and γ-Irradiated Frozen Crushed Garlic - American Chemical Society

Jul 15, 2014 - ... Technology and Research Group on Food Safety Control against Climate Change, Chung-Ang ... The garlic samples produced in Korea and...
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Physical‑, Chemical‑, and Microbiological-Based Identification of Electron Beam- and γ‑Irradiated Frozen Crushed Garlic Hyo-Young Kim,† Jae-Jun Ahn,† Hafiz Muhammad Shahbaz,† Ki-Hwan Park,§ and Joong-Ho Kwon*,† †

School of Food Science and Biotechnology, Kyungpook National University, Daegu 702-701, Korea School of Food Science and Technology and Research Group on Food Safety Control against Climate Change, Chung-Ang University, Ansung 456-756, Korea

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ABSTRACT: Identification of frozen crushed garlic, commercially available in the Korean market, was performed using four different analytical techniques (three screening and one confirmation). The garlic samples produced in Korea and China were irradiated (electron-beam and γ-rays) at 0, 1, 4, and 7 kGy. Non-irradiated samples showed a relatively moderate population of aerobic bacteria and yeasts/molds around 105 CFU/g. Irradiation treatments unequivocally reduced the microbial/fungal populations with dose increments. Microbiological screening through direct epifluorescent filter technique/aerobic plate count (DEFT/APC) method effectively differentiated the non-irradiated and irradiated samples. An electronic nose method positively differentiated the odor patterns of samples based on chemical sensing. However, photostimulated luminescence technique (PSL) exhibited poor sensitivity. Minerals separated from irradiated samples produced thermoluminescence (TL) glow curves in the specific temperature range of 150−250 °C. In conclusion, TL confirmatory analysis gave the most promising results in detecting the irradiation status of garlic samples irrespective of the production origin and type of ionizing radiation treatment. KEYWORDS: frozen crushed garlic, electron beam- and γ-irradiation, DEFT/APC, photostimulated luminescence, thermoluminescence

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INTRODUCTION Food irradiation has been approved as a safe and effective technology for a wide range of specific applications. The microbiological safety of products has gained much attention from researchers, regulators, industry, consumers, and the media due to the widespread incidence of foodborne illnesses. One of the principal uses of food irradiation is to kill the microorganisms that cause spoilage or deterioration of the product.1 There has been a significant advancement in food irradiation processing during the past few decades. This has enhanced the international trade of irradiated foods and the implementation of different regulations relating to the use of this technology across the world. Irradiated foods should be clearly labeled to let consumers freely buy food according to their choice. Detection methods are required to confirm the authenticity of a food product to meet applied regulations. Thus, the demand for reliable methods capable of differentiating between irradiated and non-irradiated food products has increased.2,3 Several detection methods are widely used that identify the radiation-induced minor changes related to physical, chemical, or microbiological food properties.4 However, most of these methods are time-consuming and require expensive equipment. Therefore, cost-effective and time-efficient screening methods have gained popularity to screen heavy food lots with minimal sample preparation. Although these screening techniques may not be able to provide clear judgment, they can contribute by indicating possible irradiation treatment.5,6 The European Committee for Standardization (CEN) has approved the use of PSL (EN 13751)7 and TL (EN 1788)8 as physical detection methods for irradiated food as screening and confirmatory approaches, respectively. In addition, method DEFT/APC (EN 13783)9 has also been recognized as a biological screening © 2014 American Chemical Society

