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Characterization and identification of gamma-irradiated sauces by electron spin resonance spectroscopy using different sample pretreatments
Food Chemistry 138 (2013) 1878–1883
Contents lists available at SciVerse ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Characterization and identification of gamma-irradiated sauces by electron spin resonance spectroscopy using different sample pretreatments Kashif Akram a,b, Jae-Jun Ahn a, Joong-Ho Kwon a,⇑ a b
School of Food Science and Biotechnology, Kyungpook National University, Daegu 702-701, Korea Institute of Food Science and Nutrition, University of Sargodha, Sargodha 40100, Pakistan
a r t i c l e
i n f o
Article history: Received 2 July 2011 Received in revised form 27 October 2012 Accepted 8 November 2012 Available online 17 November 2012 Keywords: Sauces Irradiated food Radical identification Electron spin resonance Cellulose radical
a b s t r a c t Tomato ketchup, barbeque sauce, sweet chili sauce, and spaghetti sauce were gamma irradiated at 0, 1, 5, and 10 kGy. Electron spin resonance (ESR) technique was used to characterize the irradiated sauces, targeting radiation-induced cellulose radicals and using a modified sample pretreatment method. The samples were first washed with water, and then the residues were extracted with alcohol. The non-irradiated sauces exhibited the single central signal, whose intensity showed a significant increase on irradiation. The ESR spectra from the radiation-induced cellulose radicals, with two side peaks (g = 2.02012 and g = 1.98516) equally spaced (±3 mT) from the central signal, were also observed in the irradiated sauces. The improvements in the central (natural) and radiation-induced (two side peaks corresponding to the cellulose radicals) signal intensities were obvious, when compared with routine freeze-drying and alcoholic-extraction techniques. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Food irradiation has proved its worth for the provision of safe and healthy food with minimum compromise on nutritional and sensory characteristics (Akram & Kwon, 2010; Diehl, 2002). Sauces, in different forms, are being used extensively. However, health and safety concerns, especially those related to microbial contamination, have key importance for the processers and health experts. Various studies have found spoilage and disease-causing microorganisms, and the effectiveness of irradiation to address these problems in different sauces (Kim et al., 2000; Song, Kim, Park, Shin, & Byun, 2001). The safety and wholesomeness of irradiated food are endorsed by international organizations (IAEA, Codex, and WHO) concerned, but there is still lack of consensus for the applied regulations and general acceptability of this technology (Arvanitoyannis, 2010; Marchesani, Mangiacotti, & Chiaravalle, 2012). In this scenario, there is a key importance of simple, reliable, and routine analytical identification techniques in compliance with regulations and consumer’s right of choice (Akram, Ahn, & Kwon, 2012b). On irradiation of foods of plant origin, unpaired electrons in the radicals may be used as probes for electron spin resonance (ESR) based non-destructive identification with very small sample
⇑ Corresponding author. Tel.: +82 53 950 5775; fax: +82 53 950 6772. E-mail address: [email protected] (J.-H. Kwon). 0308-8146/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2012.11.044
weight (EN 1787, 2000). In this case, irradiation induces two side peaks (g = 2.020 and g = 1.985) whose hyperfine constants were found to be the same as that of irradiated cellulose. Therefore, these side peaks are attributable to the radiation-induced cellulose radicals (Akram, Ahn, Kim, & Kwon, 2012c; EN 1787, 2000). Irradiation detection using ESR technique is possible without any complicated pretreatment in food with low moisture contents (Akram, Ahn, & Kwon, 2012a; Bercu, Negut, & Duliu, 2012). However, high moisture content in different foods limits the application of ESR for detection purposes, and thus many researchers have tried to address this problem using freeze-drying (Yordanov, Aleksieva, Dimitrova, Georgieva, & Tzvetkova, 2006), alcoholic-extraction (de Jesus, Rossi, & Lopes, 1999; Delincee & Soika, 2002), oven-drying (Jo & Kwon, 2006), or other techniques (Kikuchi, Shimoyama, Ukai, & Kobayashi, 2011). Recently, applicability and reliability of alcoholic-extraction pretreatment to detect radiation-induced cellulose radicals in foods of high moisture content were compared with different techniques (Ahn, Akram, Jo, & Kwon, 2012a; Kikuchi et al., 2010; Yordanov & Aleksieva, 2009). In the present study, irradiated sauces were examined by ESR spectroscopy using an improved method of sample pretreatment for complex formula food containing high moisture content. The modified protocol for ESR detection involves two steps: (i) washing the sample with distilled water and (ii) alcoholic extraction (de Jesus et al., 1999) of the residue. The method was also compared with freeze-drying and alcoholic-extraction pretreatment techniques for its general applicability.
