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Identification of gamma ray and electron-beam irradiated wheat after different processing treatments
Journal of Cereal Science 56 (2012) 347e351
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Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Identification of gamma ray and electron-beam irradiated wheat after different processing treatments Gui-Ran Kim, Kashif Akram, Jae-Jun Ahn, Joong-Ho Kwon* Department of Food Science and Technology, Kyungpook National University, Daegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2011 Received in revised form 17 February 2012 Accepted 23 February 2012
Photostimulated luminescence (PSL) and thermoluminescence (TL) methods were employed to monitor radiation-induced markers in gamma ray and electron-beam irradiated wheat after steaming (100 C, 30 min) & puffing (1.176 MPa, 15 min) treatments. Steamed wheat samples, regardless of radiation source, gave photon counts (PCs) below 700 (negative) for non-irradiated samples whereas irradiated samples gave PCs in the range of 13,564e29,959 (positive) showing the effectiveness of PSL as a screening test. However, puffed irradiated wheat samples gave PCs in the range of 1100e2138 (intermediate) showing the more drastic effect of puffing treatment than steaming. In TL analysis, the most drastic effect on TL glow curve intensity was also found by puffing treatment, but for all other samples, identification was possible as a maximum peak for irradiated samples appeared in the range of 150e250 C. TL ratio (TL1/TL2) was calculated after 1 kGy re-irradiation of TL1-tested minerals, to enhance the reliability of TL1 results. For the non-irradiated steamed samples, regardless of radiation source, TL ratio was less than 0.1 whereas ratios were more than 0.1 for all irradiated samples except puffed ones. The results proved the potential of luminescence techniques to characterize irradiated samples; however heat processing conditions showed an apparent effect on luminescence properties of irradiated samples. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Irradiation Processing Thermoluminescence Photostimulated luminescence Wheat
1. Introduction Wheat is the main cereal crop with its worldwide consumption in a variety of products. (Berghofer et al., 2003). Different processing techniques such as milling, pasta making (extrusion) and cooking are being employed extensively to obtain value added products from wheat grains (Cubadda et al., 2009). A survey undertaken in Australia has shown higher microbiological count of both spoilage and pathogenic microbes in wheat and wheat flour samples where similar results were also reported earlier for the wheat consumed in various part of the world (Berghofer et al., 2003). It is also estimated that annually, about 10e30% of grains produced worldwide are lost due to insect and rodent damage, which also results in loss of weight and nutrients, increased susceptibility to microbes, reduced market value and drastic effects on overall quality attributes (Singh et al., 2009). Food irradiation has proved its technological feasibility to improve microbial quality and reduce post harvest losses due to
Abbreviations: CEN, European Committee for Standardization; e-beam, electron-beam; KFDA, Korea Food and Drug Administration; PSL, Photostimulated luminescence; TL, Thermoluminescence; WHO, World Health Organizations. * Corresponding author. Tel.: þ82 53 950 5775; fax: þ82 53 950 6772. E-mail address: [email protected] (J.-H. Kwon). 0733-5210/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2012.02.013
insect pest attack during storage (Koksel et al., 1998). An irradiation treatment up to 5 kGy (to improved hygienic quality) could be applied without a significant effect on physiochemical properties of wheat; however a dose 1 kGy could serve the purpose of insect disinfestations (KFDA, 2008; Koksel et al., 1998; Smith and Pillai, 2004). The safety of irradiated food to any applied dose for technological benefits is guaranteed by leading world health organizations (WHO, 1999). Even though this technology is now permitted in more than 55 countries for commercial use, there is still a lack of international consensus for its general use (Farkas and Mohácsi-Farkas, 2011). Different national and international regulations are being enforced with mandatory labeling of irradiated food all over the world. Hence, identification methods are necessary to obtain acceptability of irradiated food in international trade (Arvanitoyannis, 2010). The European Committee for Standardization (CEN) has adopted 10 different protocols for the identification of irradiated food but, unfortunately, none has the potential to be applied for all food items (Chauhan et al., 2009; Delincée, 2002). Among others, two luminescence techniques, photostimulated luminescence (PSL) and thermoluminescence (TL) are standardized by the European Union as screening and confirmatory methods, respectively (EN 13751, 2009; EN 1788, 2001). These methods proved successful in identification of many different food materials containing enough
348
G.-R. Kim et al. / Journal of Cereal Science 56 (2012) 347e351
inorganic material especially silicates (Chauhan et al., 2009). The reliability of these methods depends upon quality and quantity of inorganic dust friction of food, whereas the luminescence property is also sensitive to adverse processing conditions (Alberti et al., 2007; Delincée, 2002, 1998). The aim of the present study was to investigate the effect of different processing conditions on luminescence characteristics of gamma and e-beam irradiated wheat and to check the applicability of CEN protocols for the identification of heat processed wheat products. 2. Materials and methods 2.1. Sample preparation and treatments Wheat (Triticum aestivum L.) of Korean origins was purchased from local markets in Korea. The samples were packed in polyethylene bags and irradiated using a Co-60 gamma irradiator (point source AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, ON, Canada) at the Korea Atomic Energy Research Institute, Jeongeup, South Korea and an electron-beam accelerator (ELV-4, 2.5 MeV, Fujifilm, Tokyo, Japan) at EB-TECH Co., Daejeon, South Korea. The absorbed doses of 0, 1, & 5 kGy at a dose rate of 2.1 kGy/h (KFDA, 2008) from both radiation sources were confirmed by using alanine dosimeters with a diameter of 5 mm (Bruker Instruments, Rheinstetten, Germany). The free-radical signals were analyzed by Bruker EMS 104 EPR analyzer (Bruker Instruments, Rheinstetten, Germany). To check the effect of processing conditions, two different types of treatments were employed. The irradiated wheat samples were steamed (100 C) for 30 min or puffed for 15 min at a pressure of 1.176 MPa using a commercial extruder (SYP 4506, Shinyoung mechanics Co. Ltd, South Korea).