method for irradiation detection of foodstuffs. An electronic nose (E-nose) system can be used as a screening technique based on the analysis of volatile compound profiles of food samples. Garlic (Allium sativum L.) is an important vegetable used in cooking as a spice worldwide. It possesses a variety of biological and pharmacological properties.10 China and South Korea are among the leading garlic-producing countries in the world.11 Garlic is one of the most important vegetable crops used as a spice in South Korea. Recently, the market for fresh ready-touse vegetables has experienced solid growth, stimulated largely by consumer demand, which led to minimally processed prepeeled and crushed garlic products.12 A research paper has cited the application of different analytical methods for the detection of garlic bulbs irradiated mainly to inhibit sprouting.13 The recommended irradiation dose range for the frozen foods is 3−7 kGy for the purpose of preventing foodborne illness by destruction of non-spore-forming pathogenic bacteria.14 To our best knowledge, there has been no attempt to examine the microbiological quality and irradiation identification characteristics of frozen crushed garlic. This study was carried out to investigate the potential of analytical screening techniques such as DEFT/APC, PSL, and E-nose for the identification of the irradiation status of frozen crushed garlic commercially available in the Korean market. The results of screening analyses were further confirmed by a validated TL technique. In addition, the efficacy of electron Received: Revised: Accepted: Published: 7920

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beam- and γ-irradiation on the total aerobic bacteria and yeasts/ molds in garlic samples was also evaluated.

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of the instrument were tested at the start of each set of measurements using a reference light source. The PCs were analyzed with respect to lower and upper threshold limits. The accumulated PCs were classified as “‘negative’” (T1 = 700 counts/60 s), “‘intermediate”’ (between 700 and 5000 counts/60 s), or “‘positive’” (T2 = 5000 counts/60 s).7 PSL measurements were performed under subdued light conditions in accordance with EN 137517 recommendations. Thermoluminescence Analysis. The silicate minerals were separated from the non-irradiated and irradiated garlic samples by a density gradient method. TL measurements were made by a 4500 TL Reader (Harshaw, Erlangen, Germany) equipped with controlledtemperature ramping and automatic reheating facility at the Centre for Instrumentation, Kyungpook National University. The TL glow curves (glow 1) were recorded on both irradiated and non-irradiated samples with a reading time of 80 s and a heating range of 50−400 °C at a linear heating rate of 5 °C/s. The TL oven was purged under a continuous nitrogen flux (2 mL/min) to reduce spurious TL signals. Each sample disk containing tested minerals was subjected to a reirradiation step at a standard dose of 1 kGy for normalization of the glow 1 results. The second TL glow curve (glow 2) was recorded to obtain the integrated TL signal ratios (TL1/TL2). Mineral isolation, sample preparation, and TL measurements were performed according to EN 1788.8 Statistical Analysis. Origin 8.0 (Microcal Software Inc., Northampton, MA, USA) and SAS program (version 9.3, SAS Institute, Cary, NC, USA) were used for data analysis. All analyses were performed in triplicates, and values were reported as the means (±SD).

