J Radioanal Nucl Chem DOI 10.1007/s10967-014-3421-6
Thermoluminescence characterization of isolated minerals to identify oranges exposed to c-ray, e-beam, and X-ray for quarantine applications Deokjo Jo • Bhaskar Sanyal • Ju-Woon Lee Joong-Ho Kwon
•
Received: 15 April 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract Identification of irradiated fruits is of paramount importance to address the limitation of irradiation technology because of varying national and international regulations. Thermoluminescence (TL) analysis was carried out to identify oranges irradiated with c-ray, electron beam and X-ray. Nonirradiated samples exhibited background TL signals, but all the irradiated samples showed defined TL glow curves characterized by a prominent peak at 158–163 °C. Characterizations of the irradiated standard minerals showed that feldspars were the major contributors to the TL emission and stable TL signals revealed a successful detection of irradiated oranges even after a prolonged storage. Keywords Food irradiation detection Thermoluminescence Feldspars Quartz
Introduction Large quantities of fruits, vegetables, and nuts are produced in South Korea. However, fruits and nuts imported
Bhaskar Sanyal—On leave from Food Technology Division, Bhabha Atomic Research Centre, Mumbai, India. D. Jo B. Sanyal J.-H. Kwon (&) School of Food Science and Biotechnology, Kyungpook National University, Daegu 702-701, Korea e-mail:
[email protected] B. Sanyal Food Technology Division, Bhabha Atomic Research Centre, Mumbai, India J.-W. Lee Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeonbuk 580-185, Korea
from the United States are finding increasing markets in South Korea because of their quality, relatively low cost, and variety. In 2010, South Korea’s fresh fruit and vegetable imports rose significantly because of some recovery from the economic downturn of the previous year. Fresh fruit imports from the United States were worth $205 million, up by 56 % from that of the previous year, and oranges were the major item in this trade [1]. Growth is likely to continue over the next decade. However, the import of oranges from other countries is associated with the risk of migration of potentially damaging organisms such as insect pests to new areas. To overcome this barrier to international trade known as quarantine barrier, technological solutions are being developed worldwide. Food irradiation technology could be one of the potential solutions to this problem. The safety, wholesomeness, and nutritional adequacy of irradiated foods are now well documented and accepted by all major health and food authorities [2]. Food irradiation technology is being used commercially in more than 55 countries around the world to improve hygiene quality [3]. Further developments in the design and adaptation of uses of machine radiation sources (e-beam facilities and X-ray machines) [4–6] are also being investigated in quarantine treatments of various food commodities, including citrus fruits such as oranges. However, various national and international regulations with mandatory labeling requirements restrict the general use of this technology. The acceptability of irradiated food commodities, especially in the commercial domain of international trade, needs reliable identification methods to enforce regulations and traceability [7]. The development of reliable methods to distinguish irradiated foods from non-irradiated foods is therefore essential in view of the growing interest by the food industry in irradiation technology.
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Detection methods are classified into three basic categories; chemical, physical, and biological [8]. Significant progress has been made in the development of detection methods for irradiated foods [9–12]. Thermoluminescence (TL) is one of the most important physical techniques for examining spices, herbs, and dried fruits [13–15]. There is an increasing interest among researchers in extending the use of this technique to different types of food commodities because of its sensitivity and efficiency in detecting irradiation after prolonged storage. This method can be used for any food materials from which silicate minerals can be isolated [16]. The success of this technique depends on the quantity and quality of the isolated polyminerals, which contain various amounts of different constituents such as quartz, feldspars, and clay particles. [10, 12, 17–19]. The intensities of the TL outputs from irradiated samples therefore vary, and different TL glow curve structures are observed. The characteristics such as shape, emission wavelength, and intensity of a TL glow curve are critically dependent on both the nature of the material and the defects within it. However, the differences among the TL signal shapes for different minerals do not change the validity of the detection tests for irradiated foods [20]. Several reports on the intensities of TL glow peaks are available for inorganic dust particles collected from different irradiated foods [21, 22]. However, limited information is available on the TL properties of the isolated polyminerals and the detection efficiencies of the TL technique for food subjected to electron beam and X-ray irradiation. The purpose of the present study was to determine the TL glow curve structures of inorganic minerals isolated from irradiated oranges and to characterize the TL properties in comparison with those of the standard minerals. The efficacy of TL measurements in identifying imported oranges exposed to c-ray, electron beam, and X-ray irradiations for quarantine applications was assessed even after a prolonged storage.