Table 1 Effects of steaming and puffing treatments on PSL count of gamma and e-beam irradiated wheat. (unit: photon count/60s). Processing Radiation Irradiation dose (kGy) method source 0 1 Control Steaming Puffing
g-ray e-beam g-ray e-beam g-ray e-beam
391 362 297 316 251 243
13 ()a,b 25 () 32() 19 () 5 () 8 ()
24725 17056 13564 16438 1207 1100
5
7014 (þ) 1847 (þ) 1994 (þ) 1734 (þ) 249 (M) 169 (M)
54524 23835 25499 29959 2138 1568
10417 (þ) 9934 (þ) 2304 (þ) 3402 (þ) 146 (M) 294 (M)
a
Mean Standard deviation (n ¼ 3). Threshold value : T1 ¼ 700 (non-irradiated), T2 ¼ 5000 (irradiated), () < T1, T1 < (M) < T2, (þ) > T2. b
50e400 C at a temperature increase rate of 6 C/s under continuous nitrogen (99.999%) flushing. To normalize TL1 response, the tested minerals were re-irradiated at 1 kGy and the second TL glow curve (TL2) was measured. Finally, the TL ratio (integrated area of TL1/TL2 between 150 and 250 C) was calculated and used as recommended by EN 1788 (2001). 2.4. Statistical analysis All the measurements were obtained from three different packs (n ¼ 3) and results were presented as means (standard deviation). The data were analyzed using Origin 6.0 (Microcal Software Inc., Northampton, Mass., USA). 3. Results and discussion 3.1. PSL characteristics
2.2. Photostimulated luminescence (PSL) measurements The PSL measurement was performed as described in EN 13751 (2009) using a SURRC PPSL Irradiated Food Screening System (serial 0021, SURRC, Scottish Universities Research and Reactor Centre, Glasgow, U.K). The control and processed samples (5 g) were placed in a disposable petri dish (50 mm diameter; Bibby sterlin type 122, Glasgow, UK) and measured without any other preparation. The PSL signals emitted from the samples per second were automatically accumulated and presented as photon counts (PCs)/60s. The results were interpreted in accordance with protocol EN 13751 (2009) using lower threshold (T1, 700 counts/60s) and upper threshold (T2, 5000 counts/60s) values. The signals between the two thresholds were classified as intermediate, which requires further investigations for the confirmation of the tested samples. The experiments were conducted under subdued lighting. 2.3. Thermoluminescence (TL) measurements TL analysis was conducted in accordance with the protocol EN 1788 (EN, 2001). In brief, the samples (100 g, n ¼ 3) were first washed with distilled water over a nylon sieve (150 mm) to separate contaminating minerals. Sodium polytungstate (Sometu-Europe, Berlin, Germany) of density 2 g/mL was used to remove organic impurities, producing an inorganic (mineral) fraction. The inorganic minerals were treated with 1 N HCl (to dissolve carbonates) and 1 N NH4OH (neutralization). The final silicate minerals were suspended in acetone and loaded on TL discs (diameter 10 mm). The prepared discs were stored overnight in a laboratory oven at 50 C to remove unstable TL signals. TLD system (Harshaw TLD4500, Thermo Fisher Scientific Inc., Waltham, MA, USA) was used for the measurement of TL glow curve in the temperature range of
Table 1 shows the results of the PSL photon counts (PCs) for the control and processed wheat samples (steamed, puffed) after e-beam or gamma irradiation. The PCs of all non-irradiated samples (control & processed) showed negative results (<700 PCs), whereas dose dependent higher PSL count (>5000 PCs; positive), was experienced in all irradiated (e-beam & gamma ray) control samples. Regarding processing, steamed samples were positive with higher PSL counts depending on the applied dose. The most drastic effect was seen by the puffing treatment where the irradiated samples gave a PSL count in the range of 1100e2138 PCs (threshold value T1eT2, M), indicating an intermediate value requiring further investigations to confirm the identification result (EN 13751, 2009). Lower values of PSL counts from the sample irradiated at doses <1 kGy may be expected after puffing treatments showing the limitation of the PSL technique. The sensitivity of the PSL count to adverse conditions was also reported by different researchers (Alberti et al., 2007; Goulas
Table 2 TL glow curves (150e250 C) and TL ratios (TL1/TL2) of inorganic dust minerals separated from gamma and e-beam irradiated wheat. TL glow parameter
Radiation source
Irradiation dose (kGy)
TL1
g-ray
0.079 0.079 33.60 33.60 0.002 0.002
TL2 TL ratio
e-beam g-ray e-beam g-ray e-beam
0
1
0.013c 0.013c 6.613a 6.610a 0.001c 0.001c
11.970 8.279 43.137 25.975 0.258 0.286
5
8.091b 7.582bc 14.018a 17.855a 0.091b 0.095b
32.953 30.890 38.740 31.645 0.856 0.974
4.530a 3.867a 6.629a 0.286a 0.071a 0.113a
Means (n ¼ 3) within the rows of same parameter followed by the same superscript letters are not significantly different (P 0.05).