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2. Materials and methods
Tomato ketchup (KTP), barbeque sauce (BBQ), sweet chili sauce (SWC), and spaghetti sauce (SPT) were purchased from local market in Daegu, South Korea and stored at room temperature. The samples were irradiated (0, 1, 5, and 10 kGy at dose rate of 2.1 kGy/h) using a Co-60 gamma-ray source (AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, ON, Canada) at the Korean Atomic Energy Research Institute (KAERI), in Jeongeup, Korea. Alanine dosimeters with a diameter of 5 mm (Bruker Instruments, Rheinstetten, Germany) were used to calibrate the applied dose, and the free-radical signals were measured by a Bruker EMS 104 EPR analyzer (Bruker Instruments, Rheinstetten, Germany).
Approximately 0.1 g of the pulverized (<1 mm) sample was placed in a quartz ESR tube (5 mm dia.). The tube was then sealed with a plastic film, and stored in a desiccator at 40 ± 5% relative humidity. ESR signals were measured as described in the European standard (EN 1787, 2000). The X-band ESR spectrometer (JESTE200, Jeol Co., Tokyo, Japan) was used at room temperature under the following conditions: power, 0.4 mW; frequency 9.10– 9.21 GHz; centre field, 324 ± 2 mT; sweep width, 10–25 mT; modulation frequency, 100 kHz; modulation width, 1–2 mT; amplitude, 50–400; sweep time, 30 s; and time constant, 0.03 s. The ESR signal measurements (line width and height) were conducted using a data system ESPRIT-425 (Jeol Co.). Three measurements were recorded and mean values (±standard deviation) were reported. The results were analyzed using Microsoft excel (Microsoft Office 2007 version) and Origin 6 software.
2.2. ESR spectroscopy
3. Results and discussion
Three different sample pretreatments were employed before ESR measurements:
The ESR spectra of the non-irradiated samples exhibited a single peak, whereas irradiation treatment induced two side peaks with enhanced intensity of main ESR signal.
2.1. Samples and irradiation
i. FD: Freeze-drying (Bondiro, Ilsin Bio Base, Yangju, Kyunggido, Korea) of the sauce samples (Yordanov et al., 2006). ii. AE: Alcoholic-extraction of the samples as described by de Jesus et al. (1999) iii. WAE: Washing of sauce sample for 20 min with distilled water using a nylon sieve (150 mm) and the left over residues were used after alcoholic-extraction as described above
3.1. Spectral features of non-irradiated samples All the non-irradiated samples showed a single central signal (g = 2.004), as reported by many workers in different foods of plant origin (Engin, Aydas, & Polat, 2011; Tabner & Tabner, 1993). The ESR signal (g = 2.005 ± 0.001) in dried plant samples is due to the
1000 1500
FD AE WAE
KTP
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FD AE WAE
BBQ
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Fig. 1. The effects of different sample pretreatments (FD, freeze-drying; AE, alcoholic-extraction, and WAE, water washing and alcoholic-extraction) on ESR signals from nonirradiated sauces (KTP, tomato ketchup; BBQ, barbeque sauce; SWC, sweet chili sauce, and SPT, spaghetti sauce).
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Fig. 2. The effects of different sample pretreatments (FD, freeze-drying; AE, alcoholic-extraction, and WAE, water washing and alcoholic-extraction) on ESR signals from 10 kGy-irradiated sauces (KTP, tomato ketchup; BBQ, barbeque sauce; SWC, sweet chili sauce, and SPT, spaghetti sauce).
organic radicals (Kikuchi et al., 2010) that were attributed as semiquinone radicals in previous studies (Ahn, Akram, Kim, & Kwon, 2012b; Calucci et al., 2003). Intensity of this signal was the lowest in sweet chili sauce and the highest in ketchup sauce. Different
sample pretreatments also showed a clear effect on the signal intensity, where the highest intensity was observed in the WAE samples and the lowest of the same was obvious in the FD samples (Figs. 1 and 3)
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Fig. 3. The comparison of ESR signal intensities (g1 = left, g0 = central, g2 = right) of non-irradiated and irradiated sauces (KTP, tomato ketchup; BBQ, barbeque sauce; SWC, sweet chili sauce, and SPT, spaghetti sauce) showing the effects of different irradiation doses (0, 1, 5, and 10 kGy) and sample pretreatments (FD, freeze-drying; AE, alcoholicextraction, and WAE, water washing and alcoholic-extraction).
