Uptake and retainment of radionuclides in edible mushrooms – from fruiting to cooking

Abstract

Purpose: The purpose of this study was to study the uptake, distribution and transfer of Mn-54, Zn-65, U-238, U-235, U-234, and Po-210 in mushrooms and their respective caps and stalks, the effects of chelates and ligands, and the potential release of radionuclides during a cooking procedure. Introduction: Fungi are organisms that are composed of a fruiting body (mushroom) and tubular hyphae, that branches out into complex networks called mycelium. Mushrooms gain their nutriments and elements by excreting organic acids that solubilizes matter, which can then be absorbed. This process can lead to certain elements being accumulated in the mushroom, as such they can be considered to be bioindicators. Among the extensively studied radionuclides that can be accumulated in mushrooms are Cs-137 and Sr-90. However, few studies have investigated the accumulation of naturally occurring radionuclides from the uranium and thorium decay series. Furthermore, studies of chelates and ligands on the uptake and distribution of radionuclides is also lacking. As for decontaminating mushrooms prior to consumption, previous studies that have investigated various cooking effects on the metal content in mushrooms have seen both an increase and decrease in metal concentrations following cooking. Stir-frying with butter, which is a common cooking procedure in Sweden, has to our knowledge, not been studied extensively. Method: Black Pearl, pink- and grey oyster, and lion’s mane mushroom were included in this study. The substrates of the kits were spiked with solutions of naturally occurring radionuclides containing U-238, U-235, U-234 and Po-210 (among others), and solutions of Mn-54 and Zn-65. The mushrooms with Mn-54 and Zn-65 were analysed with gamma spectrometry, and those with naturally occurring radionuclides were analysed by alpha spectrometry. The activity concentration in the substrates were determined by the substrate weight and amount of activity added. The uptake, distribution and transfer of radionuclides in mushrooms were calculated. To study the effects of chelates and ligands, the substrate of lion's mane was added with sodium carbonate, and Black Pearl was added with EDTA. As a means of studying the cooking effects on U-238, U-234 and Po-210 content, grey oyster mushrooms were purchased from a local supermarket and stir-fried in butter and a little salt. The samples were analysed by alpha spectrometry and the annual ingestion effective dose of cooked and uncooked samples were calculated. Results: The results showed that Black Pearl had a higher uptake of Mn-54 and Zn-65 in the cap than stalk, and that there was no difference in transfer coefficients between Mn-54 and Zn 65. For pink oysters, the uptake of Mn-54 was higher than Zn-65 in the first harvests, but similar in the second harvest, and the activity concentrations in fruiting bodies were higher than the substrate. For grey oysters, there was an accumulation of Po-210 in cap and uranium isotopes in stalk. For lion’s mane, the uptake of uranium isotopes were much higher than for grey oysters. Sodium carbonate could be effective in increasing the uptake of uranium in lion’s mane, however, there was likely a negative effect on mushroom growth. The results also showed that there was a considerable variability in radionuclide uptake between the mushrooms, between and within growing kits. The cooking effects on the activity concentration of uranium isotopes and Po-210 were seen to decrease and increase, respectively.