G.-R. Kim et al. / Journal of Cereal Science 56 (2012) 347e351
18000
28000
0 kGy 1 kGy 5 kGy
15000
0 kGy 1 kGy 5 kGy
24000
20000
TL intensity (a.u.)
12000
TL intensity (a.u.)
349
9000
6000
16000
12000
3000
8000
4000
0
0 50
100
150
200
250
300
350
50
100
Temperature ( C)
6000
200
250
300
350
300
350
300
350
Temperature ( C) 4500
0 kGy 1 kGy 5 kGy
7000
150
0 kGy 1 kGy 5 kGy
4000 3500
TL intensity (a.u.)
TL intensity (a.u.)
5000 4000 3000 2000
3000 2500 2000 1500 1000 500
1000
0
0
-500 50
100
150
200
250
300
50
350
100
120
0 kGy 1 kGy 5 kGy
200
250
0 kGy 1 kGy 5 kGy
120
100
80
TL intensity (a.u.)
TL intensity (a.u.)
100
150
Temperature( C)
Temperature ( C)
60
40
80
60
40
20
20
0
0 50
100
150
200
250
Temperature ( C)
300
350
50
100
150
200
250
Temperature( C)
Fig. 1. Glow curves of minerals separated from gamma (left) and e-beam (right) irradiated wheat (above, non-processed; middle, steamed; below, puffed).
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G.-R. Kim et al. / Journal of Cereal Science 56 (2012) 347e351
et al., 2008). The PSL count was also different on the basis of irradiation sources, where e-beam irradiation gave better results, but in the case of irradiated puffed sample, the PCs were higher in g-ray treated samples. The PSL response was not linear with the absorbed dose and processing conditions. The PSL sensitivity varies depending upon the presence of minerals accidentally accumulated on the surface of the dispensed sample layers (Bayram and Delincée, 2004; EN 13751, 2009). The PSL method can only be applied to screen larger sample lots; however, a confirmatory analysis as TL is needed to obtain more reliable result (Chauhan et al., 2009).
Table 4 TL glow curve integral (150e250 C) and TL ratios (TL1/TL2) of minerals separated from gamma and e-beam wheat after puffing.
3.2. TL characteristics
4. Conclusions
In TL analysis, employed after density separation of silicate minerals (EN 1788, 2001), all non-irradiated samples showed a TL glow curve of low intensity (Table 2) with maximum peak after 280 C (Fig. 1, above), giving the clear indication of absence of irradiation history. The results were further confirmed by their low TL ratio (<0.1). The irradiated samples gave a characteristic TL glow curves with maximum peaks in the range of 150e250 C. On steaming, the samples showed a decrease in intensity (Table 3) with little shift in TL glow curve peak maxima toward the higher temperature (Fig. 1, middle), but peaks were still in the range of 150e250 C, providing enough information for identification of irradiation. The TL ratio of irradiated control and steamed sample was also >0.1 confirming the reliability of the results. In the non-processed samples, the TL glow curve shape and intensity results were informative in the case of e-beam irradiation, but contrary results were found in both processed samples. The most severe effect of high temperature and pressure during processing was found in the case of the puffed sample, where a drastic decrease in TL glow curve intensity was coupled with the eradication of the characteristic TL peak proving the irradiation history of samples (Fig. 1, below). It was still possible to discriminate between irradiated and non-irradiated samples considering the glow curve shape except for the 1 kGy e-beam irradiated sample. However, all detection characteristics were lost and TL ratios (Table 4) were lower than 0.1 except the 5 kGy gamma irradiated sample (0.104), which was also too low to give a reliable indication for irradiation treatment. Lee et al. (2008) also reported the drastic effect of roasting treatment on the TL characteristics of irradiated sesame seeds, where glow curve and intensity provided useful information. The use of TL glow curve shape and intensity only for identification of irradiation was also reported by other researchers (Sanderson et al., 1998). However, the irradiated wheat samples after puffing treatment showed comparable TL properties as those for non-irradiated samples, making detection impossible.
