Up-to-date status of food irradiation

The last decade has witnessed significant advancement of the acceptance of food irradiation processing. At present 37 countries have approved one or more food items for human consumption and 25 countries have commercialized this process. More developing countries are showing keen interest to introduce irradiation processing in order to reduce post-harvest food losses, to increase export potentials and to ensure safety of food to their people. Although progress towards acceptance of food irradiation by the industry is slow, actual market trials have shown that once consumers have understood this technology, they are willing to buy irradiated foods.This paper deals with the latest developments in the field of food irradiation with particular reference to legislation, consumer acceptance, commercialization and potential application in developing countries. This paper also deals with the role played by the International Organizations, aimed at facilitating the acceptance of food irradiation.     

Identification of irradiated pulses by thermoluminescence of the contaminating minerals

Thermoluminescence (TL) response of contaminating minerals from six samples of pulses commonly consumed in Pakistan has been studied for identification of irradiation treatment. The samples were irradiated by Co-60 gamma rays at 0.3, 0.5 and 1.0 kGy, or by 10 MeV electrons using an accelerator at 0.75 and 2.2. kGy. Generally, the TL intensity for minerals separated from irradiated samples was higher than for unirradiated samples. To normalize the results, separated minerals deposited on stainless steel discs were re-irradiated by a normalizing dose and TL response was redetermined. The ratio of the area of the first glow curve to the second glow curve was more than 0.8 for all irradiated samples and less than 0.33 for most of the unirradiated samples. For those unirradiated samples where the ratio of the glow curves was more then 0.03, the shapes of the glow curves were compared. Taking this criterion into consideration, all 21 unirradiated and irradiated samples of pulses were identified correctly. Therefore, a normalization procedure by re-irradiation of minerals and analysis of TL glow curve shapes lead to unequivocal identification of radiation treatment of pulses.     

Suitability of the thermoluminescence method for detection of irradiated foods

Irradiated foods can be detected by thermoluminescence (TL) of contaminating minerals. Altogether about 300 lots of herbs, spices, berries, mushrooms and seafood were studied by the TL method. Irradiated herbs and spices were easily differentiated from unirradiated ones two years after irradiation of a 10 kGy dose. The mineral composition of seafood was variable; and while calcite was suitable for the TL analysis, aragonite and smectite gave unreliable results. Control analyses during two years confirmed the reliability of TL method.     

Applicability of the Sunna dosimeter for food irradiation control

The quick development concerning the commercial application of food irradiation in the USA recently resulted in growing marketing of irradiated red meat as well as irradiated fresh and dried fruits. These gamma and electron irradiation technologies require specific dosimetry systems for process control. The new version of the Sunna dosimeter has been characterized in gamma, electron and bremsstrahlung radiation fields by measuring the optically stimulated luminescence (osl) at 530 nm both below and above 1 kGy, i.e. for disinfestation and for meat irradiation purposes. No humidity and no significant dose rate effect on the green osl signal was observed. The temperature coefficient was determined from 0°C up to about 40°C and to stabilize the osl signal after irradiation a heat treatment method was introduced. Based on these investigations the Sunna ‘gamma’ film is a suitable candidate for dose control below and above 1 kGy for food irradiation technologies.     

Potential use of the activation energy value calculated from the thermoluminescence glow curves to detect irradiated food

We herein report on the calculation of the activation energy (E _a) from the thermoluminescence (TL) glow curves performed by the initial rise method that allows us to discriminate between irradiated and non-irradiated sesame seeds. E _a values of natural TL (0.68 ± 0.03 eV) and gamma-induced TL (never lower than 0.82 ± 0.02 eV) appear as a complementary criterion to be used differentiating between irradiated and non-irradiated foodstuffs with the position and the intensity of the main peak of the TL emission. In addition, E _a values taken from irradiated sesame samples at different doses (1, 5 and 10 kGy) and stored up to 15 months after being processed were compared to a ‘positive’ Spanish blend (i.e. at least one component was commercially irradiated).     

