Original papers
Detection of Commercially Irradiated Potatoes by Thermoluminescence and Photostimulated Luminescence Analyses
2014 Volume 20 Issue 3 Pages 555-561
Details
2014 Volume 20 Issue 3 Pages 555-561
Photostimulated luminescence (PSL) and thermoluminescence (TL) analyses were performed to identify commercially-irradiated potatoes. Increased PSL levels were observed in irradiated potatoes, regardless of their origin: the integrated photon counts (PCs) of untreated potatoes grown in 9 different fields ranged between −303 and 9194, whereas those for potatoes irradiated at 50 Gy were 23107 counts or more. Although the PSL from irradiated potatoes declined after exposure to room light, the PSL of ≥ 50-Gy-irradiated potatoes was stable after 4 months of storage in the dark. The integrated net PCs ranged from 41868 to 127226 counts in the commercially-irradiated potatoes (n = 10) after 2 months of storage. These results indicate that PSL analysis of the potato surface is useful for confirming the irradiation history before sale at a retail shop. TL analysis of potato surface soils enabled identification of commercially irradiated potatoes even after exposure to room light.
Inhibiting the sprouting of potato tubers using gamma rays is a major application of food irradiation. A minimum dose of 50 – 150 Gy is sufficient to prevent sprouting of tubers, extend shelf life, and maintain quality (ICGFI 1997). In Japan, the use of gamma rays from cobalt-60 at an absorbed dose of less than 150 Gy for the purpose of inhibiting sprouting was approved under the Japanese Food Sanitation Law in 1972.
Commercial irradiation of potatoes has been performed since 1974. Potatoes are harvested in autumn, irradiated at < 150 Gy at the Shihoro Isotope Irradiation Center in the winter, and shipped to domestic markets in the spring. Retailers sell the potatoes in plastic bags with labels stating that the product has been irradiated. About 6000 metric tons of Japanese potatoes were irradiated in 2010 (Kume and Todoriki, 2013).
Although the labelling of irradiated potatoes has been well implemented in the Japanese market, analytical detection methods to ensure proper labelling should be established to maintain consumer confidence.
Hayashi et al. demonstrated the application of impedance measurements for the detection of irradiated potatoes, including Japanese commercially-irradiated potatoes (Hayashi et al., 1992). In addition, thermoluminescence (TL) analysis of the minerals from the surface soil of the tuber has been recognized as the most promising method; the applicability of TL analysis have been reported for German and Korean varieties of potatoes (Schreiber et al., 1997; Kwon et al., 2002). Nakauma et al. demonstrated the application of TL analysis for Japanese potatoes of different origins and for potatoes commercially irradiated at the Shihoro Isotope Irradiation Center (Nakauma et al., 2004).
Although TL analysis is useful, the process requires complicated mineral separation and is time consuming. In contrast, direct measurement of photostimulated luminescence from the food matrix is a simple and rapid screening technique for identifying herbs and spices (Sanderson et al., 2003a) and shellfish (Sanderson et al., 2003b). The possibility of applying PSL measurements to low-dose-irradiated tubers has been recently reported (Ahn, 2012a; Ahn, 2012b).
In the present study, we examined the applicability of PSL analysis for the rapid screening of commercially-irradiated potatoes in Japan.
Samples Non-irradiated commercial potatoes were purchased from local retail shops at Tsukuba City, Japan. Potatoes cultivated in the following nine regions in Japan were obtained: A: Hokkaido (Memuro), B: Hokkaido (Kucchan 1), C: Hokkaido (Kucchan 2), D: Hokkaido (Shihoro), E: Kagoshima 1, F: Kagoshima 2, G: Hokkaido (Tokoro), H: Aomori, I: Hokkaido.
Commercially-irradiated potatoes treated at the Shihoro Isotope Irradiation Center were purchased through a retailer in Tsukuba. A 10-kilogram commercial carton of potatoes was stored for 2 months at 5°C in the dark before analysis. Other lots of potatoes were obtained directly from the Shihoro Isotope Irradiation Center; tubers were sampled from the centre position or the end position (farther from the radiation source) of the irradiation container (164 cm width × 98 cm depth × 150 cm height). Non-irradiated samples harvested from the same field were used as a control. These samples were also stored at 5°C in the dark.
Gamma-ray irradiation Untreated potato tubers from retail shops were irradiated with gamma rays from a cobalt-60 source (Gammacell 220; MDS Nordion International Co., Ltd., Ottawa, Ontario, Canada) at the National Food Research Institute of Japan. The dose rate was 600 Gy/h. The tubers were irradiated at doses of 50 Gy and 150 Gy at room temperature. An alanine pellet dosimeter (Bruker Biospin Ltd., Rheinstetten, Germany) was attached to the surface of each tuber, and the absorbed dose was determined using an electron spin resonance (ESR) spectrometer (Bruker EMX; Bruker Biospin Ltd.). After irradiation, the potatoes were stored at 5°C in the dark before analysis, unless otherwise stated.