The PSL screening was effective for all studied samples, except puffed wheat, where intermediate results were found. Comparatively better PSL results were experienced in the case of e-beam irradiated samples. In TL analysis, all non-irradiated samples showed the TL glow curve of low intensity without any characteristic peak in the temperature range of 150e250 C. The TL ratios of all non-irradiated samples were also lower than 0.1. For the identification of irradiated samples, TL glow curve shape, intensity, and TL ratio (TL1/TL2) provided key information for the identification of both types of irradiation regarding the non-processed and irradiated puffed samples. In these samples, high intensity TL peaks were found in the range of 150e250 C whereas TL ratios were >0.1. The most drastic effect was observed after puffing treatment, which showed significant decrease in intensity with eradication of TL signals showing the irradiation history of the samples. The TL ratios of puffed samples were also too low to confirm the TL results. The PSL and TL methods are effective techniques to screen and confirm the irradiation, respectively; however, adverse processing conditions can hamper their effective application.
Table 3 TL glow curve integral (150e250 C) and TL ratios (TL1/TL2) of minerals separated from gamma and e-beam irradiated wheat after steaming. TL glow parameter Radiation Irradiation dose (kGy) source 0 1 TL1 TL2 TL ratio
g-ray e-beam g-ray e-beam g-ray e-beam
0.029 0.036 6.352 7.844 0.005 0.005
0.012e 0.011e 0.003b 0.645b 0.002d 0.002d
1.321 1.714 5.741 7.773 0.230 0.221
5
0.050d 5.323 0.037a 0.007c 3.367 0.033b 0.379c 10.015 0.772a 0.360b 8.094 0.288b 0.007c 0.533 0.037a 0.009c 0.417 0.011b
Means (n ¼ 3) within the rows of same parameter followed by the same superscript letters are not significantly different (P 0.05).
TL glow parameter Radiation Irradiation dose (kGy) source 0 1 TL1 TL2 TL ratio
g-ray e-beam g-ray e-beam g-ray e-beam
0.013 0.012 0.373 0.289 0.035 0.041
0.004c 0.003c 0.015bc 0.002c 0.011dc 0.003dc
0.031 0.043 0.985 0.904 0.032 0.047
5
0.004b 0.004ab 0.013a 0.009a 0.004d 0.004bc
0.045 0.017 0.434 0.333 0.104 0.052
0.007a 0.003c 0.064b 0.029c 0.001a 0.004b
Means (n ¼ 3) within the rows of same parameter followed by the same superscript letters are not significantly different (P 0.05).
Acknowledgments This research was supported by Technology Development Program for Food, Ministry for Food, Agricultrure, Forestry and Fisheries, Republic of Korea. References Alberti, A., Corda, U., Fuochi, P., Bortolin, E., Calicchia, A., Onori, S., 2007. Lightinduced fading of the PSL signal from irradiated herbs and spices. Radiation Physics and Chemistry 76, 1455e1458. Arvanitoyannis, I.S., 2010. Consumer behavior toward Irradiated food. In: Irradiation of Food Commodities: Techniques, Applications, Detection, Legislation, Safety and Consumer Opinion. Academic Press Publications, London, pp. 673e698. Bayram, G., Delincée, H., 2004. Identification of irradiated Turkish foodstuffs combining various physical detection methods. Food Control 15, 81e91. Berghofer, L.K., Hocking, A.D., Miskelly, D., Jansson, E., 2003. Microbiology of wheat and flour milling in Australia. International Journal of Food Microbiology 85, 137e149. Chauhan, S.K., Kumar, R., Nadanasabapathy, S., Bawa, A.S., 2009. Detection methods for irradiated foods. Comprehensive Reviews in Food Science and Food Safety 8, 4e16. Cubadda, F., Aureli, F., Raggi, A., Carcea, M., 2009. Effect of milling, pasta making and cooking on minerals in durum wheat. Journal of Cereal Science 49, 92e97. Delincée, H., 2002. Analytical methods to identify irradiated food: a review. Radiation Physics and Chemistry 63, 455e458. Delincée, H., 1998. Detection of food treated with ionizing radiation. Trends in Food Science and Technology 9, 73e82. EN 13751, 2009. Foodstuffs-detection of Irradiated Food Using Photostimulated Luminescence, European Committee of Standardization (CEN). Brussels, Belgium. EN 1788, 2001. Foodstuffs-thermoluminescence Detection of Irradiated Food from Which Silicate Minerals Can Be Isolated. European Committee of Standardization (CEN). Brussels, Belgium. Farkas, J., Mohácsi-Farkas, C., 2011. History and future of food irradiation. Trends in Food Science and Technology 22, 121e126.