3.2. Spectral features of irradiated samples On irradiation, a dose-dependent increase in intensity of the central signal was observed in all samples except FD samples, which showed a minimum change following irradiation (Figs. 2 and 3). The radiation-induced two side peaks (g = 2.0201 and g = 1.9851) were also clearly found in all alcoholic-extracted samples at 5 kGy irradiation. However, a significant increase in the signal intensity was found by including a water-washing step. These side peaks were not clear in the 5 kGy-irradiated FD samples of BBQ and SWC sauces (Table 1). The clear increase in intensities of these side peaks was observed at 10 kGy irradiation, where the effect of different sample pretreatments was more comprehensible (Fig. 2). The radiation-induced side peaks were equally spaced at about ±3 mT from the main signal, which has been associated with the radicals produced by irradiation in cellulose-containing foods (Raffi et al., 2000). Deighton, Glidwell, Goodman, and Morrison (1993) reported that the left peak is only due to radiation induced-cellulose radicals, whereas the peak at right side, induced by lignin radicals, is sensitive to irradiation as well as drying processes (de Jesus, Rossi, & Lopes, 1996). Table 1 shows that the distance (g1–g2 = 6.0336 ± 0.0567) and g-values (g1 = 2.0242 ± 0.0003 and g2 = 1.9866 ± 0.0003) of two side peaks did not vary significantly with the changes in the samples and pretreatments, which represented that nature of radiation-induced free radicals remained unchanged following different pretreatments. However,
the difference in ESR signal intensity was prominent. The same trend was observed by de Jesus et al. (1999) in fruit pulp samples with different sample pretreatments. The intensities of the side peaks in comparison to the main signal (signal ratio) also changed with different sample pretreatments (Table 2). The signal ratios also varied with the irradiation doses and the sample types. In general, the FD samples showed a good signal ratio of side peaks (1:4:1) when compared with that of AE and WAE samples. But the FD spectrum was not as clear as that of the AE and WAE samples and thus identification was difficult in 5 kGy-irradiated FD samples. This might be attributed to a significant increase in intensity of the main peak in AE and WAE samples, where side peaks also exhibited a significant increase in signal intensity. The intensities of the side signals, corresponding to radiation-induced cellulose radicals, were reported in about 5% in irradiated Foeniculi fructus and 50% in irradiated citrus fruits out of the total intensity of the central ESR signal (Tabner & Tabner, 1993; Yamaoki, Tsujino, Kimura, Mino, & Ohta, 2009). 3.3. Dose-dependence of ESR signals from irradiated sauces It is observed that all the ESR peaks showed somewhat linear increase with the irradiation dose as illustrated in Fig. 3. The main peak, which also appeared in non-irradiated samples, showed a good increase in WAE and AE samples on irradiation. However, the FD samples showed minimum number of changes in intensity
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Table 1 The ESR signal information of the non-irradiated and irradiated sauces after different sample pretreatments. Samplea
KTP
BBQ
SWC
SPT
a b c d
Treatmentb
g-Valuec g0
g1
g2
0
FD AE WAE
2.0062 2.0068 2.0061
–d – –
– – –
– – –
1
FD AE WAE
2.0068 2.0069 2.0067
– – –
– – –
– – –
5
FD AE WAE
2.0060 2.0061 2.0063
2.0245 2.0241 2.0244
1.9866 1.9866 1.9865
5.896 6.067 6.067
10
FD AE WAE
2.0066 2.0064 2.0069
2.0245 2.0249 2.0243
1.9866 1.9870 1.9868
6.067 6.006 6.079
0
FD AE WAE
2.0064 2.0061 2.0064
– – –
– – –
– – –
1
FD AE WAE
2.0062 2.0062 2.0061
– – –
– – –
– – –
5
FD AE WAE
2.0069 2.0061 2.0063
– 2.0238 2.0247
– 1.9863 1.9862
– 5.957 6.011
10
FD AE WAE
2.0066 2.0066 2.0066
2.0244 2.0242 2.0239
1.9866 1.9863 1.9861