MATERIALS AND METHODS

Garlic Samples and Irradiation Treatments. Garlic samples were purchased from retail markets in Daegu, South Korea. The samples were harvested in two different regions including Korea and China. The crushed garlics were originally vacuum-packed in polyethylene bags (packaging sizes: Korea, 500 g; and China, 1 kg) and stored at −18 °C. The bag units containing the samples were divided into two equal sets and labeled with the specific irradiation dose/type. Furthermore, the samples were placed in insulated polystyrene boxes and individually carried to the irradiation processing plants. The transportation was done under a frozen state at −20 °C. One set of samples was irradiated under γ-rays 60Co radiation source (100 kCi point source AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, ON, Canada) at the Korea Atomic Energy Research Institute, Jeongeup, South Korea. Likewise, electron beam (e-beam) irradiation was performed on the second set of samples using an ebeam accelerator (Model ELV-4, acceleration voltage of 2.5 MeV, Fujifilm, Tokyo, Japan) stationed at EB-TECH Co., Daejeon, South Korea. The irradiation was performed at room temperature with doses of 0, 1, 4, and 7 kGy. The dose rate was 2.5 kGy/h from both radiation sources. The bags were rotated crosswise while receiving the radiation treatments. A ceric/cerous dosimeter (Harwell, Didcot, UK) was used to verify the absorbed dose in e-beam irradiated samples. On the other hand, an alanine dosimeter (Bruker Instruments, Rheinstetten, Germany) was used to confirm the absorbed dose in γ-irradiated samples. The samples were stored at −18 °C until required for further analysis. All of the planned experiments were replicated three times and completed within 2 weeks. Total Aerobic Bacteria and Yeasts/Molds Count. Garlic samples were analyzed for the total aerobic bacteria and yeasts/ molds count. Briefly, 5 g of the crushed garlic was mixed with 45 mL of sterile peptone water. Serial dilutions in peptonized water were made and plated on plate count agar (Difco Lab, Franklin Lakes, NJ, USA) for the total aerobic bacteria and on potato dextrose agar (Difco Lab), acidified by means of sterile 10% solution of tartaric acid, for yeasts and molds. To count total aerobic bacteria, plates were incubated at 37 °C for 24−48 h. Plate reading and colony count determinations were performed according to the Bacteriological Analytical Manual Online.15 Yeasts and molds colonies were counted 5 days after incubation at 30 °C. Colonies were counted, and the count was described as colony-forming units per gram (CFU/g). All plating was performed in triplicates to ensure accuracy. DEFT/APC Analysis. The DEFT method calculates the total number of contaminating microorganisms in a food sample regardless of viability. On the other hand, the APC method enumerates the number of viable microorganisms that can form colonies on an agar plate. A reduction of viable microbiological counts in irradiated food could be a potential screening parameter.9 The first step includes membrane filtration to capture the microorganisms, followed by its staining with a fluorochrome acridine orange. Then the membrane is rinsed after staining and fixed on a microscope slide for counting with an epifluorescent microscope. The method is quick and full analysis can be completed in 25−30 min. The sample preparation, performance, and calculations were done according to EN 137839 followed by different scientists.16,17 Electronic Nose Analysis. A zNose (Electronic Sensor Technology, Newbury Park, CA, USA) equipped with a surface acoustic wave (SAW) sensor and VaporPrint (Misrosense 4.88) software was used to evaluate the volatile compound profiles of non-irradiated and irradiated garlic. Principal component analysis (PCA) was applied to understand the trend in distribution of the numerical data. Photostimulated Luminescence Analysis. PSL photon counts (PCs) registered in 60 s were measured by a SURRC PPSL irradiated food screening system (SURRC, Glasgow, UK). Samples were distributed into 50 mm diameter Petri dishes (Bibby Sterilin type 122, Glasgow, UK). Sample preparation, handling, and the sensitivity

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RESULTS AND DISCUSSION Effect of Irradiation on Total Aerobic Bacterial and Yeasts/Molds Count. The counts of total bacteria and yeasts/ molds from countable plates are given in Figure 1. No significant differences in aerobic bacterial and yeasts/molds count were observed among two sources of untreated crushed garlics. The non-irradiated samples from China or Korea contained a relatively moderate population of aerobic bacteria, that is, approximately 105 CFU/g. Treatment with irradiation, either e-beam or γ-rays, unequivocally reduced the microbial population with dose increment from 1 to 7 kGy in a logarithmic manner. These results illustrate that the counts of total aerobic bacteria were reduced by 2-logarithmic cycles by a γ-ray dose of 4 kGy. Furthermore, a dose of 7 kGy was enough to reduce the bacterial count to nondetectable levels in locally harvested samples. In contrast, the count of aerobic bacteria was still high (104 CFU/g) in imported garlic samples. No considerable changes in bacterial count were observed in samples from China after the e-beam treatment. In general, the effect of both irradiation treatments on total aerobic bacterial count was almost similar on all examined samples. However, the garlic samples of domestic origin showed a little more sensitivity to the irradiation disinfestation as shown in Figure 1. The total aerobic bacterial values of untreated samples of the present study are equivalent to counts reported by Hong and Kim12 for crushed garlic harvested in Korea. Similarly, the initial yeasts/molds count in untreated samples was about 105 CFU/g as shown in Figure 1B. The yeasts/molds count decreased significantly with increasing irradiation dose. Irradiation treatment at 4 kGy effectively reduced the plate count of yeasts/molds to about 102 CFU/g in both types of samples. Further γ-ray application at 5 kGy dose completely decontaminated the locally produced garlic samples. On the other hand, e-beam irradiation succeeded in reducing the fungal count only by 1 log CFU/g by using a higher dose of 7 kGy. Therefore, the effect of γ-irradiation was more pronounced on 7921

