J Sci Food Agric 1998, 76, 149È155

Dose Dependence and Fading Effect of theThermoluminescence Signals in c-IrradiatedPaprikaVirgilio Correcher,* Jose� L Mun8 iz and Jose� M Go� mez-Ros

Centro de Investigaciones Energe� ticas, Medioambientales y Tecnolo� gicas (CIEMAT), Ed 2,Av Complutense 22, 28040-Madrid, Spain

(Received 31 August 1995 ; revised version received 28 October 1996 ; accepted 15 May 1997)

Abstract : The thermoluminescence (TL) method can be used to discriminate irra-diated and non-irradiated paprika. This study reports the polymineral composi-tion of the dust adhered to paprika ; radiation-speciÐc luminescence is emitted bythe inorganic material (mainly quartz, feldspars and calcite). On the basis of theshape of the TL curves (or glow curves) some physical parameters are evaluated.Natural TL curves, from non-irradiated samples, show three very low intensitypeaks while induced TL curves, from irradiated paprika, seem to consist of Ðveoverlapping peaks. TL spectra reveal a very important di†erence in intensity andin the position of the peaks between irradiated and non-irradiated paprika.Fading observations of TL after irradiation at 5, 10 and 21 kGy, show the samebehaviour in all the cases : an initial rapid decay to maintain a certain stabilityfrom 300È400 h onwards. 1998 SCI.(

J Sci Food Agric 76, 149È155 (1998)

Key words : thermoluminescence ; irradiation ; fading ; doseÈresponse ; paprika ;spices

INTRODUCTION

Seasonings, including paprika, are commonly used indiets. Cultivation of paprika and the industry involvedmaintains many Spanish families, but a big problem isencountered : contamination by microorganisms.

Irradiation represents a good alternative to ethyleneoxide or other solvents (toxic and carcinogenic in manycases) for the eradication of high contamination levels.There are many countries (USA, Netherlands, Belgium,Japan, etc) that have adopted this preservation method(FAO/WHO/IAEA/ITC-UNCTAD/GATT 1989). Since1993, the sale of irradiated food is permitted in allEurope. Some regulatory authorities consider it neces-sary to have a method for the detection of irradiatedfoods (CEE 1988). The Expert Committee of FAO/WHO/IAEA joined in Geneva in 1981 recommended,for commercial food processing, a range of radiation

* To whom correspondence should be addressed.

doses from 0É03 to 50 kGy. A maximum dose of 10 kGycurrently recommended for non-medical uses and forherbs, spices and seasonings. One of the importantaspects accepted in that meeting was the need for spe-ciÐc labelling for irradiated products to protect con-sumer choice.

In this sense, thermoluminescence (TL) seems to be agood method (not very expensive and very fast) to dis-criminate between irradiated and non-irradiated food(Sanderson et al 1989a). The method is based on theemission of light from a solid sample (insulator orsemiconductor) when it is heated after being irradiatedby some kind of radiation such as X-rays, c-rays, beamsof electrons, etc (McKeever 1985).

During heating, the TL signal is detected by a photo-multiplier tube and recorded as a function of tem-perature or wavelength. The resulting curve is called aTL curve or glow curve ; the luminescent intensity andthe shape of this glow curve are functions of radiationdose and heating rate.

1491998 SCI. J Sci Food Agric 0022-5142/98/$17.50. Printed in Great Britain(

150 V Correcher, J L Mun8 iz, J M Go� mez-Ros

This paper focuses on (i) the conÐrmation of thevalidity of TL as a method to discriminate between irra-diated and non-irradiated seasonings (paprika in thiscase) ; (ii) the characterisation of the TL glow peaksobtained from inorganic dust separated from paprika ;(iii) the determination of the dependence of TL intensitywith di†erent doses ; and (iv) the modelling, from irradi-ated samples, of the stability of TL signal with time.