G.-R. Kim et al. / Journal of Cereal Science 56 (2012) 347e351 Goulas, A.E., Stahl, M., Riganakos, K.A., 2008. Effect of various parameters on detection of irradiated fish and oregano using the ESR and PSL methods. Food Control 19, 1076e1085. KFDA, 2008. Food Code. Korea Food & Drug Administration, Seoul, pp. 2-1-9w2-1-10. Koksel, H., Sapirstein, H.D., Celik, S., Bushuk, W., 1998. Effects of gamma-irradiation of wheat on gluten proteins. Journal of Cereal Science 28, 243e250. Lee, J., Kausar, T., Kim, B.K., Kwon, J.H., 2008. Detection of g-irradiated sesame seeds before and after roasting by analyzing photostimulated luminescence, thermoluminescence, and electron spin resonance. Journal of Agricultural and Food Chemistry 56, 7184e7188.
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Sanderson, D.C.W., Carmichael, L.A., Fisk, S., 1998. Establishing luminescence methods to detect irradiated foods. Food Science and Technology Today 12, 97e102. Singh, C.B., Jayas, D.S., Paliwal, J., White, N.D.G., 2009. Detection of insect-damaged wheat kernels using near-infrared hyperspectral imaging. Journal of Stored Products Research 45, 151e158. Smith, J.S., Pillai, S., 2004. Irradiation and food safety. Food Technology 58, 48e55. WHO, 1999. High-dose Irradiation: Wholesomeness of Food Irradiated with Doses Above 10 kGy Report of a Joint FAO/IAEA/WHO Study Group on High Dose Irradiation. WHO Technical Report Series. World Health Organization, Geneva.
Contents lists available at SciVerse ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Identification of gamma ray and electron-beam irradiated wheat after different processing treatments Gui-Ran Kim, Kashif Akram, Jae-Jun Ahn, Joong-Ho Kwon* Department of Food Science and Technology, Kyungpook National University, Daegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 December 2011 Received in revised form 17 February 2012 Accepted 23 February 2012
Photostimulated luminescence (PSL) and thermoluminescence (TL) methods were employed to monitor radiation-induced markers in gamma ray and electron-beam irradiated wheat after steaming (100 C, 30 min) & puffing (1.176 MPa, 15 min) treatments. Steamed wheat samples, regardless of radiation source, gave photon counts (PCs) below 700 (negative) for non-irradiated samples whereas irradiated samples gave PCs in the range of 13,564e29,959 (positive) showing the effectiveness of PSL as a screening test. However, puffed irradiated wheat samples gave PCs in the range of 1100e2138 (intermediate) showing the more drastic effect of puffing treatment than steaming. In TL analysis, the most drastic effect on TL glow curve intensity was also found by puffing treatment, but for all other samples, identification was possible as a maximum peak for irradiated samples appeared in the range of 150e250 C. TL ratio (TL1/TL2) was calculated after 1 kGy re-irradiation of TL1-tested minerals, to enhance the reliability of TL1 results. For the non-irradiated steamed samples, regardless of radiation source, TL ratio was less than 0.1 whereas ratios were more than 0.1 for all irradiated samples except puffed ones. The results proved the potential of luminescence techniques to characterize irradiated samples; however heat processing conditions showed an apparent effect on luminescence properties of irradiated samples. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Irradiation Processing Thermoluminescence Photostimulated luminescence Wheat
1. Introduction Wheat is the main cereal crop with its worldwide consumption in a variety of products. (Berghofer et al., 2003). Different processing techniques such as milling, pasta making (extrusion) and cooking are being employed extensively to obtain value added products from wheat grains (Cubadda et al., 2009). A survey undertaken in Australia has shown higher microbiological count of both spoilage and pathogenic microbes in wheat and wheat flour samples where similar results were also reported earlier for the wheat consumed in various part of the world (Berghofer et al., 2003). It is also estimated that annually, about 10e30% of grains produced worldwide are lost due to insect and rodent damage, which also results in loss of weight and nutrients, increased susceptibility to microbes, reduced market value and drastic effects on overall quality attributes (Singh et al., 2009). Food irradiation has proved its technological feasibility to improve microbial quality and reduce post harvest losses due to
Abbreviations: CEN, European Committee for Standardization; e-beam, electron-beam; KFDA, Korea Food and Drug Administration; PSL, Photostimulated luminescence; TL, Thermoluminescence; WHO, World Health Organizations. * Corresponding author. Tel.: þ82 53 950 5775; fax: þ82 53 950 6772. E-mail address: [email protected] (J.-H. Kwon). 0733-5210/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2012.02.013