6.013 6.001 6.015
0
FD AE WAE
2.0067 2.0070 2.0068
– – –
– – –
– – –
1
FD AE WAE
2.0060 2.0065 2.0067
– – –
– – –
– – –
5
FD AE WAE
2.0064 2.0061 2.0067
– 2.0241 2.0239
– 1.9865 1.9861
– 6.003 6.102
10
FD AE WAE
2.0067 2.0063 2.0061
2.0242 2.0244 2.0247
1.9869 1.9868 1.9866
6.073 6.067 6.067
0
FD AE WAE
2.0067 2.0062 2.0066
– – –
– – –
– – –
1
FD AE WAE
2.0066 2.0069 2.0066
– – –
– – –
– – –
5
FD AE WAE
2.0066 2.0065 2.0064
2.0241 2.0239 2.0240
1.9854 1.9871 1.9863
5.892 6.018 6.116
10
FD AE WAE
2.0062 2.0065 2.0068
2.0239 2.0238 2.0242
1.9875 1.9864 1.9867
5.947 6.018 6.116
Dose (kGy)
g1–g2 Distance (mT)
KTP = tomato ketchup, BBQ = barbeque sauce, SWC = sweet chili sauce, SPT = spaghetti sauce. FD = freeze-drying, AE = alcoholic-extraction, WAE = water-washing and alcoholic-extraction. g Value (g1 = left, g0 = central, g2 = right) = 71.448 microwave (GHz)/magnetic field (mT). Signal not detected.
of main ESR signal following irradiation. The side peaks, emerged at 5 kGy-treated samples (except FD samples of BBQ and SWC sauces), increased at 10 kGy irradiation. The dose-dependent results were most prominent in WAE samples of tomato ketchup. The dose-dependent increase in radiation-induced cellulose radical signals, analyzed after different sample pretreatments, was also reported by de Jesus, Rossi, and Lopes (2000) in the flesh of irradiated vegetables.
4. Conclusion The alcoholic-extraction of irradiated sauces showed good applicability of ESR technique as compared to the freeze-drying sample pretreatments. Alcoholic-extraction method showed an improvement in ESR spectral features, particularly in signal intensity, using an easy water-washing step. The spectra associated with radiation-induced cellulose radicals, with two side peaks of g
K. Akram et al. / Food Chemistry 138 (2013) 1878–1883 Table 2 ESR signal ratios (intensity) of irradiated sauces after different sample pretreatments. Sample
Dose (kGy)
KTP
5
10
BBQ
5
10
SWC
5
10
SPT
5
10
a
Treatmenta
Signal ratio (gn intensity/g1 intensity) Left (g1)
Centre (g0)
Right (g2)
FD AE WAE FD AE WAE
1.0 1.0 1.0 1.0 1.0 1.0
3.9 12.9 10.7 4.0 10.4 8.4
1.2 0.9 0.9 1.2 1.0 1.0
FD AE WAE FD AE WAE
– 1.0 1.0 1.0 1.0 1.0
– 15.9 9.5 2.1 5.4 4.3
– 0.9 0.9 1.0 1.0 1.0
FD AE WAE FD AE WAE
– 1.0 1.0 1.0 1.0 1.0
– 19.3 20.4 4.2 17.7 18.9
– 1.4 1.1 1.0 1.1 1.2
FD AE WAE FD AE WAE
1.0 1.0 1.0 1.0 1.0 1.0
4.0 18.7 23.4 9.2 16.2 20.8
1.1 1.4 1.0 1.4 1.2 1.1
Same as mentioned for Table 2.
values 2.020 and 1.985, were observed at 5 kGy gamma irradiation. The most promising results were shown by tomato ketchup samples, where the effects of different sample pretreatments were also evident. These results suggest that paramagnetic radicals in cellulose produced by irradiation could be used effectively to identify gamma-irradiated sauces. Acknowledgements This research was supported by Technology Development Program for Food, Ministry for Food, Agriculture, Forestry and Fisheries, Republic of Korea. References Ahn, J. J., Akram, K., Jo, D., & Kwon, J. H. (2012a). Investigation of different factors affecting the electron spin resonance-based characterization of gammairradiated fresh, white, and red ginseng using different sample pretreatments. Journal of Ginseng Research, 36, 308–313. Ahn, J. J., Akram, K., Kim, H. K., & Kwon, J. H. (2012b). Electron spin resonance spectroscopy for the identification of irradiated food with complex ESR signals. Food Analytical Methods. http://dx.doi.org/10.1007/s12161-012-9440-4. Akram, K., Ahn, J. J., Kim, G. R., & Kwon, J. H. (2012c). Identification of irradiated spaghetti sauces using different physical techniques. Journal of Food Quality, 35, 292–297. Akram, K., Ahn, J. J., & Kwon, J. H. (2012a). Identification and characterization of gamma-irradiated dried Lentinus edodes using ESR, SEM, and FTIR analyses. Journal of Food Science, 77, 690–696.