Experimental Materials and irradiation treatment Navel oranges, imported from the United States, were purchased from a local market in Daegu, Korea. K-Feldspars and quartz were procured from Sigma-Aldrich (St. Louis, MO, USA) as standard TL materials to study the TL responses when exposed to different radiation sources. Samples were exposed to c-ray, electron beam, and X-ray in the dose range of 0–2 kGy. Gamma irradiation was carried out using a Co-60 source (100 kCi, AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, Canada) at the Korean Atomic Energy Research Institute, Jeongeup, South Korea. Electron beam and X-ray irradiation were carried out using an electron accelerator (ELV-4, 10 MeV,
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Fujifilm, Tokyo, Japan) at EB-Tech, Daejeon, South Korea. The dose rate of c irradiation was 1.5 kGy/h. In case of electron beam the dose rates for 0.5 and 2 kGy accumulated doses were of the order of 1.11 and 3.14 kGy/s, respectively. For X-ray irradiation the dose rate was of the order of 0.42 kGy/s. The absorbed doses were measured using an alanine-electron paramagnetic resonance dosimetry system, with an EMS 104 EPR analyzer (Bruker Biospin, Rheinstetten, Germany). Orange samples were stored at 4 ± 1 and 20 ± 1 °C during the experiments. Analysis using X-ray diffraction (XRD) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) XRD was used to determine the compositions of the minerals isolated from oranges. The inorganic dust was separated from the sample by washing with distilled water and characterized using a multipurpose X-ray diffractometer (MP XRD, X’Pert Pro, PANalytical, Almelo, Netherlands). The X-ray diffractometer was calibrated using silicon powder (corundum) as a standard reference material. The analysis was conducted under the following conditions: X’celerator (ultrafast) detector; 0–60° scan angle; 11.9°/s scan rate; Gonio scan axis; continuous scan mode; and Cu ˚ . The patterns were Ka radiation of wavelength 1.540598 A obtained between 0 and 60°h at a 2°h. The peaks were identified by comparison with reference data. SEM/EDX analysis was performed to determine the polymineral compositions of the isolated minerals. A field-emission scanning electron microscope (S4300; Hitachi, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer was used at an accelerating voltage of 15 kV. TL analysis Silicate minerals were separated from the orange samples by rinsing the fruit surface on a nylon sieve (125 lm) with distilled water, and sequential treatments with sodium polytungstate (2.0 g/cm3), 1 N HCl and 1 N NH4OH following recommendation of EN 1788 [14]. The mass of the aliquots for TL measurements was in the range of 0.8–0.9 mg in case of isolated minerals and the same for the standard minerals was 1 ± 0.2 mg. The grain size of the samples were less than 125 lm because of the sieving (mesh 125 lm) during sample preparation. The isolated minerals were analyzed using a TL reader (Harshaw 4500, Thermo Fisher Scientific Inc., Waltham, MA, USA) under dark conditions [14], by heating the sample from ambient temperature to 400 °C with a heating rate of 5 °C/s. During TL measurements no additional filter was used except the inbuilt infrared cutoff filter available in the TL reader. All the measurements were repeated for 3 times. The results
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were shown with the average of three measurements with the associated standard deviation. The significance of data was analyzed using a Duncan’s multiple range test (SAS Institute Inc. SAS User’s Guide. Statistical Analysis System Institute, Cary, NC, USA (1990).