insect pest attack during storage (Koksel et al., 1998). An irradiation treatment up to 5 kGy (to improved hygienic quality) could be applied without a significant effect on physiochemical properties of wheat; however a dose 1 kGy could serve the purpose of insect disinfestations (KFDA, 2008; Koksel et al., 1998; Smith and Pillai, 2004). The safety of irradiated food to any applied dose for technological benefits is guaranteed by leading world health organizations (WHO, 1999). Even though this technology is now permitted in more than 55 countries for commercial use, there is still a lack of international consensus for its general use (Farkas and Mohácsi-Farkas, 2011). Different national and international regulations are being enforced with mandatory labeling of irradiated food all over the world. Hence, identification methods are necessary to obtain acceptability of irradiated food in international trade (Arvanitoyannis, 2010). The European Committee for Standardization (CEN) has adopted 10 different protocols for the identification of irradiated food but, unfortunately, none has the potential to be applied for all food items (Chauhan et al., 2009; Delincée, 2002). Among others, two luminescence techniques, photostimulated luminescence (PSL) and thermoluminescence (TL) are standardized by the European Union as screening and confirmatory methods, respectively (EN 13751, 2009; EN 1788, 2001). These methods proved successful in identification of many different food materials containing enough
348
G.-R. Kim et al. / Journal of Cereal Science 56 (2012) 347e351
inorganic material especially silicates (Chauhan et al., 2009). The reliability of these methods depends upon quality and quantity of inorganic dust friction of food, whereas the luminescence property is also sensitive to adverse processing conditions (Alberti et al., 2007; Delincée, 2002, 1998). The aim of the present study was to investigate the effect of different processing conditions on luminescence characteristics of gamma and e-beam irradiated wheat and to check the applicability of CEN protocols for the identification of heat processed wheat products. 2. Materials and methods 2.1. Sample preparation and treatments Wheat (Triticum aestivum L.) of Korean origins was purchased from local markets in Korea. The samples were packed in polyethylene bags and irradiated using a Co-60 gamma irradiator (point source AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, ON, Canada) at the Korea Atomic Energy Research Institute, Jeongeup, South Korea and an electron-beam accelerator (ELV-4, 2.5 MeV, Fujifilm, Tokyo, Japan) at EB-TECH Co., Daejeon, South Korea. The absorbed doses of 0, 1, & 5 kGy at a dose rate of 2.1 kGy/h (KFDA, 2008) from both radiation sources were confirmed by using alanine dosimeters with a diameter of 5 mm (Bruker Instruments, Rheinstetten, Germany). The free-radical signals were analyzed by Bruker EMS 104 EPR analyzer (Bruker Instruments, Rheinstetten, Germany). To check the effect of processing conditions, two different types of treatments were employed. The irradiated wheat samples were steamed (100 C) for 30 min or puffed for 15 min at a pressure of 1.176 MPa using a commercial extruder (SYP 4506, Shinyoung mechanics Co. Ltd, South Korea).
Table 1 Effects of steaming and puffing treatments on PSL count of gamma and e-beam irradiated wheat. (unit: photon count/60s). Processing Radiation Irradiation dose (kGy) method source 0 1 Control Steaming Puffing
g-ray e-beam g-ray e-beam g-ray e-beam
391 362 297 316 251 243
13 ()a,b 25 () 32() 19 () 5 () 8 ()
24725 17056 13564 16438 1207 1100
5
7014 (þ) 1847 (þ) 1994 (þ) 1734 (þ) 249 (M) 169 (M)
54524 23835 25499 29959 2138 1568
10417 (þ) 9934 (þ) 2304 (þ) 3402 (þ) 146 (M) 294 (M)
a
Mean Standard deviation (n ¼ 3). Threshold value : T1 ¼ 700 (non-irradiated), T2 ¼ 5000 (irradiated), () < T1, T1 < (M) < T2, (þ) > T2. b
50e400 C at a temperature increase rate of 6 C/s under continuous nitrogen (99.999%) flushing. To normalize TL1 response, the tested minerals were re-irradiated at 1 kGy and the second TL glow curve (TL2) was measured. Finally, the TL ratio (integrated area of TL1/TL2 between 150 and 250 C) was calculated and used as recommended by EN 1788 (2001). 2.4. Statistical analysis All the measurements were obtained from three different packs (n ¼ 3) and results were presented as means (standard deviation). The data were analyzed using Origin 6.0 (Microcal Software Inc., Northampton, Mass., USA). 3. Results and discussion 3.1. PSL characteristics