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Contents lists available at SciVerse ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Characterization and identification of gamma-irradiated sauces by electron spin resonance spectroscopy using different sample pretreatments Kashif Akram a,b, Jae-Jun Ahn a, Joong-Ho Kwon a,⇑ a b
School of Food Science and Biotechnology, Kyungpook National University, Daegu 702-701, Korea Institute of Food Science and Nutrition, University of Sargodha, Sargodha 40100, Pakistan
a r t i c l e
i n f o
Article history: Received 2 July 2011 Received in revised form 27 October 2012 Accepted 8 November 2012 Available online 17 November 2012 Keywords: Sauces Irradiated food Radical identification Electron spin resonance Cellulose radical
a b s t r a c t Tomato ketchup, barbeque sauce, sweet chili sauce, and spaghetti sauce were gamma irradiated at 0, 1, 5, and 10 kGy. Electron spin resonance (ESR) technique was used to characterize the irradiated sauces, targeting radiation-induced cellulose radicals and using a modified sample pretreatment method. The samples were first washed with water, and then the residues were extracted with alcohol. The non-irradiated sauces exhibited the single central signal, whose intensity showed a significant increase on irradiation. The ESR spectra from the radiation-induced cellulose radicals, with two side peaks (g = 2.02012 and g = 1.98516) equally spaced (±3 mT) from the central signal, were also observed in the irradiated sauces. The improvements in the central (natural) and radiation-induced (two side peaks corresponding to the cellulose radicals) signal intensities were obvious, when compared with routine freeze-drying and alcoholic-extraction techniques. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Food irradiation has proved its worth for the provision of safe and healthy food with minimum compromise on nutritional and sensory characteristics (Akram & Kwon, 2010; Diehl, 2002). Sauces, in different forms, are being used extensively. However, health and safety concerns, especially those related to microbial contamination, have key importance for the processers and health experts. Various studies have found spoilage and disease-causing microorganisms, and the effectiveness of irradiation to address these problems in different sauces (Kim et al., 2000; Song, Kim, Park, Shin, & Byun, 2001). The safety and wholesomeness of irradiated food are endorsed by international organizations (IAEA, Codex, and WHO) concerned, but there is still lack of consensus for the applied regulations and general acceptability of this technology (Arvanitoyannis, 2010; Marchesani, Mangiacotti, & Chiaravalle, 2012). In this scenario, there is a key importance of simple, reliable, and routine analytical identification techniques in compliance with regulations and consumer’s right of choice (Akram, Ahn, & Kwon, 2012b). On irradiation of foods of plant origin, unpaired electrons in the radicals may be used as probes for electron spin resonance (ESR) based non-destructive identification with very small sample
⇑ Corresponding author. Tel.: +82 53 950 5775; fax: +82 53 950 6772. E-mail address: [email protected] (J.-H. Kwon). 0308-8146/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2012.11.044
weight (EN 1787, 2000). In this case, irradiation induces two side peaks (g = 2.020 and g = 1.985) whose hyperfine constants were found to be the same as that of irradiated cellulose. Therefore, these side peaks are attributable to the radiation-induced cellulose radicals (Akram, Ahn, Kim, & Kwon, 2012c; EN 1787, 2000). Irradiation detection using ESR technique is possible without any complicated pretreatment in food with low moisture contents (Akram, Ahn, & Kwon, 2012a; Bercu, Negut, & Duliu, 2012). However, high moisture content in different foods limits the application of ESR for detection purposes, and thus many researchers have tried to address this problem using freeze-drying (Yordanov, Aleksieva, Dimitrova, Georgieva, & Tzvetkova, 2006), alcoholic-extraction (de Jesus, Rossi, & Lopes, 1999; Delincee & Soika, 2002), oven-drying (Jo & Kwon, 2006), or other techniques (Kikuchi, Shimoyama, Ukai, & Kobayashi, 2011). Recently, applicability and reliability of alcoholic-extraction pretreatment to detect radiation-induced cellulose radicals in foods of high moisture content were compared with different techniques (Ahn, Akram, Jo, & Kwon, 2012a; Kikuchi et al., 2010; Yordanov & Aleksieva, 2009). In the present study, irradiated sauces were examined by ESR spectroscopy using an improved method of sample pretreatment for complex formula food containing high moisture content. The modified protocol for ESR detection involves two steps: (i) washing the sample with distilled water and (ii) alcoholic extraction (de Jesus et al., 1999) of the residue. The method was also compared with freeze-drying and alcoholic-extraction pretreatment techniques for its general applicability.