MATERIALS AND METHODS

Samples of paprika were acquired at random from com-mercial sources in Spain, without knowing the pro-ducer. Since this technology for commercial foodprocessing is not yet being applied in Spain, it was pre-sumed that these samples were unirradiated. The TLmethod requires that the inorganic components beseparated from the sample. The reason for this require-ment is that the polymineral phase emits radiation-induced TL, whereas the organic phase producesnon-speciÐc signals. Therefore, TL intensity of wholesamples depends on the degree of mineral contami-nation (Go� ksu- and Regulla 1989 ; SandersonO� gelmanet al 1989b).

Preparation of samples

Separation of the organic and inorganic phases is pos-sible using di†erent methods (plasma ashing, Soxhletextraction, etc) but they are not very useful for dailyroutine. Therefore, a practical method of separation hasbeen developed on the basis of the partial one describedby Pinnioja et al (1993). Paprika contains along withthe organic phase an important amount of inorganicphase. The whole sample of paprika was immersed in abeaker with carbon tetrachloride and stirred for(CCl4),3È4 h. was selected because it is an inert solvent ;CCl4it does not damage the sample and has a good densityfor the aim (1É59 g cm~3). Subsequent centrifugationshowed clearly two di†erent layers, the organic matterÑoating and the inorganic material at the bottom of thebeaker. The two parts were separated by decantationand the mineral phase was washed several times using

Since this phase still had some organic matter inCCl4 .it, the sample was treated with a mixture of hydrogenperoxide and sodium hypochlorite (NaClO)(H2O2)(1 : 1) to remove as much organic matter as possible. Inthis method, no acid treatment was employed avoidingto eliminate carbonates.

Finally, the samples were dried and stored at roomtemperature in a desiccator with silica gel in the pres-ence of sunlight to simulate similar conditions to that ofproduction. This method of separation yielded about4% of mineral material (20 g per 500 g from the wholesample of paprika). When the inorganic phase was iso-

lated, the samples were carefully crushed with a pestleand mortar and sieved to obtain a size of the grainunder 50 lm. The powder was spread on stainless alu-minium discs, about 5 mg on each one.

Instrumentation and data manipulation

Two-dimensional (2D) TL glow curves were obtainedheating the samples from room temperature to 500¡C ata heating rate of 5¡C s~1 under a nitrogen atmosphereusing an automated computer-controlled 2D-TL systemTL-DA-10 developed in Laboratory (Roskilde,RisÔDenmark) (Botter-Jensen 1988). The 3D TL system con-sists of two spectrometers and two position-sensitivephotomultiplier tubes used to detect simultaneouslyover the spectral range of 200È800 nm, and over arange of 30È400¡C at heating rate of 2É5¡C s~1 undervacuum (Lu† and Townsend 1993).

The composition of TL dosimeters present in thepolymineral phase was determined by employing anX-ray di†ractometer model Siemens D-5000, using the

radiation of Cu with a Ni Ðlter at a setting of 40 kVKaand 20 mA.

In order to simulate industrial food preservation pro-cesses, every aliquot of inorganic dust (that was used asTL dosimeter) was irradiated at room temperature atdi†erent doses, from 1 to 21 kGy, using a c source of60Co at a dose rate of 1É34 kGy h~1. This irradiationwas performed in the Irradiation Unit of CIEMAT.

The analysis of the curves were made using the glowcurve analysis (GCA) program developed in CIEMATbased on a Levenberg-Marquardt minimisation algo-rithm.

RESULTS AND DISCUSSION

X-ray di†raction analysis

In a previous study, after separating the mineral phasefrom the whole sample, its composition was determinedusing X-ray di†raction and, although these are qualit-ative results, it is interesting to note the sample wasformed mainly by quartz K-feldspars (orthoclase-(SiO2),

Ca,Na-feldspars (albite- andKAlSi3O8-), NaAlSi3O8-calcite halite (NaCl)anorthite-CaAl2Si2O8-), (CaCO3),

and a few amount of clays (Table 1).These mineral phases are also typical in other herbs

and spices (oregano, mint, sage, etc) previously studiedin the laboratory of Universidad Auto� noma of Madrid(Correcher et al 1992 ; Caldero� n et al 1995) and it issupposed they may contribute to give a good TLresponse because all of them can act as good TL dosim-eters (Horowitz 1984 ; Aitken 1985).

