Keywords

1 Introduction

Radiocesium (134Cs + 137Cs) concentrations in foods are of great concern in Japan since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, as people wish to avoid receiving additional internal doses from ingestion of the radionuclide. Food monitoring has been carried out since March 2011; provisional regulation values for total radiocesium concentration were applied at the time, and the values were 200 Bq kg−1 for water, milk, and daily products, and 500 Bq kg−1 for other food materials. From April 1, 2012, standard values for radiocesium have been employed in Japan, and the concentration in marketed raw food materials has been limited up to 100 Bq kg−1, except baby foods (50 Bq kg−1) and drinking water (10 Bq kg−1). If any food exceeds these limits, the food name together with the producing district has been reported immediately by the Ministry of Health, Labour, and Welfare (MHLW). Every month, radioactivities of more than 20,000 samples are being measured and the recent data of February 2015 [1] have shown that foods exceeding the standard limits were mostly from the wild (i.e., not commercially grown or raised), i.e., meats of wild boar, Sika deer, Asian black bear, Japanese rock fish (marine), Japanese eel and char (freshwater). Not only meats but also edible wild plants in spring and autumn in 2014, e.g., giant butterbur flower-bud, Japanese angelica-tree shoot (taranome), and various mushrooms species, exceeded the limits.

One of the public’s main concerns is how to decrease ingestion of radiocesium from foods, especially foods they have collected from the wild as well as from their home-grown garden fruits because radioactivities in these foods have not been measured for many cases. Unfortunately, radiocesium removal data by food processing, including culinary preparation, for crops commonly consumed in Japan have been limited because of little interest in the topic before the FDNPP accident. To remedy this, Japanese researchers have begun collecting such data [2–12]. Recently, the Radioactive Waste Management Funding and Research Center (RWMC) compiled the radiocesium removal ratios by food processing using open source data published mainly in 2011 and 2012 [13]. However, it is necessary to add more data to update the information.

In the present study, we focused on food processing effects on radiocesium removal in food plants from the wild to add more information and we compared obtained values with previously compiled values in the IAEA Technical Report Series No. 472 [14]. We also measured potassium-40 (40K) concentrations in the same samples for comparison with radiocesium.

2 Materials and Methods

The following samples were collected in Chiba and Fukushima Prefectures in 2013 and 2014: giant butterbur (Petasites japonicus: flower-bud, 2; leaf blade, 4; petioles, 4), Japanese mugwort (Artemisia indica var. Maximowiczii: young shoots, 4), field-horsetail (Equisetum arvense: fertile stem, 3), water dropwort (Oenanthe javanica: young shoots, 1), Moso bamboo (Phyllostachys heterocycla f. pubescens: young shoots, 8), chestnut (Castanea crenata: nuts, 3), persimmon (Diospyros kaki: fruits, 2), loquat (Eriobotrya japonica: fruits, 4), and Japanese apricot (Prunus mume, 1). Sampling dates are listed in Tables 18.1, 18.2, and 18.3. Immediately after the collection, samples were transferred to a laboratory and weighed to obtain the fresh weight.

Table 18.1 Food processing retention factor (F r) of 137Cs and 40K for giant butterbur tissues collected in 2013–2014
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Table 18.2 Food processing retention factor (F r) of 137Cs and 40K for eight bamboo shoots collected in 2013
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Table 18.3 Food processing retention factor (F r) of 137Cs and 40K for mugwort (M), field horsetail (F), water dropwort (W), and chestnut (C) collected in 2013–2014
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In order to obtain the food processing effect, giant butterbur, Japanese mugwort, and water dropwort samples were separated into three portions to make raw, washed, and boiled (2.5 min) subsamples. One giant butterbur petiole sample was soaked in water for 1 h at room temperature after boiling. Field horsetail, young bamboo shoot, and chestnut samples were separated into two portions to make raw and boiled subsamples (boiling times depended on samples). All samples were weighed before processing. Washing was carried out with tap water in a washing bowl by changing the water five times, and then, finally, the samples were rinsed with reverse osmosis (RO) water. For the boiling process, edible parts of the plants were cooked in RO water after washing with tap water five times. For fruits, a whole fruit was washed with running tap water and rinsed with RO water. Then the water was removed with paper towels from the fruits, and each tissue part (skin, flesh, and seeds) was separated and weighed.

Then, all samples were oven-dried at 80 °C to decrease the sample volume. Each oven-dried sample was pulverized and mixed well, and then transferred to a plastic container (U8 container). Radioactivity concentration in each sample was measured by a Ge detecting system (Seiko EG&G) and the gamma spectrum was analyzed using Gamma Station software (Seiko EG&G) to obtain activity on wet mass basis (Bq kg−1-wet mass). The detection limit was about 0.5–1.0 Bq kg−1-wet mass with the counting time of 40,000–80,000 s. The radiocesium concentrations in samples were usually low and sometimes 134Cs could not be detected 2–3 years after the accident. Therefore, in this study, only 137Cs data are presented together with 40K.

Food processing retention factor (F r) was determined by using the following equation as defined in IAEA TRS-472 [14]:

$$ {F}_{\mathrm{r}} = {\mathrm{A}}_{\mathrm{after}}\left(\mathrm{Bq}\right)\ /\ {\mathrm{A}}_{\mathrm{before}}\left(\mathrm{Bq}\right) $$

where Aafter is the total activity of 137Cs or 40K retained in the food after processing (Bq), and Abefore is the total activity in the food before processing (Bq). The wet mass of the processed subsample before processing (Wbp, kg) was recorded, thus, Abefore was calculated using the following equation:

$$ {\mathrm{A}}_{\mathrm{before}} = {\mathrm{C}}_{\mathrm{raw}}\times {\mathrm{W}}_{\mathrm{bp}} $$

where Craw is the radioactivity concentration of 137Cs or 40K in the raw subsample (Bq kg−1-wet mass).