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irradiation implies that the higher the initial population of spoilage microorganism, the higher the population will be after the irradiation treatment depending upon several other factors. Furthermore, and of course, if spoilage has already initiated, radiation processing can do nothing to reverse it.1 Screening Analyses. The garlic samples were screened for their irradiation status using three different types of screening techniques based on microbiological, chemical, and physical principles. Microbial Population Characterization by DEFT/APC Analysis. Prior to decontamination treatment, the microbial counts measured by DEFT and APC were of similar order due to the presence of microbes in the samples. However, there is a significant reduction in the APC count as compared to the DEFT count after the application of a decontamination treatment such as irradiation.10,16 The DEFT and APC values of non-irradiated and irradiated garlic samples are shown in Figure 2. Moderate APC count

Figure 1. Total aerobic bacteria (A) and yeasts/molds (B) counts of electron beam- and γ-irradiated frozen crushed garlic produced in Korea and China.

Figure 2. Logarithmic microbial counts of non-irradiated and electron beam- and γ-irradiated frozen crushed garlic produced in Korea and China.

the yeast and mold loads in all samples corresponding to total aerobic count. Comparable reductions in the microflora resulting from ionizing radiations were reported by Pezzutti et al.18 for dehydrated garlic sold in the Argentinean retail market. Irradiation exposure between 5 and 10 kGy reduced the microbial load to acceptable levels in dehydrated garlic. Microbial contamination of the vegetables may occur during production, harvesting, and distribution processes. In addition, the degree of contamination depends on the environmental conditions from growing to marketing and the physiological condition of the product.19 The microbiological results in the present study indicated that the frozen crushed garlic samples available in the Korean market from both sources were hygienically safe for human consumption. The count of the total aerobic bacteria in frozen processed foods should not exceed 106 CFU/g according to the microbiological specifications established by the Ministry of Food and Drug Safety, South Korea.20 It is worth noting that garlic, either freeze-dried or fresh-crushed, also has high antibacterial properties.21 A review of different studies confirmed the presence of potential foodborne pathogens such as Salmonella, Escherichia coli, Clostridium perfringens, Bacillus cereus, and toxigenic molds in powdered garlic grown/processed in different countries.22 Additionally, a given irradiation dose will be able to destroy a certain proportion of the microbial population regardless of the number of microorganisms present. This characteristic of

(>105 CFU/g) was detected in the untreated samples from both sources. The DEFT count remained almost unchanged despite the irradiation treatment. On the other hand, APC values (mesophilic bacteria) progressively decreased with the dose increment either by e-beam or γ-ray processing. There was a clear reduction of mesophilic bacterial population (viable microorganisms) to the level of 102 CFU/g on exposure to 4 kGy γ-irradiation. The log DEFT/APC ratio was determined for each sample. The log DEFT/APC ratio of garlic samples increased with the gradual increase in the irradiation dose. For example, the log DEFT/APC ratio of non-irradiated sample from China was 1.05, whereas it was 1.27 for 1 kGy irradiated samples. In general, the log DEFT/APC ratio was 1.2 logarithmic units. Therefore, DEFT/APC analysis provided clear separation of the non-irradiated and irradiated garlic samples of both origins. On the basis of the present results, it is suggested that a DEFT/APC ratio of 1.2 logarithmic units could be used as a standard for the screening of frozen crushed garlic samples available in the Korean market as non-irradiated or irradiated. Oh et al.23 successfully evaluated the applicability of the DEFT/APC method for the screening of irradiated spices produced in Korea. Other studies showed that a clear screening 7922