Saturation and fading on the T L signals in c-irradiated paprika 151

TABLE 1Relative abundance of polymineral material characterized by

X-ray di†raction

Mineral phase assigned Relative amount

Quartz Very abundantHalite AbundantCalcite Less abundantCa/Na/K-feldspars PresentClays Traces

ConÐrmation of the validity of TL method

Starting with these data, samples of dust separated fromthe paprika were exposed to di†erent radiation doses.In Fig. 1, comparisons of the TL glow curves from non-irradiated polymineral phase (natural thermolumine-scence glow curve, NTL) with glow curves obtained200 h after irradiation (induced thermoluminescence,ITL) are displayed.

2D-NTL glow curves (temperature in ¡C vs intensityin arbitrary units (au)), all had the same shape (24replicates). An interesting aspect to emphasize is the lowcoefficient of variation of the NTL intensity results, iethe variation between curves is in the range of 5È10%.It is also important to indicate that, as has been seen forother spices, only low light levels are shown by theseNTL glow curves. The integrated areas between 80 and450¡C, were never higher than 60 000^ 4000 au. In allcases, the main di†erence between these glow curves isthat the TL intensities from polymineral dust samples,exposed to a 60Co source, exceeded signiÐcantly the TLintensities of non-irradiated samples, with light levelsthat were never lower than 500 000^ 25 000 au (limitsof integration area between 80 and 450¡C). The mea-surements performed on irradiated samples revealedthat all of them (from 1 to 21 kGy) could be clearlydiscriminated from non-irradiated ones on the basis of

TABLE 2Ratio of ITL/NTL (integration area between 80 and 450¡C)for polymineral dust from paprika, mint, oregano and sage

(Measurements took place 200 h after c-irradiation)

Dose (kGy) Ratio IT L /NT L

Paprika Mint Oregano Sage

1 5 6 55 175 7 14 131 43

10 12 16 161 6418 13 15 169 6521 13 17 194 69

Fig 1. 2D-TL glow curves of irradiated paprika at di†erentdoses (from 1 to 21 kGy) using a gamma source of 60Co at adose rate of 1É34 kGy h~1. The measurements were madeafter 200 h of storage since the treatment of the samples in

presence of sunlight and at room temperature.

Fig 2. Isometric data for the TL of a polymineral sample frompaprika. These plots of 3D-glow curves were got at a heatingrate of 2É5¡C s~1 of (a) non-irradiated, and (b) 18 kGy gamma

irradiated paprika after 5000 h of storage.

152 V Correcher, J L Mun8 iz, J M Go� mez-Ros

Fig 3. Decomposition of 2D-TL glow curves from inorganicphases of (a) non-irradiated, and (bÈf ) irradiated paprika. Theywere obtained at a heating rate of 5¡C s~1 in nitrogen atmo-

sphere at vacuum.

the intensity of the TL signal, and sometimes also con-sidering the position of the peaks. In fact, ITL glowcurves exhibit the presence of a very intense peak at lowtemperature (160¡C) related to c-irradiation. The di†er-ences in the TL intensities from the unirradiated andirradiated dust are evident. In Table 2, the ratio ofinduced to natural TL signal (ITL/NTL) is shown forthe paprika sample measured 200 h after irradiation.Paprika results are compared with oregano, mint andsage measurements that took placed under the sameconditions (Beneitez et al 1994). Of course, in practice,the unirradiated area is unknown and hence, to make aclear identiÐcation, it is necessary to perform a previousnormalisation step (Schreiber et al 1993).