For fruit samples, distribution percentages of wet mass, 137Cs and 40K in skin, flesh, and seeds were calculated and these distributions were compared.

3 Results and Discussion

The F r values of 40K and 137Cs for each wild edible plant sample are shown in Tables 18.1, 18.2, and 18.3. Some F r data exceeded 1.0 although that is impossible from the above equation; however, because of the low concentrations in samples giving a large counting error, and the samples for raw and processed samples being not completely the same, values of more than 1.0 were inevitable.

The mean F rs of 137Cs by washing for butterbur leaf blade (n = 4), petiole (n = 4), mugwort (n = 4), and water dropwort (n = 1) were 0.98, 1.0, 0.91, and 0.90, respectively and that of 40K were 1.1, 1.0, 0.98, and 0.96, respectively. From this result, it was clear that washing plants is not effective to remove both 40K and 137Cs, because both elements are distributed inside the plant body. Boiling provided different removal effects by plant tissues. The mean F rs of 137Cs for leaf-blade of giant butterbur (n = 4), mugwort (n = 4), water dropwort (n = 1), and fertile stem of field horsetail (n = 3) were 0.40, 0.41, 0.61, and 0.42, respectively, i.e., about 40–60 % of the 137Cs was removed by this process. Similar results were observed for 40K for these samples. Unfortunately, however, the number of samples treated in 2013–2014 was not enough for statistical analysis between F rs of 40K and 137Cs in each plant species. Only analytical results for bamboo shoots (n = 8) are reported here; when ANOVA test was carried out, no statistical difference was observed between F rs of 40K and 137Cs. For some sample types, all the data from 2012 (reported in [2]) to 2014 were summarized (Tables 18.1, 18.2, and 18.3) and the values for 40K and 137Cs did not show any statistical differences by ANOVA test. Thus, we concluded that K could be used as an analogue for radiocesium to calculate F rs.

Compared to leafy samples, bamboo shoots and petioles of giant butterbur (except soaking in water for 1 h) showed slightly higher F rs of 137Cs of 0.74 and 0.73, respectively; since these tissues are thicker than leaf tissues, it is reasonable to expect that more radiocesium would be retained in these tissues. However, when the giant butterbur sample soaked in water for 1 h showed lower F r values, therefore, boiling followed by soaking would effectively remove 137Cs, although K was also removed by this process. For chestnut, mean values of F r of 137Cs and 40K were 0.95 and 0.87, respectively. Removal of these two elements by boiling of chestnut was difficult, because the hard shell prevented extracting 137Cs and 40K from the nuts into the boiling water. In total, most of the F r values presented in this study and our previous data [2] were within the range of values compiled by IAEA [14], as shown in Fig. 18.1.

Fig. 18.1
figure 1

Comparison of food processing retention factor (F r) of 137Cs for several plant materials collected in 2011–2014 by boiling. Both previous [2] and present study’s data are plotted. The shaded area shows Fr range for vegetables, berries, and fruits boiled in water reported in TRS-472 by IAEA [14]

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The results for fruits are listed in Table 18.4. For persimmon and loquat fruits, compared to the relative proportions of skin mass to the total fruits, partitioning percentages of 137Cs and 40K in the skin samples were 1.4–2.2 and 1.2–2.0 times higher, respectively, and those in flesh were 0.8–1.1 and 0.6–0.9, respectively. Thus, 137Cs concentrations in skin were higher than those in flesh for both species. On the other hand, Japanese apricot showed higher percentage of 137Cs distribution than wet mass proportion, although 40K distributions were similar to those of wet mass proportion. In this study, we measured only one sample, however, similar results were obtained in 2012 [13], as listed in the same table.

Table 18.4 Relative proportions (%) of wet mass, 137Cs and 40K in fruit tissues (skin, flesh, and seeds) to whole fruits for four species collected in 2013–2014 at harvest
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In 2011, loquat and persimmon fruits were also measured and the 137Cs distribution percentages in skin samples [15] were higher than those observed in 2013 and 2014 for both species. It was assumed that immediately after the 137Cs was taken up through these trees’ aboveground parts, it transferred to their growing tissues including the fruits; during development of fruits, fruit flesh mass increases at the middle-late ripening stage [16], but probably 137Cs supply reduced due to the smaller amount of 137Cs uptake through tree surface. By this process, 137Cs distributions in fruit parts differed from those we observed in 2013 and 2014, although more studies are necessary to understand the mechanisms of Cs transfer in fruit trees.

4 Conclusions

The effects of washing and boiling of wild edible plants, and peeling of home-grown fruits were studied. It was found that washing plant surface is not effective in removing both 40K and 137Cs, because both elements are in the plant tissues. Food processing retention factor, F r, decreased by boiling, and the average values ranged from 0.40 to 0.61 for leaf-blade of giant butterbur, mugwort, water dropwort, and fertile stem of field horsetail. Bamboo shoots and petioles of giant butterbur (except soaking in water for 1 h), however, showed slightly higher F rs of 137Cs of 0.59–0.94, because these tissues are thicker than leaf tissues, and thus, it is reasonable to expect that more radiocesium would be retained in these tissues. For fruits of persimmon and loquat, 137Cs concentrations in skin were higher than those in flesh, but different trend was observed in Japanese apricot. Interestingly, the distributions of 137Cs in skin, flesh, and seeds observed in this study differed from those observed in 2011. The mechanism is not clarified yet; therefore, further studies are necessary to understand the radiocesium transfer to fruits after direct deposition to fruit tree surfaces.