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of the different irradiated (0−10 kGy) spices is possible by studying changes in microbial profile employing DEFT/APC methods.17 Previously, the DEFT/APC method gave satisfactory screening results to indicate irradiation treatment of minimally processed vegetables. In general, a log DEFT/APC difference of around 2.0 is required for the useful application of this technique. However, the main drawback of the DEFT/ APC method is that it not specific to irradiation treatment.16 E-Nose Screening Analysis. Irradiation can alter the odor profile of a food. Gardner and Bartlett24 initially reported that the volatile profiles of samples based on the odor patterns are more useful indicators rather than the specific volatile compounds during E-nose analysis. The differences and similarities of odor patterns among different samples have been represented by the distinct fingerprint of the volatile profile.25 Figure 3 shows the PCA of irradiated garlic samples using Enose measurement. A prominent screening of the irradiated samples was observed depending on the irradiation dose. It was clear that there were large differences in odor patterns between irradiated and non-irradiated garlics. In the case of the irradiated samples, different electronic fingerprints were observed in comparison with the non-irradiated ones, establishing the possibility of detection of the irradiated samples. PCA helped further to understand the trend in distribution of the numerical data.26 Published studies have reported the use of Enose analysis for irradiated spices,17 fresh mushrooms,26 and anchovy sauce.25 Radiation-induced changes in odor pattern on food cannot be considered as radiation-specific detection markers, and the results need to be confirmed by a validated technique.17 Photoluminescence Screening Analysis. The development of a PSL novel screening has been aimed at resolving the practical limitations of laborious mineral separation in the TL method. Many foods are contaminated with mineral debris, which can store energy in charge carriers at structural, interstitial, or impurity sites upon irradiation. The PSL technique employs light rather that heat as a stimulus for liberating the trapped energy induced by radiation in solid materials. However, false-positive and false-negative results may be observed on the basis of the quality and quantity of the minerals present on the surface of the food sample.7 All non-irradiated garlic samples gave PCs in the negative PSL range [5000 PCs), thereby easily screening them out as irradiated. Furthermore, the PSL response was linear with the absorbed irradiation doses for imported samples (Table 1). On the contrary, the irradiated garlic samples from Korea exhibited a rather unusual behavior. Table 1 shows either intermediate or false-negative PSL counts were recorded for γ- or e-beamtreated domestic samples. The PCs of 1 and 4 kGy γ-irradiated garlics from Korea were 625.5 ± 306.3 and 1511.6 ± 923.4, respectively. According to European standard protocol (EN 13751),7 negative calibrated PSL results are indicative of insufficient PSL sensitivity. Therefore, the varying results from our data emphasize the application of PSL as a screening analysis. This behavior was in agreement with that reported by Cutrubinis et al.13 for the PSL screening analysis of γ-irradiated (25, 50, and 100 Gy) garlic bulbs collected from the Brazilian,

Figure 3. Principal component analysis of E-nose determinations on non-irradiated and electron beam- and γ-irradiated frozen crushed garlic produced in Korea and China.

German, and Romanian markets. The PSL technique showed limitations as PCs values were found below the lower threshold limit, possibly due to the low dose or irradiation. PSL sensitivity relies on the quantity and type of minerals within the individual sample.7 Therefore, the insufficient PSL screening in domestic garlic bulbs can be ascribed to the presence of a low amount of silicate minerals. Ahn et al.27 studied how different light conditions (natural light, artificial light, and dark room) can affect the PCs values for irradiated garlic bulbs during storage. In general, the PCs of all garlic samples decreased as storage time proceeded. The control garlic samples showed intermediate PSL PCs at day 0 7923

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Table 1. Photostimulated Luminescence Properties of Electron Beam- and γ-Irradiated Frozen Crushed Garlic Produced in China and Korea photon count/60 sa radiation source

dose (kGy)

electron beam

0 1 4 7

garlic sample produced in China 329.1 11603.2 3105.0 17412.9

± ± ± ±

122.7 (−)b bx 18512.2 (+) abxy 786.2 (M) abxy 23747.8 (+) axy

γ-ray

0 1 4 7

327.1 13840.3 14124.8 35612.3

± ± ± ±

106.4 (−) bx 17732.8 (+) bx 4281.1 (+) bx 25533.9 (+) ax

garlic sample produced in Korea 251.5 395.4 345.9 466.8

± ± ± ±

118.3 116.4 149.6 226.7

283.6 625.5 1511.6 2488.5

± ± ± ±

95.6 (−) cx 306.3 (−) cy 923.4 (M) by 1291.1 (M) ay

(−) (−) (−) (−)

bx abxy abxy axy

Mean ± SD (n = 10). Values within the same row with different letters (a−c) are significantly different at p < 0.05. Values within the same column with different letters (x, y) are significantly different at p < 0.05. bThreshold value: T1 = 700, T2 = 5000, (−) T2. a

