent agreement on theresults for all samples and confirmed that both referee sam-ples had been irradiated. Both laboratories, independently,correctly classified the control samples, including theblends of irradiated and non-irradiated chilli powder,down to a 1% blend concentration. Examples of glowcurves from CVUA, Karlsruhe, produced during TL anal-yses are presented in Figs. 1–8. Owing to the time requiredto set up new procedures for this case it proved impossibleto complete the referee analysis within a time scale that per-mitted subsequent formal legal action. However, the HomeAuthority and the Food Standards Agency were informedof the outcome in order to pursue the matter with the

importer. The experience gained in the case has enabledthe Government Chemist to deal with subsequent refereesamples concerning irradiation within a legally acceptabletime scale.

2.6. Guarana – a further referee sample

During the completion of the chilli powder analysis areferee sample of guarana capsules was received from Bathand North East Somerset District Council. This samplewas the subject of an adverse report from their Public Ana-lyst as a non-permitted irradiated product. Guarana is acaffeine-containing herb, which is sold as a natural stimu-lant and is claimed to have health benefits. The owner dis-puted the Public Analyst’s findings and the sample was sentto the Government Chemist for referee analysis.

For this sample the procedure that had been used for thechilli powders was altered somewhat. Because SURRC andCVUA, Karlsruhe, had produced results which were inexcellent agreement in the chilli powder cases, it wasdecided that a single expert laboratory analysis would besufficient providing that once again a suitable number ofcontrol samples were used and the procedures were wit-nessed by Government Chemist staff.

Fig. 1. CVUA, Karlsruhe TL Glow 1 for non-irradiated chilli powder control sample. *Temperature interval I is defined by LiF, TLD-100� as describedin Annex B of BSEN1788 as an alternative to the calibration of the absolute temperature scale. Irradiated silicate minerals show a maximum intemperature interval I.

Fig. 2. CVUA, Karlsruhe TL Glow 2 for non-irradiated chilli powder control sample. TL glow ratio ¼ 4832 CTS11;969;142 CTS

¼ 0:0004.


2.7. Control samples of guarana

A survey carried out on behalf of the Food StandardsAgency (Food Standards Agency, 2002) had previouslyrevealed that half the guarana samples tested had been irra-diated. In addition the first commercial sample of guarana

(Chromadex Guarana 1) purchased for control purposeswas found by SURRC to be heterogeneous and unsuitablefor use as a blank with an atypically low and variable min-eral content, and equivocal TL results. However, sub-sam-ples of this material were irradiated at 3, 5 and 10 kGy byIsotron PLC, to provide positive controls. SURRC were

Fig. 3. CVUA, Karlsruhe Glows 1 and 2 for chilli powder control sample wholly irradiated at 5 kGy. TL glow ratio ¼ 48;950;686 CTS41;102;628 CTS

¼ 1:19.

Fig. 4. CVUA, Karlsruhe TL Glows 1 and 2 for chilli powder control sample blended at 1% concentration. TL glow ratio ¼ 1;197;951 CTS26;970;062 CTS

¼ 0:04.


able to supply blank material confirmed as non-irradiatedby TL analysis and a further guarana product was pur-chased from Chromadex (Chromadex Guarana 2).

3. Results

CVUA, Karlsruhe analysed the referee and control sam-ples and reported the referee sample as irradiated. The

results are presented in Table 3. The positions of the max-ima in the Glow curves (Glow 1) show clearly that irradia-tion has taken place. The PSL results were negative for the‘‘referee’’ sample, but it should be noted that the PSLmethod was only used for screening, and that TL was usedfor confirmation. In addition, the threshold values (700(lower) and 5000 (upper) counts per 60 s) have only beenvalidated for herbs, spices and seasonings. Negative results

Fig. 5. CVUA, Karlsruhe TL Glows 1 and 2 for referee sample (D3007363) of chilli powder. TL glow ratio ¼ 43;217;570 CTS53;015;570 CTS

¼ 0:82.

Fig. 6. CVUA, Karlsruhe TL Glows 1 and 2 for non-irradiated guarana powder control sample (SURRC blank). TL-ratio ¼ 15;738 CTS1;465;765 CTS

¼ 0:011.


for irradiated samples with insufficient PSL sensitivity mayoccur. The sensitivity of any individual sample is affectedby the quantities and types of minerals present in it. BSEN 13751:2002 states that false negative results can occurwhen there is low PSL sensitivity.