PSL analysis The tubers were cut into slices that fit into a Petri dish (d = 50 mm; h = 15 mm), which was the dedicated holder used for PSL measurements. The skin of the tuber slice faced upwards.
PSL measurements were carried out using a PSL system (ES-7340A, JREC, Ltd., Ibaraki, Japan). The PSL signals from the control and irradiated samples were recorded for 100 s after the initial self-luminescence (background) counts were measured for 10 s. The results were expressed as “integrated net counts” for the recorded 90 s. The background counts of the initial 10 s were extrapolated as the background level of photon counts (PCs) for the subsequent 90 s. Before and after each measurement, an empty Petri dish was tested to ensure that it was free from contamination. All the samples were analysed at 25 ± 1°C and 45 ± 2% humidity. All handling and measurements of the samples were done in a safelight laboratory room following EN 13751 (EN13751 2009) recommendations. All measurements were repeated a minimum of 4 times.
TL analysis The silicate minerals for TL analysis were separated from the surface of the potato tubers according to the procedure described in the EN1788 methods (EN1788 2001).
Whole tubers were placed into a 500 mL beaker and covered with a sufficient volume of distilled water. The solution was sonicated for 5 min and discarded by decantation, after which the mineral-rich fractions remaining at the bottom of the beaker were obtained. The minerals were transferred to a centrifuge tube along with sodium polytungstate (SPT) solution (d = 2 g mL−1) and centrifuged for 2 min at 1000 × g. Then, the minerals were treated with 4 N HCl, followed by purification with acetone. Finally, about 1–2 mg of the minerals was weighed, transferred to a stainless steel disc, and stored for 16 h at 50°C in a laboratory oven for the TL measurements.
TL measurements were performed using a TL reader (QS3500, Bicron, Harshaw, AZ) with computer-controlled temperature ramping and automatic reheat capability. Before analysis, the mineral samples were protected from light exposure. The TL glow curves (Glow 1) were recorded for each sample disc from 70 – 400°C at a heating rate of 6°C s−1, and the TL oven was purged of oxygen using nitrogen at a flow rate of 2 mL min−1. For normalization of the Glow 1 results, each sample was irradiated with a standard dose of 250 Gy and preheated for 16 h in an oven at 50°C. The second TL glow curve (Glow 2) was then recorded under the same conditions as given above. The luminescent intensities, TL1 and TL2, were integrated for Glow 1 and Glow 2, respectively, in the temperature range of 150 – 250°C, and the TL area ratio (TL1/TL2) was calculated for both the irradiated and non-irradiated samples to verify the reliability of the detection results from TL1. All measurements were repeated a minimum of 4 times.
Storage under artificial light The influence of light exposure on the samples was examined. Potato slices in Petri dishes were irradiated at 150 Gy, and then exposed to room light (fluorescent lamp 18W: FL20SSN/18, NEC Co. Ltd, Tokyo, Japan) for a certain period. The samples were positioned at a distance of 50 cm from the lamp. PSL measurements were performed after 2 – 240 h of exposure to the light. For TL analysis, 50- or 150-Gy-irradiated tubers were stored in the dark at 5°C for 2 months, followed by exposure of the whole tubers to the light for 24 h. The minerals were then separated from the tubers.
Statistical analysis Statistical analyses were performed using Microsoft Excel 2007 software (Microsoft Corporation, Redmond, WA, USA). Multiple comparison of Tukey’s test was used to analyse all results. A significance level of p > 0.05 between groups was taken as being not statistically different.
PSL signals from the irradiated potato surface Figure 1 shows the time course of PSL signals for the control and irradiated potato tubers in lot G. The PSL signal from the irradiated samples at 50 Gy and 150 Gy markedly increased immediately after LED excitation, and then diminished with time. The PSL counts were higher in the samples irradiated at 150 Gy than in those irradiated at 50 Gy. In contrast, the PSL signal of the non-irradiated samples (Fig. 1 bottom) was very small; a slight increase in the PCs was observed after LED excitation. Because the potato surfaces were covered with soil, the integrated net PCs of the non-irradiated samples were higher than those of other foodstuffs (Chen et al., 2011); these PSL were attributed to natural environmental radiation. Ahn et al. also reported false positive PSL results (7000 counts/60 sec) for minerals separated from the control potato sample (Ahn 2012a).