Figure 4. Thermoluminescence glow curves of minerals separated from electron beam- and γ-irradiated frozen crushed garlic produced in Korea and China.

localized states within the band gap. The physical process of TL is based on the emission of light photon upon recombination of the opposite charge carriers when stimulated by thermal means. The emitted light is detected by a sensitive photon counter and amplified using a photomultiplier tube. The light intensity is finally recorded as a function of temperature to construct the TL glow curve.4,8 All of the irradiated garlics from different origins of production were reliably detected by the TL method. The TL spectra of minerals from garlic samples produced in China and Korea are shown in Figure 4. Minerals separated from irradiated samples produced TL glow curves in the recommended temperature interval between 150 and 250 °C. The TL glow curves from irradiated samples peaked at approximately 180 °C with high intensity. Furthermore, there were no interference signals at around 180 °C. The glow curve structure was very clear with well-defined peaks. In addition,

that decreased upon different storage conditions. In another study, Ahn et al.17 found unsatisfactory PSL screening values during the analysis of irradiated powder samples of different spices and fresh paprika. The presence of a low quantity of the minerals in food samples and insensitivity to light stimulation could lead to poor PSL output. Despite varying results, PSL is a versatile technology to get a valuable indication about the irradiation status of various food stuffs. However, there is a need to confirm the results obtained from the PSL method using a more validated technique that can ensure the irradiation status of the suspected samples.5,7 Confirmation Analysis. Thermoluminescence Characteristics of Isolated Minerals. The TL technique offers the possibility of a very accurate identification of non-irradiated and irradiated inorganic dust, that is, quartz or feldspar. During irradiation, free charge carriers, namely, electrons and holes, are produced in the crystals, which may be trapped by 7924

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different irradiated spices. The authenticity of TL for the identification of irradiated foods has been confirmed by various studies.3,4,6,30 In conclusion, the frozen crushed garlics of different origins of production available in the Korean market were hygienically safe according to the set microbiological quality standards. In general, γ-irradiation resulted in greater reduction in total bacterial or yeasts/molds count in garlics regardless of their origin of harvest. Therefore, irradiation processing may satisfy the demand of consumers for high-quality minimally processed garlic.

the TL intensity was proportional to dose level and the origin of cultivation. Garlic from China comparatively produced TL glow curves with high intensity at all irradiation doses. Thus, the difference in intensity may be related to the mineral composition. All of the non-irradiated garlics produced a residual geologically derived TL signal (natural TL) of maximum peak at approximately 300 °C or above with low intensity regardless of the area of production (Figure 4). Normalization of TL glow curves through a re-irradiation step with a standard dose of 1 kGy can greatly improve the reliability of TL results. The TL glow ratio (Tl1/TL2) was calculated for all samples as shown in Figure 5. The TL ratio

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AUTHOR INFORMATION

Corresponding Author

*((J.-H.K.) Phone: +82 53 950 5775. Fax: +82 53 950 6772. Email: [email protected]. Funding