2.2. Photostimulated luminescence (PSL) measurements The PSL measurement was performed as described in EN 13751 (2009) using a SURRC PPSL Irradiated Food Screening System (serial 0021, SURRC, Scottish Universities Research and Reactor Centre, Glasgow, U.K). The control and processed samples (5 g) were placed in a disposable petri dish (50 mm diameter; Bibby sterlin type 122, Glasgow, UK) and measured without any other preparation. The PSL signals emitted from the samples per second were automatically accumulated and presented as photon counts (PCs)/60s. The results were interpreted in accordance with protocol EN 13751 (2009) using lower threshold (T1, 700 counts/60s) and upper threshold (T2, 5000 counts/60s) values. The signals between the two thresholds were classified as intermediate, which requires further investigations for the confirmation of the tested samples. The experiments were conducted under subdued lighting. 2.3. Thermoluminescence (TL) measurements TL analysis was conducted in accordance with the protocol EN 1788 (EN, 2001). In brief, the samples (100 g, n ¼ 3) were first washed with distilled water over a nylon sieve (150 mm) to separate contaminating minerals. Sodium polytungstate (Sometu-Europe, Berlin, Germany) of density 2 g/mL was used to remove organic impurities, producing an inorganic (mineral) fraction. The inorganic minerals were treated with 1 N HCl (to dissolve carbonates) and 1 N NH4OH (neutralization). The final silicate minerals were suspended in acetone and loaded on TL discs (diameter 10 mm). The prepared discs were stored overnight in a laboratory oven at 50 C to remove unstable TL signals. TLD system (Harshaw TLD4500, Thermo Fisher Scientific Inc., Waltham, MA, USA) was used for the measurement of TL glow curve in the temperature range of
Table 1 shows the results of the PSL photon counts (PCs) for the control and processed wheat samples (steamed, puffed) after e-beam or gamma irradiation. The PCs of all non-irradiated samples (control & processed) showed negative results (<700 PCs), whereas dose dependent higher PSL count (>5000 PCs; positive), was experienced in all irradiated (e-beam & gamma ray) control samples. Regarding processing, steamed samples were positive with higher PSL counts depending on the applied dose. The most drastic effect was seen by the puffing treatment where the irradiated samples gave a PSL count in the range of 1100e2138 PCs (threshold value T1eT2, M), indicating an intermediate value requiring further investigations to confirm the identification result (EN 13751, 2009). Lower values of PSL counts from the sample irradiated at doses <1 kGy may be expected after puffing treatments showing the limitation of the PSL technique. The sensitivity of the PSL count to adverse conditions was also reported by different researchers (Alberti et al., 2007; Goulas
Table 2 TL glow curves (150e250 C) and TL ratios (TL1/TL2) of inorganic dust minerals separated from gamma and e-beam irradiated wheat. TL glow parameter
Radiation source
Irradiation dose (kGy)
TL1
g-ray
0.079 0.079 33.60 33.60 0.002 0.002
TL2 TL ratio
e-beam g-ray e-beam g-ray e-beam
0
1
0.013c 0.013c 6.613a 6.610a 0.001c 0.001c
11.970 8.279 43.137 25.975 0.258 0.286
5
8.091b 7.582bc 14.018a 17.855a 0.091b 0.095b
32.953 30.890 38.740 31.645 0.856 0.974
4.530a 3.867a 6.629a 0.286a 0.071a 0.113a
Means (n ¼ 3) within the rows of same parameter followed by the same superscript letters are not significantly different (P 0.05).
G.-R. Kim et al. / Journal of Cereal Science 56 (2012) 347e351
18000
28000
0 kGy 1 kGy 5 kGy
15000
0 kGy 1 kGy 5 kGy
24000
20000
TL intensity (a.u.)
12000
TL intensity (a.u.)
349
9000
6000
16000
12000
3000
8000
4000
0
0 50
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200
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300
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50
100
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6000
200
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350
Temperature ( C) 4500
0 kGy 1 kGy 5 kGy
7000
150
0 kGy 1 kGy 5 kGy
4000 3500
TL intensity (a.u.)
TL intensity (a.u.)
5000 4000 3000 2000
3000 2500 2000 1500 1000 500
1000
0
0
-500 50
100
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120
0 kGy 1 kGy 5 kGy
200
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0 kGy 1 kGy 5 kGy
120
100
80
TL intensity (a.u.)
TL intensity (a.u.)
100
150
Temperature( C)
Temperature ( C)
60
40
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60
40
20
20
0
0 50
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300
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50
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Temperature( C)
Fig. 1. Glow curves of minerals separated from gamma (left) and e-beam (right) irradiated wheat (above, non-processed; middle, steamed; below, puffed).
350
G.-R. Kim et al. / Journal of Cereal Science 56 (2012) 347e351
et al., 2008). The PSL count was also different on the basis of irradiation sources, where e-beam irradiation gave better results, but in the case of irradiated puffed sample, the PCs were higher in g-ray treated samples. The PSL response was not linear with the absorbed dose and processing conditions. The PSL sensitivity varies depending upon the presence of minerals accidentally accumulated on the surface of the dispensed sample layers (Bayram and Delincée, 2004; EN 13751, 2009). The PSL method can only be applied to screen larger sample lots; however, a confirmatory analysis as TL is needed to obtain more reliable result (Chauhan et al., 2009).