Results and discussion Compositional analysis of isolated minerals TL can be used to identify irradiated foods from which silicate minerals can be isolated [14]. This inorganic material consists mainly of different amounts of minerals, i.e. quartz, silicates (muscovite, clinochlore, and feldspars), and carbonates (calcite and dolomite), as found in herbs and other species studied previously [17, 19, 23]. The polymineral components need to be isolated from the organic components because the inorganic phase emits radiation-induced TL, whereas the organic phase produces non-specific signals. In order to assess the possibility of using TL to identify irradiated oranges, investigation of the compositions of the isolated minerals is therefore of paramount importance. In view of this, the polyminerals separated from the oranges were studied using XRD and SEM/ EDX. The results of qualitative studies were interesting in terms of the relative abundances of polyminerals. The XRD results in Fig. 1a show major contributions from Feldspars (F) and quartz (Q). However, the XRD results did not clarify the elemental compositions of the inorganic minerals. To confirm the compositional characteristics of the isolated polyminerals further, SEM/EDX analysis was carried out. The SEM image in the inset of Fig. 1b shows the morphology of the extracted polyminerals and the elemental weight percentages are shown in EDX spectrum. Both results showed that the separated polyminerals consisted mainly of quartz (SiO2) and K-feldspars (KAlSi3O8-). In addition to quartz and feldspars, traces of Fe, Na, Ca, and Mg were also identified, and were probably present as oxides and carbonates. Silicon in the form of quartz and aluminum as feldspar were the two major constituents of the polyminerals isolated from the oranges, and their known TL sensitivities indicated that TL could be used in irradiation detection. Similar observations on the compositions of minerals isolated minerals from rice [12], herbs and spices such as oregano, mint, sage [18], and paprika [19], and potatoes [24] have been reported. TL characteristics of minerals exposed to different radiation sources With the advent of food irradiation technology, the use of ionizing radiation from different sources for phytosanitary
Fig. 1 X-ray diffraction patterns of silicate minerals, quartz-Q and feldspars-F isolated from oranges (a) and elemental compositions of the separated minerals using energy-dispersive X-ray spectroscopy analysis, scanning electron microscopy image is in the inset (b)
applications is increasing. Gamma ray, electron beam and X-ray radiation all have equal potential uses in quarantine treatment of fruits in the commercial domain. Few reports are available on the TL behaviors of polyminerals isolated from food and exposed to different sources of ionizing radiation. In the present context, identification of irradiated fruits treated with different ionizing radiations is therefore of major interest. In view of this, the TL characteristics of the inorganics separated from oranges before and after irradiation with c- and X-rays and an electron beam were studied. Figure 2 shows the TL glow curves of the minerals isolated from imported oranges irradiated with c- and X-rays and electrons in the dose range 0.5-2 kGy. No differences were observed among the TL glow curve structures obtained using different sources of ionizing radiation. In all cases, dose-dependent enhancement of the TL intensity was observed. The glow curves of the irradiated samples were characterized by a prominent glow peak in the range 158–163 °C with considerable enhancement of the TL intensity. The area under the glow curve was around 100–600 times more than that of the curve for a non-irradiated sample, depending on the delivered dose. Higher TL glow values (TL1) for irradiated samples have been reported in previous studies of spices and herbs [16], chestnuts [25], and rice [12]. The non-irradiated sample
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been reported in the literature for polyminerals isolated from non-irradiated foods and allied products [24, 26]. TL glow curve (TL1) measurements clearly identified the irradiated orange samples for all the radiation sources. The TL response also depends on the nature and quantity of mineral particles deposited on the aluminum disc used for TL measurements and the amount of radiation [27]. In other words, different types of mineral show varying degrees of luminescence at the same irradiation dose. To minimize the scattering of the luminescence results, a normalization method was used for unequivocal detection of irradiated food samples, as recommended in the literature [12, 21]. To confirm successful isolation of minerals from the oranges and irradiation detection, the same minerals were re-irradiated after the first measurements (TL1) with the respective radiation sources at a normalization dose of 1 kGy. The ratio of the areas of the first and second glow curves (TL1/TL2) for non-irradiated samples was \0.1, whereas the ratio for the irradiated sample was [0.1, as shown in Table 1. Higher values of TL1/TL2 for irradiated samples were in good agreement with the European Standard EN 1788 [14]. Normalization by re-irradiation at a dose of 1 kGy enhanced the reliability of the detection results. Since the TL glow is dependent on the nature and amount of adhered mineral particles, the dose–response curves of the orange sample measurements are completely reproducible. However, in the case of irradiated samples, dose-dependent increases in the TL intensities were observed. Behavior of TL signals during storage
Fig. 2 Thermoluminescence glow curves of silicate minerals separated from oranges irradiated with different ionizing radiations