Effect of light exposure on PSL signal Figure 2 shows the PSL signals of samples (lot G) irradiated at 150 Gy followed by illumination with an artificial room light for 0, 2, 6, and 12 h. When the potato surfaces were exposed to room light, the intensities of the PSL signals from the irradiated samples decreased in accordance with the exposure time to the light. After 12 h exposure, the PSL response became very small and essentially the same as that of the non-irradiated control. Table 1 summarizes the average integrated net PCs of samples exposed to light for up to 240 h. The mean values of the integrated net PCs significantly decreased as the duration of light exposure increased. The value of the non-irradiated control samples was determined to be 1805 ± 640 (n = 12). Compared with this control value, the integrated net PCs for the samples exposed to light for 12 h or longer were not significantly larger (p > 0.05) according to Welch’s t test. On the contrary, the integrated net PCs of the samples kept in the dark were the same as those of the initial counts (p > 0.05).
Typical PSL signals obtained from potato surfaces irradiated at 0 (control), 50, and 150 Gy after 24 h storage in the dark (top). The bottom is a magnified view of the data for the control sample (bottom).
Potatoes from origin G (Tokoro, Hokkaido) were subjected to irradiation in a laboratory irradiator.
Effect of light illumination on the PSL signals of potatoes irradiated at 150 Gy.
Potato samples in Petri-dishes were irradiated at 150 Gy. The samples were then exposed to light for 0 h (●), 2 h (●), 6 h (●), and 12 h (ο).
| Time (h) | Artificial light | Under darkness |
|---|---|---|
| 0 | 127213 ± 19349 a | 127213 ± 19349 a |
| 2 | 43967 ± 6882 b | 176999 ± 44325 a |
| 6 | 8991 ± 1987 c | 145257 ± 17030 a |
| 12 | 1911 ± 431 c | 137241 ± 19375 a |
| 24 | 1816 ± 222 c | 140204 ± 22724 a |
| 72 | 484 ± 211 c | 124355 ± 43909 a |
| 168 | 342 ± 220 c | 147683 ± 34182 a |
| 240 | 332 ± 225 c | 124736 ± 31367 a |
Mean value ± S.D. (n = 4)
a-c: Values with the same character are not significantly different (p > 0.05).
Average of integrated net PCs for non-irradiated (control) samples was 1805 ± 640 (n = 12).
Table 2 shows the integrated net PCs of tubers irradiated at 50 and 150 Gy during long-term storage in the dark. The values were not significantly different at each dose level (p > 0.05) and remained unchanged from the initial levels (24 h after irradiation) after 4 months of storage in the dark. It was reported that the PCs of irradiated minerals from potato, garlic, and onion measured using a PSL system decreased in light; however, all irradiated samples gave positive PC values even after 2 years, except when stored under natural light (Ahn et al., 2012a; Ahn et al., 2012b). In the present study, we directly measured the soil covering the potato surface without separating the minerals. This protocol is more simple and rapid; however, the decline in PCs due to light would be much larger compared with the previous studies. Identification of the potatoes following irradiation would be difficult after storage under artificial light using the irradiation levels employed in these experiments.
PSL measurements of potatoes from different regions and of commercially-irradiated potatoes Because PSL was observed in non-irradiated samples due to natural environmental radiation, the range of the integrated PCs in non-irradiated control samples was examined using potatoes from 9 regions, along with their response to radiation. Figure 3 shows the results of the integrated net PCs of the 9 lots stored for 24 h in the dark after irradiation. The PCs of non-irradiated samples (open symbols) varied between −303 and 9194, depending on the field in which they were cultivated; these samples provided fewer than 10000 counts in 90 s. Although the sensitivity of the surface soil to radiation varied among the lots, the PSL counts of the irradiated potatoes increased in a dose-dependent manner, irrespective of the region of origin. The minimum PC of the 50-Gy-irradiated samples was 23,107 (lot A), and all the counts from the 50 Gy irradiated samples (solid symbols) exceeded 20000 counts in 90 s. Therefore, no overlap of the PSL counts was observed between the non-irradiated and 50-Gy samples. The integrated net PCs ranged from 41868 to 127226 counts in the commercially-irradiated potatoes (n = 10) after 2 months storage.
In European Standard EN 13751, the lower (T1) and upper (T2) threshold PCs are set for evaluating the irradiation history (EN13751 2009). According to this standard, when we used these threshold values (T1 = 10000 counts and T2 = 20000 counts), the values for all the commercially-irradiated samples were higher than T2 and were categorized as positive, although further analysis for the different original samples is required to establish the validated threshold values.
Normally, potatoes are stored and shipped in dark and cold conditions. It is therefore possible to use PSL analysis to check the irradiation of commercial potatoes before selling them in a market.
| Storage (month) | Control | 50 Gy | 150 Gy |
|---|---|---|---|
| 0* | 1822 ± 716**a | 47753 ± 8568 b | 115907 ± 35928 c |
| 1 | 36114 ± 12800 b | 100897 ± 22092 c | |
| 2 | 34605 ± 5895 b | 121752 ± 25723 c | |
| 3 | 43016 ± 10703 b | 112111 ± 15962 c | |
| 4 | 1789 ± 622 a | 40761 ± 8415 b | 120719 ± 30335 c |
a–c: Values with the same character are not significantly different (p > 0.05).