This research was supported in 2013 by a grant (10162KFDA995) from the Ministry of Food and Drug Safety, Republic of Korea. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) WHO. Practical applications of food irradiation. In Food Irradiation − A Technique for Preserving and Improving the Safety of Food; WHO/FAO: Geneva, Switzerland, 1988; pp 33−43. (2) Molins, R. A.; Motarjemi, Y.; Käferstein, F. K. Irradiation: a critical control point in ensuring the microbiological safety of raw foods. Food Control 2001, 12, 347−356. (3) Arvanitoyannis, I. S.; Tserkezou, P. Legislation on Food Irradiation. In Irradiation of Food Commodities: Techniques, Applications, Detection, Legislation, Safety and Consumer Opinion, 1st ed.; Arvanitoyannis, I. S., Ed.; Academic Press: London, UK, 2010; pp 673−698. (4) Chauhan, S. K.; Kumar, R.; Nadanasabapathy, S.; Bawa, A. S. Detection methods for irradiated foods. Compr. Rev. Food Sci. Food Saf. 2009, 8, 4−16. (5) Shahbaz, H. M.; Ahn, J. J.; Akram, K.; Kwon, J. H. Screening methods for the identification of irradiated foods: a review. Curr. Res. Agric. Life Sci. 2013, 31, 1−10. (6) Akram, K.; Ahn, J. J.; Kwon, J. H. Analytical methods for the identification of irradiated foods. In Ionizing Radiation: Applications, Sources and Biological Effects; Belotserkovsky, E., Ostaltsov, Z., Eds.; Nova Science Publishers: New York, 2012; pp 1−36. (7) EN13751. Foodstuffs−Detection of irradiated food using photostimulated luminescence; European Committee of Standardization (CEN), Brussels, Belgium, 2009. (8) EN1788. Foodstuffs−Thermoluminescence detection of irradiated food from which silicate minerals can be isolated; European Committee of Standardization (CEN), Brussels, Belgium, 2001. (9) EN13783. Foodstuffs−Detection of irradiated food using direct epifluorescent filter technique/aerobic plate count (DEFT/APC); European Committee of Standardization (CEN), Brussels, Belgium, 2001. (10) Corzo-Martinez, M.; Corzo, N.; Villamiel, M. Biological properties of onions and garlic. Trends Food Sci. Technol. 2007, 18, 609−625. (11) FAO. STAT data; http://faostat3.fao.org (accessed July 11, 2014). (12) Hong, S. I.; Kim, D. M. Storage quality of chopped garlic as influenced by organic acids and high-pressure treatment. J. Sci. Food Agric. 2001, 81, 397−403. (13) Cutrubinis, M.; Delincee, H.; Bayram, G.; Villavicencio, A. C. H. Germination test for identification of irradiated garlic. Eur. Food Res. Technol. 2004, 219, 178−183.

Figure 5. TL ratio (TL1/TL2) of inorganic dust minerals separated from electron beam- and γ-irradiated frozen crushed garlic produced in Korea and China.

was less than threshold value of 0.1 for the non-irradiated garlic samples produced in China and Korea. On the other hand, all irradiated samples regardless of their origin and type of irradiation treatment gave a TL ratio of >0.1 that increased with the given dose (Figure 5). Therefore, the clear identification of irradiated samples was guaranteed by the analysis of shape and intensity of the first TL glow curve and TL ratios. Cutrubinis et al.13 reported the limited availability of an amount of silicate minerals from the garlic bulbs. However, TL analysis successfully detected the irradiation status of the garlic bulb samples obtained from three different countries. Recently, Ahn et al.28 investigated the effect of storage time under different light conditions on the TL properties of garlic bulbs cultivated in two different regions of Korea. A typical TL glow curve of high intensity with a maximum peak in the temperature range of 160−185 °C was prominent in all irradiated garlic samples. Additionally, no difference in the shapes of TL glow curves was experienced in all irradiated samples irrespective of the origin of cultivation. However, a significant difference in intensity was recorded relating to the origin of the cultivation. Furthermore, there was a timedependent decrease during 2 years of storage in TL intensities and TL ratios for all irradiated samples. Kim et al.29 applied the TL technique to identify different spice blends containing small amounts of different irradiated (0, 1, and 10 kGy) spice powders, such as red pepper, garlic, or ginger. Upon analysis, the blends containing even the lowest 0.5% irradiated garlic component gave typical TL glow curves that could be interpreted as positive. Ahn et al.17 also discussed the TL method as a reliable confirmatory technique to identify 7925

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Journal of Agricultural and Food Chemistry

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dx.doi.org/10.1021/jf500200r | J. Agric. Food Chem. 2014, 62, 7920−7926