Gamma-irradiated (0, 1, 5 and 10 kGy) spaghetti sauce samples were identified using photostimulated luminescence (PSL), thermoluminescence (TL) and electron spin resonance (ESR) techniques. PSL technique was used as a screening method for irradiated sauce samples, where the improved results of PSL method were observed for the freeze-dried and alcoholic-extracted samples. TL technique, through the density separation step of silicate minerals from irradiated samples, gave specific shape, intensity and occurrence of TL glow curve in a typical temperature range as well as TL ratio (TL 1/TL 2) to identify the irradiation treatment. The ESR method employed for the freeze-dried samples, showed radiation-specific cellulose signals for 5 and 10 kGy-treated samples, and the results were comparable with oven-dried samples. In general, TL technique was found the most sensitive and reliable for the identification of irradiated spaghetti sauces.
The safety of food irradiation is well documented; however, this technique lacks international consensus for general applicability. Validated identification methods have prime importance for the application of different regulations regarding the international trade of irradiated food. This study comprehensively investigated the potential of different available techniques for the identification of irradiated sauce samples. Liquid samples were treated with different modified methods to find the improved identification results. The presented results may be useful for different regulatory authorities to identify or monitor irradiated spaghetti sauces.
Different kinds of sauces are being used around the globe to enhance the taste of various food products (Martínez-Padilla and Rivera-Vargas 2006; Rengsutthi and Charoenrein 2011). Sauces are the liquid products containing different spices and having desired flow ability and consistency to improve appearance, flavor, and texture of the food (Martínez-Padilla and Rivera-Vargas 2006). With their various and extensive use, the hygienic safety has a key importance for processers and health authorities. Various studies demonstrated the effectiveness of irradiation to address the problem in different sauces associated with microbial contamination (Lee et al. 2001; Song et al. 2001; Jo et al. 2003). The major international health organizations have accepted the potential of this technology to ensure the hygienic quality of food with minimum nutritional losses (Farkas and Mohácsi-Farkas 2011).
Despite the safety assurance of food irradiation at any dose level by leading health authorities (WHO 1999), there are different national and international regulations regarding food item categories and applied doses. The labeling of irradiated food is also mandatory to safeguard the consumer's right of choice (Arvanitoyannis 2010). In this case, the commercialization of this technology needs effective and reliable identification methods in compliance with the regulations. Luminescence techniques, such as photostimulated luminescence (PSL) and thermoluminescence (TL), and electron spin resonance (ESR) are being used extensively as physical detection methods to serve the purpose (Chauhan et al. 2009).
All available methods have some limitations in terms of their specific application range, product to product variation, complex food matrix and low concentration of radiation-induced markers (Delincée 1998). Luminescence techniques (PSL and TL) mainly depend upon the mineral part of the product that is usually not inherent to food itself and varies greatly with respect to food type and its origin. PSL is an easy technique and requires no sample pretreatment generally but can only be used as a screening test and needs TL or ESR analysis to confirm the results (EN 1788 2001; EN 13751 2009). ESR technique can be applied to solid foods containing cellulose, sugar and bone. For food items with high moisture, effective drying methods are required with least/no effect on ESR spectrum, which is sensitive to adverse temperature conditions (Kikuchi et al. 2011). In this regard, extensive research on different food products is needed to ascertain the effectiveness and reliability of the method.
The objective of the present study was to check the effectiveness and reliability of luminescence (PSL and TL) and ESR techniques in detecting irradiated spaghetti sauce. Different sample pretreatments were also tried for PSL and ESR analyses to improve their results. The scope of EN protocols was also extended by investigating their potential applications to irradiated liquid food products.
Materials and Methods
Samples and Irradiation
Spaghetti sauce (Ottogi Co., Ltd. Anyang, Gyeonggi, South Korea) was purchased from a local market in Daegu, Korea and stored at 5C. The samples were irradiated (0, 1, 5 and 10 kGy) using a Co-60 gamma-ray source (AECL, IR-79, MDS Nordion International Co. Ltd., Ottawa, Canada) at the Korean Atomic Energy Research Institute (KAERI) in Jeongeup, Korea. Dosimetry was performed to confirm the required absorbed dose using alanine dosimeters of 5 mm diameter (Bruker Instruments, Rheinstetten, Germany), and Bruker EMS 104 EPR analyzer (Bruker Instruments) was used to measure the free-radical signals.