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2. Materials and methods
Tomato ketchup (KTP), barbeque sauce (BBQ), sweet chili sauce (SWC), and spaghetti sauce (SPT) were purchased from local market in Daegu, South Korea and stored at room temperature. The samples were irradiated (0, 1, 5, and 10 kGy at dose rate of 2.1 kGy/h) using a Co-60 gamma-ray source (AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, ON, Canada) at the Korean Atomic Energy Research Institute (KAERI), in Jeongeup, Korea. Alanine dosimeters with a diameter of 5 mm (Bruker Instruments, Rheinstetten, Germany) were used to calibrate the applied dose, and the free-radical signals were measured by a Bruker EMS 104 EPR analyzer (Bruker Instruments, Rheinstetten, Germany).
Approximately 0.1 g of the pulverized (<1 mm) sample was placed in a quartz ESR tube (5 mm dia.). The tube was then sealed with a plastic film, and stored in a desiccator at 40 ± 5% relative humidity. ESR signals were measured as described in the European standard (EN 1787, 2000). The X-band ESR spectrometer (JESTE200, Jeol Co., Tokyo, Japan) was used at room temperature under the following conditions: power, 0.4 mW; frequency 9.10– 9.21 GHz; centre field, 324 ± 2 mT; sweep width, 10–25 mT; modulation frequency, 100 kHz; modulation width, 1–2 mT; amplitude, 50–400; sweep time, 30 s; and time constant, 0.03 s. The ESR signal measurements (line width and height) were conducted using a data system ESPRIT-425 (Jeol Co.). Three measurements were recorded and mean values (±standard deviation) were reported. The results were analyzed using Microsoft excel (Microsoft Office 2007 version) and Origin 6 software.
2.2. ESR spectroscopy
3. Results and discussion
Three different sample pretreatments were employed before ESR measurements:
The ESR spectra of the non-irradiated samples exhibited a single peak, whereas irradiation treatment induced two side peaks with enhanced intensity of main ESR signal.
2.1. Samples and irradiation
i. FD: Freeze-drying (Bondiro, Ilsin Bio Base, Yangju, Kyunggido, Korea) of the sauce samples (Yordanov et al., 2006). ii. AE: Alcoholic-extraction of the samples as described by de Jesus et al. (1999) iii. WAE: Washing of sauce sample for 20 min with distilled water using a nylon sieve (150 mm) and the left over residues were used after alcoholic-extraction as described above
3.1. Spectral features of non-irradiated samples All the non-irradiated samples showed a single central signal (g = 2.004), as reported by many workers in different foods of plant origin (Engin, Aydas, & Polat, 2011; Tabner & Tabner, 1993). The ESR signal (g = 2.005 ± 0.001) in dried plant samples is due to the
1000 1500
FD AE WAE
KTP
500 250 0 -250 -500
FD AE WAE
BBQ
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ESR intensity (a.u.)
ESR intensity (a.u.)
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Magnetic field (mT) 1500
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SWC
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SPT
ESR intensity (a.u.)
ESR intensity (a.u.)
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Fig. 1. The effects of different sample pretreatments (FD, freeze-drying; AE, alcoholic-extraction, and WAE, water washing and alcoholic-extraction) on ESR signals from nonirradiated sauces (KTP, tomato ketchup; BBQ, barbeque sauce; SWC, sweet chili sauce, and SPT, spaghetti sauce).
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FD
ESR intensity (a.u.)
ESR intensity (a.u.)