Thermoluminescence detection of irradiated shellfish: international interlaboratory trial

An international interlaboratory trial was conducted using thermoluminescence for the detection of irradiated shellfish, aimed at validating the method for routine use. Nephrops norvegicus, mussels, brown shrimps, black tiger prawns, and king scallops were presented as nonirradiated and irradiated to 0.5 and 2.5 kGy. The protocol called for the use of 3 preparation methods: extraction of silicates from whole shellfish by acid hydrolysis and physical separation, and of carbonates from powdered shells. Homogeneity was tested on each product and each treatment. Results verified that all methods were able to distinguish between nonirradiated and irradiated samples regardless of dose. Silicate methods produced better discrimination than powdered shell, and acid hydrolysis showed some evidence of better separation between the 2 doses than the physical method. Participants received each product in each treatment category for blind analysis. Six participants returned results for acid hydrolysis, 7 for physical separation, and 5 for the powdered shell method. Their results confirmed the homogeneity testing. Qualitative results gave 100% correct classification for both silicate methods and 85.3% for powdered shell. Silicate methods are therefore preferable unless only shell is available. Overall, the results confirmed the case for validation.     

Study of effective factors in detection of irradiated food using thermoluminescence based on the models of reference minerals

In the thermoluminescence (TL) detection method for irradiated foods, accurate standards have been developed for detecting irradiated foods. The standard method describes that emission maximum temperature (T1i) and TL ratio for non-heated or non-mixed sample can be in the range of 150–250 °C and more than 0.1, respectively, when it was irradiated food. But when irradiated food is heated up to 200 °C, or mixed up with non-irradiated stuffs, T1i and TL ratio would not drop in the range. Here we examined the effects of the two processes, heating and mixing with non-irradiated food, on T1i and G1/G1k ratio (ratio of G1 and average G1 for 1-kGy-irradiated JF2, this value is modeled after TL ratio) using a model consisting of irradiated and non-irradiated geochemical standards of feldspar (JF1, JF2, PF, etc.). T1i temperatures for irradiated JF1, JF2, and PF ranged from 163 to 175 °C, while those for the non-irradiated JF2 ranged from 253 to 263 °C. T1i temperatures for 5-kGy-irradiated and preheated JF2 for 10 s, 20 s, and 30 s at 180 °C were 215, 225, and 231 °C, respectively.When JF2 was irradiated from 100 Gy to 5 kGy, the T1i was almost constant at any doses. G1/G1k ratios at 100, 200, and 500 Gy were 0.15, 0.23, and 0.60, respectively. G1/G1k ratio was proportional to the given dose at the integration temperature ranges.The TS sample, which originated from farm soil in Tanegashima Island, gave the same results as JF2. T1is for 5-kGy-irradiated and preheated JF2 for 20 s at 150, 180, and 200 °C were 197, 225, and 246 °C, respectively. Longer and higher preheating resulted in higher T1i. Longer and higher preheating extremely reduced the G1/G1k ratio, and in some cases the ratio was less than 0.1. This means TL ratio is useless in determination of the standard for irradiated food.Peak temperatures for JF2 in mixture of 5-kGy-irradiated to non-irradiated (1.25–5%) were 261–263 °C (non-irradiated portion, T1n) and 177–180 °C (irradiated portion T1i). The peak positions are almost the same as those of original components and would not be affected by the mixing ratio. But TL ratio could not be used to determine irradiated food because mixing would reduce it remarkably.Some of the glow curves were simulated by a computer program.In conclusion, T1i/n is a key factor in an irradiated food determination practice for sample containing feldspar, rather than TL ratio.     

Determination of paralytic shellfish toxins in shellfish by receptor binding assay: collaborative study