PSL measurements were employed using a PPSL system (serial; 0021, SURRC; Scottish Universities Research and Reactor Center, Glasgow, U.K.). Three different types of samples were used for PSL measurement:
- Liquid sample without any treatment
- Solid sample after freeze drying (Bondiro, Ilsin Bio Base, Yangju, Kyunggi-do, Korea)
- Solid sample after alcoholic extraction of moisture as described by De Jesus et al. (1999).
The sample (about 2 g) was deposited as a uniform thick layer in disposable Petri dish (50 mm diameter; Bibby sterilin type 122) and was measured in the sample chamber. The radiation-induced photon counts (PPSL signal) emitting per second from the irradiated samples were automatically accumulated by a personal computer up to 60s. The method, EN 13751 (2009), was used for measurement of photon counts (PCs) and interpretation of results. The PCs less than 700/60s were considered as negative (nonirradiated) and more than 5,000/60s were considered as positive (irradiated). The value between these two limits was reported as intermediate. The samples were measured in triplicate (n = 3) under the same laboratory and instrumental conditions and mean values (±SD) were reported.
The silicate minerals for TL analysis were separated from the samples (100 g) by density separation method, and TL discs were prepared as described in the EN 1788 protocol (2001). The separated minerals were deposited on clean stainless steel discs and kept overnight at 50C in a laboratory oven. TL measurements were performed using a TL reader (Harshaw 4500, Thermo Fisher Scientific Inc. Waltham, MA) at 50–400C temperature range with a heating ramp of 6C/s. A first measurement was carried out on the extracted minerals to get the first glow curve, TL1. To normalize the TL response, the samples were reirradiated at 1 kGy using the same source. The discs were again stored overnight at 50C and measured to get the second glow curve, TL2. Full-process blanks were prepared to calculate minimum detectable level (MDL), and the samples with a TL2 glow curve intensity 10 times more than the MDL were selected for the TL ratio calculation. TL1 shape, intensity and TL ratio (TL1/TL2) of the glow curve integral evaluated over the temperature range of 150–250C were used for the interpretation of results in accordance with EN 1788 (2001).
In ESR analysis of irradiated food, the food material with high moisture content must be dried before measurement. In this regard, freeze drying (Yordanov et al. 2006) and oven drying (Jo and Kwon 2006) methods were employed as pretreatments of the liquid sauce samples.
Approximately 0.1 g of the dried sample was placed in a quartz ESR tube (5 mm dia.). European standards EN 1787 (2000) was applied to measure ESR signals targeting the radiation induced-cellulose signals. X-band ESR spectrometer (JES-TE 300, Jeol Co., Tokyo, Japan) was used at room temperature under the following conditions: power, 0.4–5 mW; frequency 9.18–9.21 GHz; center field, 327 ± 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. Measurements were performed three times (n = 3), and mean values (±SD) were reported. The results were analyzed using Microsoft excel (Microsoft Office 2007 version) and Origin 8 software.
Results and Discussion
The PSL technique, using different sample pretreatments, was employed as a screening test; and the results are presented in Table 1. In case of PSL measurement of liquid samples, the nonirradiated control gave negative (<700 PCs), 1 and 5 kGy gave intermediate (700–5,000 PCs), while 10 kGy irradiated sample gave positive result (>5,000 PCs). A significant improvement was obvious in solid samples, where freeze-dried samples showed the best PSL count. In freeze-dried samples, the control was negative, while all irradiated samples were found to be positive. PSL is a rapid technique when used as screening test, but freeze drying may take several days. In this case, the sample extracted with alcohol may be a good choice. Results from alcoholic-extracted samples were much better than liquid samples and comparable with freeze-dried samples, where all irradiated samples were positive except 1 kGy-irradiated sample that gave an intermediated result. The silicate minerals present in the food samples are responsible for their luminescence characteristics. The dried samples got enriched with the mineral fraction providing relative better PSL count. Many scientists effectively used PSL as a screening test for different food products, especially spices, dried fruits and dried vegetables (Sanderson et al. 2003; Bayram and Delincée 2004).