The 3D plots of the thermoluminescence intensity (inau) against temperature (in ¡C) and wavelength (in nm)obtained from these samples in the Sussex laboratory,headed by Dr Townsend, are displayed in Fig. 2. It isseen the high intensity of the TL signal of irradiateddust with a dose of 18 kGy, after 5000 h of storage (Fig2(b)), when it is compared with the non-irradiatedone (Fig 2(a)). Here it is important to emphasise thatin irradiated samples the emission of light is in theorangeÈred region (600È700 nm) with intensitiesof 150È200 cts nm~1, which is a very strong signalcompared with the signal from unirradiated material

TABLE 3Some physical parameters of the TL peaks of non-irradiated (NTL) and irradiated (from 1 to 21 kGy) polymineral samples

obtained from GCA program (Measurements were made 200 h after irradiation)

Peak no NT L 1 kGy 5 kGy 10 kGy 18 kGy 21 kGy

TMa (¡C) 1 187 162 160 162 164 1622 277 182 179 182 183 1823 370 207 209 212 213 2104 È 244 236 241 243 2415 È 349 342 336 333 338

IMa (au) 1 332 3067 5626 6495 4781 71852 656 4630 7446 8853 6726 92823 1100 3053 6782 9042 7890 84274 È 3555 8806 9537 10808 104605 È 747 1390 2157 2636 2338

Ea (eV) 1 0É86 1É12 0É99 1É16 1É15 1É142 0É97 0É99 0É95 1É02 1É03 1É013 0É83 0É81 0É85 0É80 0É78 0É804 È 0É61 0É57 0É66 0É65 0É645 È 0É58 0É58 0É48 0É45 0É45

Area of peak (au) 1 8É84E3 5É72E4 1É16E5 1É17E5 8É77E4 1É32E52 2É20E4 1É06E5 1É76E5 1É96E5 1É48E5 2É07E53 5É72E4 9É28E4 2É00E5 2É85E5 2É56E5 2É62E54 È 1É63E5 4É12E5 4É01E5 4É63E5 4É48E55 È 5É02E4 9É13E4 1É65E5 2É09E5 1É82E5

FOMa 2É3E-2 1É5E-2 1É1E-2 1É4E-2 1É2E-2 1É6E-2

a TM, temperature of the maxima (¡C), IM, intensity of the maxima in arbitrary units (au) ; E, activation energy (eV), and FOM,factor of merit.

Saturation and fading on the T L signals in c-irradiated paprika 153

Fig 4. Fitting and fading of the signal of irradiated samples of paprika at (a) 5, (b) 10, and (c) 21 kGy after almost of 1500 h. TLsignals were integrated over the range 80È450¡C. Measurements were made after storage in presence of sunlight and at room

temperature.

(5È10 cts nm~1). TL emissions in UVÈblue regionfrom quartz and feldspars were reduced by exposureto light. This e†ect is also employed in the dating ofgeological sediments (Aitken 1985).

Glow curve analysis and dose results

In Fig 3, the whole TL curves, for di†erent doses, wereanalysed in terms of Ðrst-order kinetics glow peaks. Itwas made using the GCA program developed byGo� mez-Ros and Delgado (1989) from which it can beobtained information about several physical param-eters. As illustrated in Fig 3(a), NTL glow curve pre-sents three peaks corresponding to temperatures of187¡C, 277¡C and 370¡C, respectively.

The lowest temperature peak could be assigned tohalite although the presence of this peak is not frequentbecause at low temperatures the induction rate issmaller than the fading rate. The origin of the peakcould be due to the activation of determined traps as aresult of the UV light from the sunlight. In fact, the UVliberated charges become trapped and the TL measure-

ments, in this kind of material, provide a controlledrelease of these electrons during a heating cycle whichresults in a luminescent signal (Espan8 a et al 1992).

The second one, at 277¡C, is probably a consequenceof the presence of calcite (Aitken 1985) and K-feldspars(Duller 1994) and the third one can be assigned toquartz (Aitken 1985).