A collaborative study was conducted on a microplate format receptor binding assay (RBA) for paralytic e shellfish toxins (PST). The assay quantifies the composite PST toxicity in shellfish samples based on the ability of sample extracts to compete with (3)H saxitoxin (STX) diHCl for binding to voltage-gated sodium channels in a rat brain membrane preparation. Quantification of binding can be carried out using either a microplate or traditional scintillation counter; both end points were included in this study. Nine laboratories from six countries completed the study. One laboratory analyzed the samples using the precolumn oxidation HPLC method (AOAC Method 2005.06) to determine the STX congener composition. Three laboratories performed the mouse bioassay (AOAC Method 959.08). The study focused on the ability of the assay to measure the PST toxicity of samples below, near, or slightly above the regulatory limit of 800 (microg STX diHCl equiv./kg). A total of 21 shellfish homogenates were extracted in 0.1 M HCl, and the extracts were analyzed by RBA in three assays on separate days. Samples included naturally contaminated shellfish samples of different species collected from several geographic regions, which contained varying STX congener profiles due to their exposure to different PST-producing dinoflagellate species or differences in toxin metabolism: blue mussel (Mytilus edulis) from the U.S. east and west coasts, California mussel (Mytilus californianus) from the U.S. west coast, chorito mussel (Mytilus chiliensis) from Chile, green mussel (Perna canaliculus) from New Zealand, Atlantic surf clam (Spisula solidissima) from the U.S. east coast, butter clam (Saxidomus gigantea) from the west coast of the United States, almeja clam (Venus antiqua) from Chile, and Atlantic sea scallop (Plactopecten magellanicus) from the U.S. east coast. All samples were provided as whole animal homogenates, except Atlantic sea scallop and green mussel, from which only the hepatopancreas was homogenized. Among the naturally contaminated samples, five were blind duplicates used for calculation of RSDr. The interlaboratory RSDR of the assay for 21 samples tested in nine laboratories was 33.1%, yielding a HorRat value of 2.0. Removal of results for one laboratory that reported systematically low values resulted in an average RSDR of 28.7% and average HorRat value of 1.8. Intralaboratory RSDr based on five blind duplicate samples tested in separate assays, was 25.1%. RSDr obtained by individual laboratories ranged from 11.8 to 34.9%. Laboratories that are routine users of the assay performed better than nonroutine users, with an average RSDr of 17.1%. Recovery of STX from spiked shellfish homogenates was 88.1-93.3%. Correlation with the mouse bioassay yielded a slope of 1.64 and correlation coefficient (r(2)) of 0.84, while correlation with the precolumn oxidation HPLC method yielded a slope of 1.20 and an r(2) of 0.92. When samples were sorted according to increasing toxin concentration (microg STX diHCl equiv./kg) as assessed by the mouse bioassay, the RBA returned no false negatives relative to the 800 microg STX diHCl equiv./kg regulatory limit for shellfish. Currently, no validated methods other than the mouse bioassay directly measure a composite toxic potency for PST in shellfish. The results of this interlaboratory study demonstrate that the RBA is suitable for the routine determination of PST in shellfish in appropriately equipped laboratories.     

Analysis of shellfish by thermoluminescence and X-ray diffraction methods: Knowledge of gamma-ray treatment and mineral characterization

To investigate the detection of irradiated shellfish, seven kinds of shellfish samples were gamma irradiated at 3 and 6 kGy. The first-glow curves for all the control and irradiated shellfish samples were recorded between temperatures of 50 and 400 °C at a heating rate of 5 °C/s. All the samples, the first glow curves (TL₁) of which have been recorded, were re-irradiated at 1 kGy for the normalization of results. Their second glow curves (TL₂) were obtained at the same conditions in order to achieve the thermoluminescence (TL) ratios (TL₁/TL₂). The TL method proactively identified the whole shellfish samples by using the calcite, aragonite and quartz minerals that are present in the shells. The X-ray diffraction spectroscopy has been used as an analytical tool for the characterization of minerals that are present in the shells as inorganic materials along with biomaterial.     

Comparative identification of irradiated herbs by the methods of electron paramagnetic resonance and thermoluminescence

Non irradiated and γ-irradiated dry herbs savoury (Savoury), wild thyme (Thymus serpollorium) and marjoram (Origanum) with absorbed dose of 8 kGy have been investigated by the methods of elecrtron paramagnetic resonance (EPR) and thermoluminescence (TL). Non-irradiated herbs exhibit only one weak siglet EPR signal whereas in irradiated samples its intensity increase and in addition two satelite lines are recorded. This triplet EPR spectrum is attributed to cellulose free radical generated by irradiation. It has been found that upon keeping the samples under the normal stock conditions the life-time of the cellulose free radical in the examined samples is ∼60–80 days. Thus the conclusion has been made that the presence of the EPR signal of cellulose free radical is unambiguous indication that the sample under study has been irradiated but its absence can not be considered as the opposite evidence. In the case when EPR signal was absent the method of TL has been used to give the final decision about the previous radiation treatment of the sample.     