|Sample state||Irradiation dose (kGy)|
|Liquid||260.3 ± 57.1a (−)b||1,445.5 ± 428.5 (M)||3,258.0 ± 1,345.2 (M)||5,441.7 ± 2,096 (+)|
|Freeze dried||219.0 ± 9.9 (−)||6,960.5 ± 132.2 (+)||17,357.0 ± 5,331.5 (+)||19,588 ± 8,666.3 (+)|
|Alcoholic extracted||515 ± 42 (−)||2,955 ± 165 (M)||7,945 ± 196 (+)||17,992 ± 5,139 (+)|
In identification of irradiated food, TL technique is considered as the most promising confirmatory method to discriminate irradiated from nonirradiated samples, which could be applied to any food item with traces of silicate minerals (EN 1788 2001). All irradiated samples showed TL glow curve of high intensity in the temperature range of 150–250C (Fig. 1), whereas the TL glow curve of low intensity was found after 300C, proving only geological signals in separated minerals from nonirradiated samples. The nature (quartz, feldspar, carbonates, etc.) and the quantity of minerals on TL discs have significant impact on TL signals (Soika and Delincée 2000). Therefore, the European standard recommends the normalization step to confirm the suitability of TL techniques for the minerals on the discs. TL2 was measured after reirradiation (1 kGy) of the same TL discs having measured (TL1) samples to check suitability in terms of quality and quantity of separated minerals on the discs. TL ratios (Table 2) of all irradiated samples were more than 0.1, whereas nonirradiated samples showed TL ratios less than 0.1. This confirmed the suitability of TL method for clear identification of studied samples. Our results were in good agreement with the TL results reported for different food products especially spices, dried fruits and dried vegetables (Chauhan et al. 2009).
|Irradiation dose (kGy)||TL parameters|
|TL1 (nC)||TL2 (nC)||TL ratio|
For ESR analysis, two types of sample pretreatments (freeze drying and oven drying) were employed. Nonirradiated samples in both cases showed a single central peak at g = 2.004, which originated from semiquinone radicals (Raffi et al. 2000; Calucci et al. 2003) and reported from different plant materials (Raffi and Agnel 1989; Tabner and Tabner 1991, 1993). The intensity of this signal was lower in the oven-dried sample than in the freeze-dried sample.
Upon irradiation, an increase in intensity of the central signal was found (Figs. 2 and 3) in both types of dried samples. The overall signal intensity was lower in the oven-dried samples, which might be due to the use of high temperature (Yordanov and Aleksieva 2009). Irradiation also induced two side peaks (g = 2.0207 and g = 1.9854), which were clear in 5 and 10 kGy-irradiated samples. These radiation-induced side peaks were equally spaced at about ±3 mT from the central signal and were ascribed to radiation-induced cellulose radicals (Raffi et al. 2000). These side peaks were not clear in the 1 kGy-irradiated samples of both treatments. Mn2+ signals were also found, making the ESR spectra difficult to analyze especially at low dose (1 kGy). A similar problem was also reported by Kikuchi et al. (2010) in ESR analysis of irradiated mango. The distance between radiation-induced two-side peaks (g1–g2 = 6.002 ± 0.002 mT) and g values of side and central signals (g0 = 2.0062 ± 0.0002, g1 = 2.0245 ± 0.0001 and g2 = 1.9863 ± 0.0001) did not vary significantly with respect to different drying methods. De Jesus et al. (1999) also reported that different sample pretreatments had negligible effect on g values and mutual distance of main and side peaks in fruit pulp samples.
The PSL analysis using freeze-dried or alcoholic-extracted samples proved better alternative to the liquid state of sample. The sample drying without use of high temperature could enrich the silicate minerals in the sample, resulting in better PSL count. TL technique could provide a clear discrimination on the basis of TL glow curve shape, intensity, temperature range of TL peak maxima and TL ratio (TL1/TL2). The ESR method employed using the freeze-dried or oven-dried samples showed radiation-specific cellulose signals in 5 and 10 kGy-irradiated samples, where signal intensity was about twice in case of freeze-dried samples. Oven drying provided a time-efficient approach for the ESR analysis; however, the ESR technique was inappropriate to detect radiation-induced markers in 1 kGy-irradiated sample. The studied physical techniques have potential to identify irradiation history of the spaghetti sauce.
This research was supported by the National Research Foundation (NRF) of Korea in 2012.