Figures 3(b)È(f ) show, in ITL glow curves, the pres-ence of four closely overlapping peaks and, at highertemperature, slightly separated from the others, onewider peak.

The very intense peak at low temperature (160¡C) is aspeciÐc maximum that can be related to the presence ofquartz (Godfrey-Smith 1994) or K-feldspars (Duller1994). Also, the TL response exhibits other glow peaksat 180¡C (that can be due to the presence of calcite(Carmichael et al 1994)) ; at 210¡C (related to the pres-ence of Na-feldspars (Kirsh et al 1990) or quartz(Godfrey-Smith 1994)) ; at 240¡C (related to the presenceof K-feldspars (Kirsh et al 1994)) and at 340¡C (due tothe presence of K-feldspars (Botter-Jensen et al,unpublished)).

In Table 3 some physical parameters are displayed.The values of activation energies are lower than they

154 V Correcher, J L Mun8 iz, J M Go� mez-Ros

were expected probably due to the Ðrst-order kineticsassumption for every peak to simplify the calculations.Although this is a good approach each peak has its ownorder with a value in a speciÐc range (between 0É8 and2É3). The value of the FOM (Ðgure of merit) is lowerthan 3%, indicating a good Ðt (Balian and Eddy 1977).

Undoubtedly, one critical aspect concerns the begin-ning of saturation of signal with respect to increasingradiation doses. As displayed in Tables 2 and 3, thegrowth of TL signal with dose shows a continuedincrease of the sensitivity until 10 kGy. Afterwards, theluminescence intensity falls of due to the onset of satu-ration. In the case of paprika (Table 3), the beginning ofsaturation occurs with the glow peaks (peaks 2 to 5) allgrowing in the same way. The exception is peak 1 whichthe onset of saturation appears after doses in excess of5 kGy.

Fading of TL signal

Furthermore, some studies were done in order to checkthe evolution of the TL signals with the elapsed timesince the irradiation process took place ; ie an attemptto establish the stability of the TL signal of irradiatedsamples over time. In this sense, some measurementswere made after increasing storage periods of time, until1500 h (62 days) and for di†erent doses, to be exact5, 10 and 21 kGy (5 replicates per unit of time andper dose). The experimental fading results are shownin Fig. 4.

In every case, regardless of the storage period, it wasclearly possible to distinguish irradiated from non-irradiated samples. The behaviour of every curve,regardless of the dose absorbed by the sample, is similarin all the cases ; an initial rapid decay (around 50%) tomaintain a certain stability from 300È400 h onwards(results obtained from 400 h to 1500 h show a smallloss of signal, around 15È20%). All decay processes canbe Ðtted to a general mathematic equation of the sort :

Y \ a ] bx ] cJx ] dF(x)

where Y corresponds to the intensity of the TL signal, xis the time after irradiation process, F(x) is a function oftime and a, b, c and d are coefficients of the equation.

This response can be justiÐed regarding the process offading which corresponds (as a good approach) with aÐrst-order kinetic equation ; although in many circum-stances, the decay of a luminiscence signal is seen tofollow neither monomolecular nor bimolecular laws(McKeever 1985). Maybe this would be a good futureapproach for the quantiÐcation of dose received by thefoods.

ACKNOWLEDGEMENTS

This study was carried out with funds from CICYT(I ] D, ALI93-0072 and I ] D, ALI91-0203) and CAM

(I ] D, C141/91). Also, it would not have been possiblewithout the aid of the UAM team headed by Dr Cal-dero� n ; the University of Sussex team headed by DrTownsend and personal suggestions from Drs Delgadoand Robredo.

REFERENCES

Aitken M J 1985 (T hermoluminescence Dating. AcademicPress, London, UK.

Balian H G, Eddy N W 1977 Figure-of-merit (FOM), animproved criterion over the normalized chi-squared test forassessing goodness-of-Ðt of gamma-ray spectral peaks. NuclInstr Meth 145 389È395.

