Detection of low-dose irradiation of dry fruits by termoluminescence

This paper describes the possibility of using the thermoluminescence (TL) method for the detection of the irradiation of dried fruits using low doses of radiation. Dried fruits are used on an industrial scale as an active ingredient in phytopharmaceuticals, mainly in food supplements and in teas and herbal infusions. Market research on dried fruit shows that the number of consumers of these products is constantly growing [1, 2], and they should therefore meet the requirements to be considered as healthy and safe food.

Dried fruits, along with spices and herbs, belong to a group of products that are relatively frequently exposed to ionizing radiation for preservation purposes and to eliminate the incidental presence of pathogenic microorganisms. The recommended dose for preservation is approximately 10 kGy [3–12]. However, doses <10 kGy are often used for sanitary protection. According to Cruz-Zaragoza et al. [13], doses <0.4 kGy are used in many countries.

Despite the many concerns about the adverse effects prevailing, it can be pointed out that irradiation does not cause radioactivity in food; does not change the composition of food; does not reduce the nutritional value; and does not change the taste, texture, or appearance of food [14–21].

The European Committee for Standardization has published the standard EN 1788:2001 [22] for the detection of radiation by TL in foods from which silicate minerals can be isolated. TL is currently the most widely used method for detecting irradiation of seafood, spices, and herbs and their mixtures [23–49]. This method is also used for the detection of irradiation of fresh and dried fruits and vegetables [26–28, 31–35, 50–66].

Electron paramagnetic resonance (EPR) is also used to detect irradiation of dried fruits [67–80]. However, the EPR signals of irradiated fruits are complex and relatively broad, which can have a negative effect on the sensitivity of the method and therefore the difficulty of detecting irradiation of dried fruits with low doses of radiation. In the present work, 10 dried fruits were selected to study whether the irradiation treatment is detected by TL.

In the present work, 10 types of dried fruits (strawberries, cherries, black currants, two kinds of raisins, mulberries, apricots, figs, dates, and plums) have been selected to study the detection of the irradiation treatment by TL. The products were purchased from Warsaw, Poland.

All chemicals of the analytical grade were purchased from commercial sources: sodium polytungstate (3 Na₂WO₄·9 WO₃·H₂O) was purchased from Sometu, Germany; hydrochloric acid (HCl), ammonium hydroxide (NH₃(aq), and acetone (C₃H₆O) were purchased from Avantor Performance Materials, Poland S.A.; and silicone spray was purchased from Henax Sp. z o.o., Poland. Nitrogen (N₂) for flushing the TL heating chamber was purchased from Air Products and Chemicals, Inc., with a purity of 99.98%. The water was at grade 3.

Gamma irradiation of dried fruits was performed in the Gamma Chamber GC5000 cobalt source (BRIT, India – installed at the Institute of Nuclear Chemistry and Technology) at doses of 0.1 kGy, 0.3 kGy, and 0.5 kGy at a dose rate of 1.6 kGy/h. The absorbed dose was controlled with an alanine dosimeter according to the ISO/ASTM 51607:2013(E) Standard Practice for use of an alanine-EPR dosimetry system [81].

The conducted research aimed to determine whether the TL method is effective in detecting irradiation of dried fruits with lower doses of ionizing radiation than the previously used doses in the range of 1–10 kGy.

The results obtained from measuring the TL of the dried fruits studied are presented in Table 1 (before irradiation – dose of 0 kGy and after irradiation – dose of 0.1 kGy, 0.3 kGy, and 0.5 kGy).

According to the EN 1788:2001 standard, in the TL method, a reliable proof of food irradiation is the analysis of TL glow curves and the following two conditions:

– Condition 1: The TL glow ratio (Glow1/Glow2) measured in the temperature range of 150–250°C must be >0.1 (see Table 1, column 6).

From the 10 samples of remaining dried fruits (strawberries, cherries, blackcurrants, two kinds of raisins, mulberries, apricots, figs, dates, and plums), it was possible to isolate appropriate amounts of silicate minerals using the density method with sodium polytungstate solution. Samples of the 10 listed fruits irradiated with doses of 0.3 kGy and 0.5 kGy were unequivocally identified as irradiated. On the TL glow curves of the samples irradiated with these doses, characteristic peaks of glow could be distinguished in the temperature range from 176°C to 206°C (see Table 1, column 7), ideally within the temperature range of 150–250°C, in which according to the norm, only the glow of the irradiated samples is observed. Also, the TL Glow1/Glow2 ratios of the samples irradiated with these doses were higher than the critical value of 0.1. In the case of samples irradiated with a dose of 0.3 kGy, the ration were within the range of 0.159–0.418, while for samples irradiated with a dose of 0.5 kGy, the ratios were within the range of 0.267–0.481 (see Table 1, column 6).

Not so unambiguous results were obtained when examining samples irradiated with the lowest dose 0.1 kGy. The TL Glow1/Glow2 ratios for these samples deviated from 0.1 and were slightly lower, ranging from 0.034 to 0.077 (see Table 1, column 6).

On the other hand, in the TL glow curves of these samples, it was possible to distinguish peaks in the range of 150–250°C, recorded at temperatures from 184°C to 208°C (see Table 1, column 7 and Figs. 1–10), i.e., within in the range of 150–250°C, typical for irradiated samples.

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Moreover, the EN 1788:2001 standard states that a sample has been irradiated if the ratio of glow 1 to Glow 2 is <0.1 and a clearly defined Glow 1 is observed in the range of 150–250°C. This is sufficiently reliable evidence of irradiation of the sample [22].

The TL Glow 1 records obtained with dried fruits (strawberries, cherries, blackcurrants, Sultana raisins, mulberries, Iranian raisins, apricots, figs, dates, and plums) irradiated with doses of 0.1 kGy, 0.3 kGy, and 0.5 kGy are shown in Figs. 1–10, respectively.

The tests of dried fruits described in this paper carried out using the TL method showed the usefulness of this method to identify the irradiation of a selected group of dried fruits irradiated with doses lower than those recommended in normative documents.