exhibited negligible integral TL without any defined TL peak within the entire temperature range studied. A small hump at around 280 °C and a weak peak at 390 °C were observed, probably because of deep trapped charge-carriers formed by natural radiation. Similar observations have
TL measurements is a suitable and qualitative approach to detect irradiated food and/or food ingredients from which enough amounts of minerals can be isolated according to the EN 1788 European standard [14]. Identification of irradiated food after storage is a challenging task but important in accordance with the practical scenario. However, to the best of our knowledge, a limited information on the suitability of this protocol for the doses less than 5 kGy is available in literature. Recently, RodriguezLazcano et al. [28], showed that the identification of irradiated (1, 5 and 10 kGy) sesame seeds was possible using
Table 1 Thermoluminescence ratios of silicate minerals separated from oranges irradiated with different sources Radiation source
Irradiation dose (kGy) 0
Gamma ray E-beam X-ray a–c
0.5
1.0
0.00 ± 0.00c
0.34 ± 0.06bc
c
bc
0.00 ± 0.00
c
0.00 ± 0.00
0.20 ± 0.01
bc
0.22 ± 0.02
Values within a row followed by different superscript are significantly different
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2.0
0.64 ± 0.07b
1.55 ± 0.20a
0.40 ± 0.06
ab
0.63 ± 0.02a
0.35 ± 0.09
b
1.19 ± 0.07a
J Radioanal Nucl Chem Table 2 The effect of storage temperatures on TL signal intensity of oranges during postirradiation storage for 6 weeks
a–c
Values within a row followed by different superscript are significantly different
Irradiation source
Irradiation dose (kGy)
None
0
Gamma ray E-beam
2 2
d
Increasing rate compared with the intensity immediately after irradiation using each irradiation source
X-ray
2
Storage Temp (°C)
IRd (%)
Storage period (week) 0
6
4
1.52 ± 0.06a
1.49 ± 0.35a
-1.97
20
a
1.21 ± 0.19a
-20.39
4
104.66 ± 2.74a
84.97 ± 2.95b
-18.81
20
a
34.41 ± 0.29c
-67.12
b
1.52 ± 0.06 104.66 ± 2.74
a
4
104.82 ± 10.17
73.17 ± 4.29
-30.19
20
104.82 ± 10.17a
77.59 ± 16.42ab
-25.97
4
175.29 ± 5.60a
59.60 ± 5.37b
-65.99
20
175.29 ± 5.60a
31.60 ± 9.62c
-81.97
TL method even after a storage period of 15 months. In view of this, signal intensities of TL glow peaks of the isolated minerals from oranges irradiated at 2 kGy were measured during storage at two different temperatures of 4 and 20 °C till 6 weeks and represented in Table 2. Nonirradiated samples with weak TL peaks exhibited negligible fading of \2 % when stored at 4 °C. But samples kept at 20 °C showed enhanced fading around 20 %. However, in the case of irradiated samples considerable reduction in TL signal intensities were observed. Oranges stored at 4 °C after irradiation with gamma, electron beam and X-ray showed more stable intensity in comparison with those stored at 20 °C. Figure 3a–c show the shapes of the TL glows of the irradiated samples during storage. No noteworthy changes in glow curve structures during storage were observed except decrease in signal intensities. A shifting of the TL peaks toward higher temperature were seen for all the samples. The reduction in TL intensity could be associated with the probability of electron release from the shallower traps leading to right-shift of the peak temperature. Similar observations were also reported in literature [26, 28]. However, clear differences in TL glow curve structures between irradiated and non-irradiated samples were observed and a successful detection of oranges exposed to different types of radiations was possible even after a prolonged storage. TL characterization of isolated inorganics by comparison with marker minerals
Fig. 3 Thermoluminescence glow curves of the isolated minerals from orange exposed to different ionizing radiations and stored for 6 weeks at 4 and 20 °C
All the analyzed inorganics isolated from the oranges exhibited very complex glow curve structures, indicating that analysis is difficult using the commonly accepted physical model to explain the trap structure. Most TL peakanalysis methods have assumed that the trap depths associated with the localized states are single discrete levels. In the case of polymineral dust materials, the trap depths associated with particular defects will be spread over a range of values rather than being uniquely defined [29]. In practice, therefore, these methods are difficult to apply
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Fig. 4 Thermoluminescence glow curves of marker minerals (quartz and feldspars) irradiated with different ionizing radiations
because of the complications associated with the closely overlapping and difficult to separate TL peaks. Such problems will be maximized when the sample under study is a mixture of different phosphors [29]. We attempted to understand these TL glow curve structures of minerals isolated from oranges by comparing their TL properties with those of standard minerals. Quartz and feldspars were the major constituents in the isolated polyminerals. The TL behavior of feldspars and quartz marker minerals (1 ± 0.2 mg) were studied after irradiation with all three sources of ionizing radiation. Radiation doses of 0.1, 0.25,
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0.5, and 1 kGy were delivered and the TL glows were measured. Figure 4 shows the TL glow curve structures of the samples before and after irradiation. Non-irradiated quartz and feldspars samples showed weak TL outputs. In the case of feldspars, a broad peak was observed at around 262 °C, whereas for quartz, three weak peaks at 180, 255, and 350 °C were identified. For polyminerals isolated from orange samples, a weak peak and a poorly defined peak were observed at 275 and 390 °C, respectively. These peaks resembled the TL peaks of both quartz and feldspars, with a more prominent influence from feldspars, probably
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TL signal intensity (a.u.)