Integrated PSL PCs of potatoes from different regions and commercially-irradiated potatoes.
Untreated potatoes from 9 regions (district A-E, see Materials and Methods) were laboratory-irradiated at 0 Gy (◊), 50 Gy (●) and 150 Gy (▲). PSL was measured after 24 h of storage in the dark. In this chart, negative values for 0 Gy were omitted. PSL of commercially-irradiated potatoes obtained through a retailer (×) were analysed 2 months after purchase.
TL analysis of commercially-irradiated potatoes at the Shihoro Irradiation Center The Glow1 curves of minerals from non- irradiated potatoes showed small peaks at approximately 300°C, but those of irradiated tubers showed intense peaks at approximately 185°C (data not shown).
Figure 4a shows the distribution of TL glow ratios for potatoes commercially irradiated at the Shihoro Isotope Irradiation Center. The results for samples taken directly from the irradiation container are plotted in the upper panel, along with the results of the control potatoes harvested from the same cultivation field. The glow ratios for potatoes obtained from the retail market are shown in the lower panel (n = 20). It was found that the TL ratio of the non-irradiated samples was approximately 0.01, whereas the TL ratios of the irradiated samples were significantly larger than 0.1. These results meet the evaluation criteria for irradiation defined in EN 1788 and are consistent with previous studies (Kwon et al., 2002; Nakauma et al., 2004; Schreiber et al., 1997).
TL glow ratios of (a) commercially-irradiated and (b) laboratory-irradiated potatoes.
(a) Potatoes obtained directly from the Shihoro Isotope Irradiation Center, control (non-irradiated: Δ), potatoes from the centre position (▲), and potatoes from the end position (▲) of the irradiation container. TL glow ratios for commercially-irradiated potatoes obtained from a retailer are indicated with (×).
(b) Potatoes from origin G (Tokoro, Hokkaido) were irradiated in a laboratory irradiator at doses of 0 Gy (◊), 50 Gy (◆) and 150 Gy (◆). After irradiation, the potatoes were stored for 1 day or 2 months in the dark at room temperature. A portion of the potatoes stored for 2 months were further subjected to light illumination for 24 h.
In the Shihoro Isotope Irradiation Center, a large basket that can hold 1.5 tons of potatoes (98 cm depth for gamma-ray direction × 164 cm width × 150 cm height) was employed as an irradiation container. This basket had been used to hold the harvested potatoes in the cultivation field, and during transportation and storage. Thus, the process load remained unchanged before irradiation, thereby minimizing physical stresses to the potatoes. During irradiation, the basket is turned 180° and irradiated from the other side, ensuring that the contents are exposed to a minimum gamma-ray dose of 0.06 kGy at the centre and a maximum dose of less than 0.15 kGy on the surface of the container (Kameyama and Ito, 2000). Miyahara et al. recently carried out detailed dose mapping and confirmed that the dose distribution within the irradiation container fit the original design concept of the plant (Miyahara et al., 2009).
The TL glow ratios for the irradiated sample taken directly from the Shihoro Irradiation Center showed that samples removed from a central location in the basket provided smaller values than samples removed from an outside position. The glow ratios for irradiated potatoes obtained from retailers were distributed between these boundary values.
Effect of storage and light illumination on the TL glow ratios Figure 4b shows the change in TL glow ratios for laboratory- irradiated potatoes (50 Gy and 150 Gy) during storage. The TL glow ratios after 2 months of storage in the dark did not change significantly from those after 24 h of storage for both irradiation doses. When the potatoes were removed and exposed to room light for 24 h, the TL glow ratios significantly decreased (p > 0.05); however, the glow ratios remained > 0.1, and the Glow 1 peaks were clearly identified between the temperature range 150 – 250°C for 50-Gy-irradiated potatoes. Therefore, it is possible to identify the 50-Gy-irradiated potatoes by TL analysis, even though the PSL signals at the surface fade out after 24 h of light exposure.
The levels of PSL from the surface of non-irradiated potatoes are different depending on the field in which they are cultivated. Increased PSLs were observed in irradiated potatoes, regardless of their origin. PSL signals from irradiated potatoes decayed in a short time after exposure to light. The PSL from potatoes irradiated at 50 Gy or more could be distinguished from that of non-irradiated potatoes after 4 months of storage in the dark at 5°C. Commercially-irradiated potatoes from a retailer can be detected by PSL when stored in the dark. PSL analysis of the potato surface is useful for confirming the irradiation history before sale at a retail shop. TL measurements can detect irradiated potatoes after exposure to room light.
PSL; Photostimulated luminescence, TL; thermoluminescence, PC; Photon Count