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0
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WAE
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g1 g0
KTP
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500
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400
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400
0
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200
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g0
g2
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Magnetic field (mT)
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0
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SWC 318
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FD
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g2
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Fig. 2. The effects of different sample pretreatments (FD, freeze-drying; AE, alcoholic-extraction, and WAE, water washing and alcoholic-extraction) on ESR signals from 10 kGy-irradiated sauces (KTP, tomato ketchup; BBQ, barbeque sauce; SWC, sweet chili sauce, and SPT, spaghetti sauce).
organic radicals (Kikuchi et al., 2010) that were attributed as semiquinone radicals in previous studies (Ahn, Akram, Kim, & Kwon, 2012b; Calucci et al., 2003). Intensity of this signal was the lowest in sweet chili sauce and the highest in ketchup sauce. Different
sample pretreatments also showed a clear effect on the signal intensity, where the highest intensity was observed in the WAE samples and the lowest of the same was obvious in the FD samples (Figs. 1 and 3)
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ESR signal intensity (a.u.)
ESR signal intensity (a.u.)
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Irradiation dose (kGy)
Fig. 3. The comparison of ESR signal intensities (g1 = left, g0 = central, g2 = right) of non-irradiated and irradiated sauces (KTP, tomato ketchup; BBQ, barbeque sauce; SWC, sweet chili sauce, and SPT, spaghetti sauce) showing the effects of different irradiation doses (0, 1, 5, and 10 kGy) and sample pretreatments (FD, freeze-drying; AE, alcoholicextraction, and WAE, water washing and alcoholic-extraction).
3.2. Spectral features of irradiated samples On irradiation, a dose-dependent increase in intensity of the central signal was observed in all samples except FD samples, which showed a minimum change following irradiation (Figs. 2 and 3). The radiation-induced two side peaks (g = 2.0201 and g = 1.9851) were also clearly found in all alcoholic-extracted samples at 5 kGy irradiation. However, a significant increase in the signal intensity was found by including a water-washing step. These side peaks were not clear in the 5 kGy-irradiated FD samples of BBQ and SWC sauces (Table 1). The clear increase in intensities of these side peaks was observed at 10 kGy irradiation, where the effect of different sample pretreatments was more comprehensible (Fig. 2). The radiation-induced side peaks were equally spaced at about ±3 mT from the main signal, which has been associated with the radicals produced by irradiation in cellulose-containing foods (Raffi et al., 2000). Deighton, Glidwell, Goodman, and Morrison (1993) reported that the left peak is only due to radiation induced-cellulose radicals, whereas the peak at right side, induced by lignin radicals, is sensitive to irradiation as well as drying processes (de Jesus, Rossi, & Lopes, 1996). Table 1 shows that the distance (g1–g2 = 6.0336 ± 0.0567) and g-values (g1 = 2.0242 ± 0.0003 and g2 = 1.9866 ± 0.0003) of two side peaks did not vary significantly with the changes in the samples and pretreatments, which represented that nature of radiation-induced free radicals remained unchanged following different pretreatments. However,