10000 8000 6000 4000
Gamma ray (y = -0.0011x² + 5.79x - 145.56, r² = 0.9948) E-beam (y = 0.0036x² + 1.52x - 48.60, r² = 0.9994) X-ray (y = -0.0026x² + 8.89x - 117.17, r² = 0.9974)
2000 0 -2000 0
100
250
500
1000
Irradiation dose (Gy)
TL signal intensity (a.u.)
100 Gamma ray (y = -3E-06x² + 0.03x + 20.33, r² = 0.9723)
80
60
E-beam (y = -3E-05x² + 0.03x + 22.83, r² = 0.8816) X-ray (y = -1E-05x² + 0.02x + 25.90, r² = 0.4849)
40
20
0 0
100
250
500
1000
Irradiation dose (Gy) Fig. 5 Thermoluminescence intensities of feldspars (up) and quartz (down) irradiated with increasing radiation doses from different ionizing radiations
because of the deep trapped charge carriers formed by natural radiation [12]. After irradiation, the shapes of the glow curve structures of both the marker minerals changed, and the intensities were enhanced. However, for each mineral, no differences among the glow curve structures were observed when different sources of radiation were used. The TL glows of feldspars samples exposed to all the ionizing radiations had a prominent radiation-induced TL peak at 160 °C. A weak high-temperature peak was also observed at around 350 °C. Similar behaviors of pure feldspars samples after irradiation have been reported [30]. However, in the case of the irradiated (c-ray, electron beam, and X-ray) quartz, all three weak peaks for the nonirradiated sample at 180, 255, and 350 °C were significantly intensified. Irradiated quartz exhibits several TL peaks, depending on the geological origin. Recently, Singh et al. [31] reported that for natural quartz samples irradiated with c-radiation at different high doses, two distinct peaks in the temperature ranges approximately 218–226 and 361–395 °C were observed. In contrast, the irradiated glows of the polyminerals isolated from oranges showed a
sharp TL peak in the range 158–163 °C, similar to that of the feldspars sample. This result indicated that the TL phenomenon was mainly caused by overlapping traps in feldspars, which was one of the major components of the separated polyminerals. Feldspars are well-established source of a typical TL glow curve, particularly for food materials of plant origin [32]. The TL responses of both the standard minerals with increasing radiation doses were also studied. Figure 5 shows the dose–response behavior for all the radiation sources. The TL intensity of feldspars showed a dose-dependent increase. The solid lines represent polynomial fits for all three sources, namely c-rays, electron beams, and X-rays with r2 values of 0.9948, 0.9994, and 0.9974, respectively (Fig. 5). The integral TL outputs from the irradiated feldspars were observed to be comparable at a given dose from all three radiation sources. However, in the case of quartz, up to 0.5 kGy electron beam irradiation produced a low response compared with c- and X-ray irradiations. Unlike the feldspars sample, the TL intensities of quartz irradiated with electrons and X-rays did not show a prominent dose-dependent increase, and saturation was observed beyond 0.5 kGy. However, the c-irradiated sample exhibited slow enhancement of the TL intensity, represented by a second-order polynomial (r2 = 0.9723). In the case of polyminerals separated from oranges, quantitative estimation of the TL intensity with increasing radiation dose was not possible because of the different natures and quantities of the constituents. However, a dosedependent increase in the TL intensity similar to that for feldspars was observed.
Conclusions The first TL glow curve structures of the isolated minerals from the irradiated oranges successfully showed the irradiation history of the samples. Similarities in the shapes of the TL glow curves after gamma, electron beam and X-ray irradiation indicated that the TL emission was independent of the types of the ionizing radiations. Normalization experiments further confirmed the suitable use of TL method to identify fruit samples when exposed to commercially relevant irradiation doses. Composition analyses of the isolated minerals and comparative studies of TL characteristics with the standard marker materials confirmed the origin of TL signals in orange samples. Feldspars were found out to be the most suitable marker for the detection of irradiated samples treated for quarantine application even after a prolonged storage. Acknowledgments This study was supported by the Export Promotion Technology Development Program, Ministry of Agriculture, Food and Rural Affairs, Republic of Korea.
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