Photostimulated luminescence detection of irradiated shellfish: international interlaboratory trial

An interlaboratory trial was conducted to validate photostimulated luminescence (PSL) detection of irradiated shellfish. Five species of shellfish (Nephrops norvegicus, mussels, black tiger prawns, brown shrimps, and king scallops) were presented blind as nonirradiated and irradiated to 0.5 and 2.5 kGy. Precharacterization analysis of each product and treatment was performed on both whole (including shell) and intestinal samples. The results for whole samples (including shell) confirmed that the method was able to distinguish between nonirradiated and irradiated samples, regardless of dose. Intestinal data have identified that the method is dependent on the quantity and sensitivity of grits present within the intestinal tract, which can be assessed using calibration by normalization to 1 kGy. Five laboratories returned both initial screening and calibrated data and sample classification. All laboratories correctly identified all irradiated products using the screening criteria. There were no false positives. The results confirm the validity of the PSL method for shellfish, which has been adopted as a European standard method and by the Codex Alimentarius Commission. Calibration is required where only intestinal material is available. For whole samples with shell, screening alone is sufficient.     

Identification of irradiation treatment of spices by thermoluminescence of contaminating minerals

Five spices, cumin, coriander, clove, cinnamon and black pepper were irradiated by gamma-ray doses of 1.0 and 5.0 kGy and thermoluminescence (TL) method was used for identification of the irradiation treatment. The TL response of the minerals isolated from irradiated samples was much higher as compared to the mineral particles from unirradiated control samples. For the normalisation of results the separated minerals were reirradiated to a normalisation dose of 1.0 kGy and the TL glow curve was recorded a second time. By comparing the glow curves of irradiated and unirradiated samples, finding the ratio of the areas of first and second glow curves (TL₁/TL₂) and comparing the shapes of the glow curves, all the irradiated and unirradiated samples were identified correctly.     

Determination of brevetoxins in shellfish by the neuroblastoma assay

A neuroblastoma assay for determination of brevetoxins in shellfish was developed together with a method for sample cleanup that allows separation of brevetoxins from most of the components that cause matrix interference in the assay. This improved assay method was applied to a range of shellfish samples with different characteristics. Extracts of naturally contaminated and nontoxic shellfish together with extracts spiked with known amounts of toxin were tested. The results demonstrated that brevetoxins could be reliably detected in shellfish extracts at concentrations below the regulatory limit. Brevetoxin activity was detected in 15 of 23 samples from 5 separate toxicity incidents in which shellfish tested positive in the neurotoxic shellfish poisoning (NSP) mouse bioassay. Twelve of these positive NSP results came from 2 toxicity incidents. Yessotoxin was the major contributor to toxicity in 2 other incidents, although some samples contained both yessotoxin and brevetoxin. The sample from the remaining incident contained an unidentified toxin bioactivity, together with gymnodimine. In contrast to earlier versions of the neuroblastoma assay, gymnodimine was not detected by this modified method.     

Methods for the detection of foodstuffs treated by irradiation

The Community Bureau of Reference - BCR - a research programme on measurements and testing of the European Community has organised a two years concerted action to develop and to validate methods of detection of foodstuffs treated by irradiation. The work was carried out by 50 scientists and was coordinated by 9 specialists in physics, chemistry, biology and microbiology. The results of this collaboration were that 3 methods (ESR, TL, ACP/DEFT) are validated and 2 methods (chemical, DNA) are to be validated.     

Suitability of sterin dose indicators for control of certain food irradiation processes

Sterin indicator, a new label dosimeter was evaluated for reliability and suitability as a device for discriminating and monitoring radiation treatment. Two versions, Sterin-125 and Sterin-300 were irradiated with gamma rays at dose ranges of 75–175 Gy and 200–400 Gy respectively. The irradiated samples were evaluated subjectively using Multiple Comparison Difference technique and their stability was tested under dark and differing light and temperature conditions. The results showed that these Sterin labels are generally reliable and useful but are affected by doses lower than designated threshold and by extended exposure to light sources.     

Advanced protocol for the detection of irradiated food by electron spin resonance spectroscopy

Using ESR (electron spin resonance) spectroscopy, we found various free radicals in a pepper before and after irradiation. The representative ESR spectrum of the pepper composed of a sextet centered at g=2.0, a singlet at the same g-value and a singlet at g=4.0. This reflects the evidence of three independent radicals in the pepper before irradiation. Upon gamma ray irradiation, a new pair of signals appeared. The progressive saturation behavior (PSB) at various microwave power levels indicates quite different relaxation behaviors of those signals. For the evaluation of radiation-induced radicals and irradiation effects we propose a new protocol using the PSB method. This would call for an advanced protocol for the detection of irradiated foods.     