Table 4 TL glow curve integral (150e250 C) and TL ratios (TL1/TL2) of minerals separated from gamma and e-beam wheat after puffing.
3.2. TL characteristics
4. Conclusions
In TL analysis, employed after density separation of silicate minerals (EN 1788, 2001), all non-irradiated samples showed a TL glow curve of low intensity (Table 2) with maximum peak after 280 C (Fig. 1, above), giving the clear indication of absence of irradiation history. The results were further confirmed by their low TL ratio (<0.1). The irradiated samples gave a characteristic TL glow curves with maximum peaks in the range of 150e250 C. On steaming, the samples showed a decrease in intensity (Table 3) with little shift in TL glow curve peak maxima toward the higher temperature (Fig. 1, middle), but peaks were still in the range of 150e250 C, providing enough information for identification of irradiation. The TL ratio of irradiated control and steamed sample was also >0.1 confirming the reliability of the results. In the non-processed samples, the TL glow curve shape and intensity results were informative in the case of e-beam irradiation, but contrary results were found in both processed samples. The most severe effect of high temperature and pressure during processing was found in the case of the puffed sample, where a drastic decrease in TL glow curve intensity was coupled with the eradication of the characteristic TL peak proving the irradiation history of samples (Fig. 1, below). It was still possible to discriminate between irradiated and non-irradiated samples considering the glow curve shape except for the 1 kGy e-beam irradiated sample. However, all detection characteristics were lost and TL ratios (Table 4) were lower than 0.1 except the 5 kGy gamma irradiated sample (0.104), which was also too low to give a reliable indication for irradiation treatment. Lee et al. (2008) also reported the drastic effect of roasting treatment on the TL characteristics of irradiated sesame seeds, where glow curve and intensity provided useful information. The use of TL glow curve shape and intensity only for identification of irradiation was also reported by other researchers (Sanderson et al., 1998). However, the irradiated wheat samples after puffing treatment showed comparable TL properties as those for non-irradiated samples, making detection impossible.
The PSL screening was effective for all studied samples, except puffed wheat, where intermediate results were found. Comparatively better PSL results were experienced in the case of e-beam irradiated samples. In TL analysis, all non-irradiated samples showed the TL glow curve of low intensity without any characteristic peak in the temperature range of 150e250 C. The TL ratios of all non-irradiated samples were also lower than 0.1. For the identification of irradiated samples, TL glow curve shape, intensity, and TL ratio (TL1/TL2) provided key information for the identification of both types of irradiation regarding the non-processed and irradiated puffed samples. In these samples, high intensity TL peaks were found in the range of 150e250 C whereas TL ratios were >0.1. The most drastic effect was observed after puffing treatment, which showed significant decrease in intensity with eradication of TL signals showing the irradiation history of the samples. The TL ratios of puffed samples were also too low to confirm the TL results. The PSL and TL methods are effective techniques to screen and confirm the irradiation, respectively; however, adverse processing conditions can hamper their effective application.
Table 3 TL glow curve integral (150e250 C) and TL ratios (TL1/TL2) of minerals separated from gamma and e-beam irradiated wheat after steaming. TL glow parameter Radiation Irradiation dose (kGy) source 0 1 TL1 TL2 TL ratio
g-ray e-beam g-ray e-beam g-ray e-beam
0.029 0.036 6.352 7.844 0.005 0.005
0.012e 0.011e 0.003b 0.645b 0.002d 0.002d
1.321 1.714 5.741 7.773 0.230 0.221
5
0.050d 5.323 0.037a 0.007c 3.367 0.033b 0.379c 10.015 0.772a 0.360b 8.094 0.288b 0.007c 0.533 0.037a 0.009c 0.417 0.011b
Means (n ¼ 3) within the rows of same parameter followed by the same superscript letters are not significantly different (P 0.05).
TL glow parameter Radiation Irradiation dose (kGy) source 0 1 TL1 TL2 TL ratio
g-ray e-beam g-ray e-beam g-ray e-beam
0.013 0.012 0.373 0.289 0.035 0.041
0.004c 0.003c 0.015bc 0.002c 0.011dc 0.003dc
0.031 0.043 0.985 0.904 0.032 0.047
5
0.004b 0.004ab 0.013a 0.009a 0.004d 0.004bc
0.045 0.017 0.434 0.333 0.104 0.052
0.007a 0.003c 0.064b 0.029c 0.001a 0.004b
Means (n ¼ 3) within the rows of same parameter followed by the same superscript letters are not significantly different (P 0.05).
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