CVUA, Karlsruhe reported that, in common with previ-ously analysed guarana samples, some of the control sam-

ples exhibited unusual glow curves, which could have beendue to the type of mineral present. CVUA, Karlsruhe alsoreported Chromadex Guarana 1 as ‘‘not irradiated’’,whereas SURRC had found it to be a heterogeneous mix-ture of irradiated and non-irradiated material. The hetero-geneity of this sample coupled with its low mineral contentis the probable cause of the different results reported by the

Fig. 7. CVUA, Karlsruhe Glows 1 and 2 for guarana powder control sample wholly irradiated at 5 kGy. TL-ratio ¼ 1;465;784 CTS398;530 CTS

¼ 3:68.

Fig. 8. CVUA, Karlsruhe TL Glows 1 and 2 for referee sample (E3013411) of guarana powder. TL-ratio ¼ 363;488 CTS6;717;990 CTS

¼ 0:054.


two laboratories. The ‘blank’ from SURRC was not mea-sured by PSL by CVUA, Karlsruhe as the TL analysis usedup the entire sample. CVUA, Karlsruhe reported that, onthe basis of TL analysis, it was non-irradiated. In any case,CVUA, Karlsruhe correctly identified the irradiation statusof all control samples by TL analysis.

A certificate of analysis was issued informing theauthorised officer of the positive result for the referee sam-ple. As a direct consequence, the owner of the food thenaccepted a formal caution, for importing and selling gua-rana capsules, which had been treated with ionising radia-tion in contravention of the Food (Control of Irradiation)

Table 3Analysis of guarana powders CVUA, Karlsruhe

Sample description Type Code No. of PSLtests

PSL result No. of samplepreparations

TL result

‘‘Referee’’ sampleE3013411

Unknown 1 2 Negative 2 Irradiation has takenplace

Chromadex Guarana 1 Unknown 2 7 Negative 4 Not irradiatedIrradiated at 3 kGya Irradiated 3 9 Negative or

intermediate2 Irradiation has taken place

Irradiated at 5 kGya Irradiated 4 2 Intermediate 3 Sample is regarded asirradiated

Irradiated at 10 kGya Irradiated 5 3 Intermediate 3 IrradiatedSURRC blank Blank 6 0 – 1 Not irradiatedChromadex Guarana 2 Unknown 7 6 Negative or

intermediate2 Not irradiated

a Prepared from Chromadex Guarana 1.


Regulations 1990. The owner of the food also paid theprosecution costs, estimated at £6500.

4. Conclusion

The analyses of third portion referee samples of chillipowder and guarana powder have demonstrated the capa-bilities of the techniques applied to produce analyticalresults of the quality necessary for formal food law enforce-ment action. The experience of expert laboratories in thetechniques applied was combined with that of the Govern-ment Chemist in analytical quality assurance and other for-mal procedures to produce evidence in a way that compliedwith the requirements of the Food Safety Act 1990, asamended. It has been clearly demonstrated that the TLmethod is suitable for blends of irradiated and non-irradi-ated material down to a concentration of 1%. However, inthe case of low concentration blends, the results have to beinterpreted with extreme care as contradictory results canarise, especially if there are issues with homogeneity, lowmineral content or low sensitivity.

Acknowledgements

The work described in this paper was supported undercontract with the Department of Trade and Industry as

part of the Government Chemist Programme. Analysiswas carried out by the Scottish Universities Research andReactor Centre and the Chemisches und Veterinaruntersu-chungsamt in Karlsruhe, Germany. Our thanks also go tothe British Pepper and Spice Co. Ltd. for provision of thenon-irradiated chilli powder used in the preparation of thecontrol samples.

References

British Standards Institution, 2001. BS EN 1788:2001 Foodstuffs –Thermoluminescence detection of irradiated food from which silicateminerals can be isolated. ISBN: 0-580-38328-8.

British Standards Institution, 2002. BS EN 13751:2002 Foodstuffs –Detection of irradiated food using photostimulated luminescence.ISBN: 0-580-40587-7.

Carmichael, L. A., Sanderson, D. C. W., & Scottish Universities Researchand Reactor Centre (SURRC), 1999. A preliminary investigation of the

impact of blending on luminescence of irradiated herbs and spices. Finalreport for MAFF project FS 1925.

Food Standards Agency, 2002. Food Survey Information Sheet 25/02.Survey for irradiated foods – Herbs and spices, dietary supplements,and prawns and shrimps.

Documents

Referee analysis of suspected irradiated food