Capillary electrophoresis for the analysis of paralytic shellfish poisoning toxins in shellfish: Comparison of detection methods

Paralytic shellfish toxins (PSTs) are produced by marine and freshwater microalgae and accumulate in shellfish including mussels, oysters, and scallops, causing possible fatalities when inadvertently consumed. Monitoring of PST content of shellfish is therefore important for food safety, with currently approved methods based on HPLC, using pre‐ or postcolumn oxidation for fluorescence detection (HPLC‐FLD). CE is an attractive alternative for screening and detection of PSTs as it is compatible with miniaturization and could be implemented in portable instrumentation for on‐site monitoring. In this study, CE methods were developed for C⁴D, FLD, UV absorption detection, and MS—making this first report of C⁴D and FLD for PSTs detection. Because most oxidized toxins are neutral, MEKC was used in combination with FLD. The developed CZE‐UV and CZE‐C⁴D methods provide better resolution, selectivity, and separation efficiency compared to CZE‐MS and MEKC‐FLD. The sensitivity of the CZE‐C⁴D and MEKC‐FLD methods was superior to UV and MS, with LOD values ranging from 140 to 715 ng/mL for CZE‐C⁴D and 60.9 to 104 ng/mL for MEKC‐FLD. With the regulatory limit for shellfish samples of 800 ng/mL, the CZE‐C⁴D and MEKC‐FLD methods were evaluated for the screening and detection of PSTs in shellfish samples. While the CZE‐C⁴D method suffered from significant interferences from the shellfish matrix, MEKC‐FLD was successfully used for PST screening of a periodate‐oxidized mussel sample, with results confirmed by HPLC‐FLD. This confirms the potential of MEKC‐FLD for screening of PSTs in shellfish samples.     

Identification of gamma-irradiated Chinese herbs by thermoluminescence analysis

The feasibility of thermoluminescence (TL) to differentiate irradiated Chinese medicinal herbs from non-irradiated was investigated. Thirty different dried Chinese herbs were tested, including root, flower, ramulus, rhizome, cortex, and whole plant samples. Irradiation of Chinese herbs was associated with strong TL peaks at ~150–250 °C, while TL curves of non-irradiated herbs had very low intensities above 250 °C, which was also confirmed by the TL ratio (non-irradiated, TL₁/TL₂ < 0.1). The ability to determine the irradiation dose by the TL method was influenced by the amount and types of minerals in the samples. All levels of irradiation doses could be detected when between 0.1 and 1.0 kGy, except for three herbs at 0.1 kGy dose. Different blends with small quantities (0.1–10 %) of irradiated herbs were also tested in this study. Samples with powder mixtures containing 1 % irradiated components could be differentiated (TL₁/TL₂ > 0.1) except for sterculia lychnophora, semen cassia, flos inulae, and anemone root. TL ratios of some herbs indicated irradiation (TL₁/TL₂ > 0.1) even if the irradiated components were as low as 0.1 %. Thus we demonstrated that TL analysis had excellent sensitivity and reliability for the identification of irradiated Chinese herbs.     

Fit-for-purpose shellfish reference materials for internal and external quality control in the analysis of phycotoxins

The need for reference materials for quality control of analysis of foodstuffs has been stressed frequently. This has been particularly true in the phycotoxins field, where there is a great shortage of both pure calibration standards and reference materials. Worldwide there are very few independent bodies that produce certified reference materials for phycotoxins, the main producers currently being the National Research Council Canada and the Japanese Food Research Laboratory. Limited availability of contaminated shellfish and algae, as well as the time and knowledge necessary for the production of adequate reference materials, continuously lead to limited editions of certified reference materials and even more limited production of in-house reference materials. The restricted availability of in-house quality control materials promotes the rapid use of the limited certified reference materials, which in turn hampers the production of the suite of materials required globally for complete protection of public health. This paper outlines the various options that analysts can pursue in the use of reference materials for internal and external quality control, with a view to optimising the efforts of both reference materials users and reference materials producers. For this purpose, the logical sequence is reviewed from the discovery of a new bioactive compound in shellfish, through initial method development up to regulation for food safety purposes including accepted reference methods. Subsequently, the requirements for and efforts typically spent in the production and characterisation of laboratory reference materials, certified reference materials and other test materials used in inter-laboratory studies or proficiency testing, in the area of marine biotoxins are evaluated. Particular emphasis is put on practical advice for the preparation of in-house reference materials. The intricate link between reference material characterisation and method performance is outlined to give guidance on the appropriate in-house method validation in the rapidly developing field of phycotoxins.