Beneitez P, Correcher V, Milla� n A, Caldero� n T 1994 Thermo-luminescence analysis for testing the irradiation of spices.J Radioanal Nucl Chem 185 (2) 401È410.

Botter-Jensen L 1988 The automated Riso TL dating readersystem. Nucl T racks Radiat Meas 14 (1È2) 177È180.

Caldero� n T, Correcher V. Milla� n A, Beneitez P., Rendell H M,Larsson M, Townsend P D, Wood R A 1995 New data onthermoluminescence of inorganic dust from herbs andspices. J Phys D: Appl Phys 28 415È423.

Carmichael L, Sanderson D C W, Ni Riain S 1994 Thermolu-minescence measurement of calcite shells. Radiat Meas 23(2È3) 455È463.

CEE 1988 Propuesta de Directiva de la Comisio� n (88/C336/06) sobre tratamiento de productos alimenticios con radi-aciones ionizantes. In : Alimentos T ratados con RadiacionesIonizantes. Generales. DOCE No C336, 7.

Correcher V. Beneitez P. Milla� n A, Caldero� n T 1992 Detec-cio� n de especias irradiadas por Termoluminiscencia. In :Food Science and T echnology : Industry and Distribution (volII). Proc II Congreso Internacional de Qu•�mica de laANQUE, Burgos, Spain, pp 291È298.

Duller G A T 1994 A new method for the analysis of infraredstimulated luminescence data from potassium feldspars.Radiat Meas 23 (2È3) 281È285.

Espan8 a E, Caldero� n T, Cusso� F, Jaque F, Lifante G, Towns-end PD 1992 Detection of UV radiation in the actinic rangeof the earthÏs surface using Eu-doped NaCl crystals. NuclT racks Radiat Meas 20 (4) 605È607.

FAO/WHO/IAEA 1981 W holesomeness of Irradiated Food(WHO Technical Report 659). Geneva, Switzerland.

FAO/WHO/IAEA/ITC-UNCTAD/GATT 1989 Acceptance,control of and trade in irradiated food. Conference Proc.AIEA, Geneva, Switzerland.

Godfrey-Smith D I 1994 Thermal e†ects in the opticallystimulated luminescence of quartz and mixed feldspars fromsediments. J Phys D: Appl Phys 27 1737È1746.

Go� ksu-O� gelman H Y, Regulla D F 1989 Detection of irradi-ated food. Nature 340 23.

Go� mez-Ros J M, Delgado A 1989 Analisis de lasNume� ricoCurvas de T ermoluminiscencia. CIEMAT Report 629.Madrid, Spain.

Horowitz Y S 1984 T hermoluminescence and T hermolumines-cent Dosimetry (Vol III). CRC Press Inc, Boca Raton, FL,USA.

Kirsh Y, Kristianpoller N, Shoval S 1990 Kinetic analysis ofnatural and beta-induced TL in albite. Radiat Prot Dosim33 (14) 63È66.

Lu† B J, Townsend P D 1993 High sensitivity thermolumine-scence spectrometer. Meas Sci T echnol 4 65È71.

McKeever S W S 1985 T hermoluminescence of Solids. Cam-bridge University Press, London, UK.

Saturation and fading on the T L signals in c-irradiated paprika 155

Pinnioja S, Autio T, Niemi E, Pensala O 1993 Import controlof irradiated foods by the thermoluminescence method. Z.L ebensm Unters Forsch 196 111È115.

Sanderson D C W, Slater C, Cairns K J 1989a Detection ofirradiated food : Origins and implications for detection ofirradiation. Radiat Phys Chem 34 915È924.

Sanderson D C W, Slater C, Cairns K J 1989b Detection ofirradiated food. Nature 340 23È24.

Schreiber G A, Ziegelmann G, Quitzsch G, Helle N, Bo� gl KW 1993 Luminescence techniques to identify the treatmentof foods by ionizing radiation. Food Struct 12 385È396.