the difference in ESR signal intensity was prominent. The same trend was observed by de Jesus et al. (1999) in fruit pulp samples with different sample pretreatments. The intensities of the side peaks in comparison to the main signal (signal ratio) also changed with different sample pretreatments (Table 2). The signal ratios also varied with the irradiation doses and the sample types. In general, the FD samples showed a good signal ratio of side peaks (1:4:1) when compared with that of AE and WAE samples. But the FD spectrum was not as clear as that of the AE and WAE samples and thus identification was difficult in 5 kGy-irradiated FD samples. This might be attributed to a significant increase in intensity of the main peak in AE and WAE samples, where side peaks also exhibited a significant increase in signal intensity. The intensities of the side signals, corresponding to radiation-induced cellulose radicals, were reported in about 5% in irradiated Foeniculi fructus and 50% in irradiated citrus fruits out of the total intensity of the central ESR signal (Tabner & Tabner, 1993; Yamaoki, Tsujino, Kimura, Mino, & Ohta, 2009). 3.3. Dose-dependence of ESR signals from irradiated sauces It is observed that all the ESR peaks showed somewhat linear increase with the irradiation dose as illustrated in Fig. 3. The main peak, which also appeared in non-irradiated samples, showed a good increase in WAE and AE samples on irradiation. However, the FD samples showed minimum number of changes in intensity
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Table 1 The ESR signal information of the non-irradiated and irradiated sauces after different sample pretreatments. Samplea
KTP
BBQ
SWC
SPT
a b c d
Treatmentb
g-Valuec g0
g1
g2
0
FD AE WAE
2.0062 2.0068 2.0061
–d – –
– – –
– – –
1
FD AE WAE
2.0068 2.0069 2.0067
– – –
– – –
– – –
5
FD AE WAE
2.0060 2.0061 2.0063
2.0245 2.0241 2.0244
1.9866 1.9866 1.9865
5.896 6.067 6.067
10
FD AE WAE
2.0066 2.0064 2.0069
2.0245 2.0249 2.0243
1.9866 1.9870 1.9868
6.067 6.006 6.079
0
FD AE WAE
2.0064 2.0061 2.0064
– – –
– – –
– – –
1
FD AE WAE
2.0062 2.0062 2.0061
– – –
– – –
– – –
5
FD AE WAE
2.0069 2.0061 2.0063
– 2.0238 2.0247
– 1.9863 1.9862
– 5.957 6.011
10
FD AE WAE
2.0066 2.0066 2.0066
2.0244 2.0242 2.0239
1.9866 1.9863 1.9861
6.013 6.001 6.015
0
FD AE WAE
2.0067 2.0070 2.0068
– – –
– – –
– – –
1
FD AE WAE
2.0060 2.0065 2.0067
– – –
– – –
– – –
5
FD AE WAE
2.0064 2.0061 2.0067
– 2.0241 2.0239
– 1.9865 1.9861
– 6.003 6.102
10
FD AE WAE
2.0067 2.0063 2.0061
2.0242 2.0244 2.0247
1.9869 1.9868 1.9866
6.073 6.067 6.067
0
FD AE WAE
2.0067 2.0062 2.0066
– – –
– – –
– – –
1
FD AE WAE
2.0066 2.0069 2.0066
– – –
– – –
– – –
5
FD AE WAE
2.0066 2.0065 2.0064
2.0241 2.0239 2.0240
1.9854 1.9871 1.9863
5.892 6.018 6.116
10
FD AE WAE
2.0062 2.0065 2.0068
2.0239 2.0238 2.0242
1.9875 1.9864 1.9867
5.947 6.018 6.116
Dose (kGy)
g1–g2 Distance (mT)
KTP = tomato ketchup, BBQ = barbeque sauce, SWC = sweet chili sauce, SPT = spaghetti sauce. FD = freeze-drying, AE = alcoholic-extraction, WAE = water-washing and alcoholic-extraction. g Value (g1 = left, g0 = central, g2 = right) = 71.448 microwave (GHz)/magnetic field (mT). Signal not detected.
of main ESR signal following irradiation. The side peaks, emerged at 5 kGy-treated samples (except FD samples of BBQ and SWC sauces), increased at 10 kGy irradiation. The dose-dependent results were most prominent in WAE samples of tomato ketchup. The dose-dependent increase in radiation-induced cellulose radical signals, analyzed after different sample pretreatments, was also reported by de Jesus, Rossi, and Lopes (2000) in the flesh of irradiated vegetables.
4. Conclusion The alcoholic-extraction of irradiated sauces showed good applicability of ESR technique as compared to the freeze-drying sample pretreatments. Alcoholic-extraction method showed an improvement in ESR spectral features, particularly in signal intensity, using an easy water-washing step. The spectra associated with radiation-induced cellulose radicals, with two side peaks of g
K. Akram et al. / Food Chemistry 138 (2013) 1878–1883 Table 2 ESR signal ratios (intensity) of irradiated sauces after different sample pretreatments. Sample
Dose (kGy)
KTP
5
10
BBQ
5
10
SWC
5
10
SPT
5
10
a
Treatmenta
Signal ratio (gn intensity/g1 intensity) Left (g1)
Centre (g0)
Right (g2)
FD AE WAE FD AE WAE
1.0 1.0 1.0 1.0 1.0 1.0
3.9 12.9 10.7 4.0 10.4 8.4
1.2 0.9 0.9 1.2 1.0 1.0
FD AE WAE FD AE WAE
– 1.0 1.0 1.0 1.0 1.0
– 15.9 9.5 2.1 5.4 4.3
– 0.9 0.9 1.0 1.0 1.0
FD AE WAE FD AE WAE
– 1.0 1.0 1.0 1.0 1.0
– 19.3 20.4 4.2 17.7 18.9
– 1.4 1.1 1.0 1.1 1.2
FD AE WAE FD AE WAE
1.0 1.0 1.0 1.0 1.0 1.0
4.0 18.7 23.4 9.2 16.2 20.8
1.1 1.4 1.0 1.4 1.2 1.1
Same as mentioned for Table 2.
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