A group of researchers working at the University of Graz performed simultaneous selenium and sulphur speciation analysis in cultured mushrooms using ion-pairing reversed-phase chromatography coupled to an ICP-MS/MS detector.
Background:Selenium is an essential trace element required for a number of physiological functions in humans and animals. Organic Se species are considered to be the preferred nutritional source for Se due to their reduced toxicity. Edible fungi have strong ability to transform inorganic Se into organic forms, which makes them a valuable source of Se in the diet. Mushroom yields and their chemical composition can be affected by the substrates used in their growth. In cultivated fungi, the generally low Se content can be increased through fortifying the growth substrates with selenium. Since mushrooms contain significant amounts of proteins, ranging from 16.5% to 39% dry weight, it is expected that mushroom protein fractions would contain high levels of Se. Pleurotus pulmonarius is a commonly consumed mushroom, for which data on the Se speciation was lacking.
The new study:In order to provide the missing data for the edible mushroom Pleurotus pulmonarius the researchers designed their study to characterize Se and Se species in different parts of the mushroom at different harvesting times. The fungi culture was grown by using a nutrient medium supplemented with 1.1. mg Se l-1 in the form of Na2SeO3 in the dark for 28 days. Mycelium biomass was filtered, washed and dried at 50°C.
Mushroom fruiting bodies were grown on wheat stray after inoculation with overgrown spawn. Mushrooms were harvested at first, third and sixth day after mushroom formation. Harvested mushrooms were lyophilized and ground to a fine powder.
Total Se was determined after microwave digestion by using inductively coupled plasma mass spectrometer (ICP-MS; Agilent 7700) operated in the octopole reaction system (ORS) mode with hydrogen gas.
For speciation analysis, two different extractions were performed by either using water at 37°C for 20 h or proteinase at 37°C for 12 h. Both extracts were centrifuged at 4500 rpm and the supernatants were filtered through 0.2 µm Nylon syringe filters. For Se and S speciation, diluted mushroom extracts were subjected to ion-pairing reversed phase chromatography (IP-RP HPLC) coupled to a triple quad ICP-MS (Agilent 8800). The selenium and sulphur signals were recorded in MS/MS mode, utilizing the ORS with oxygen as the cell gas.
Total Selenium concentrations (expressed on a dry mass basis) found in mycelium, colonized substrate, and fruiting bodies varied across the range of 23–56 μg g−1. The extraction efficiency ranged between 63.3 and 104 % for the enzymatic digestion and 45.3 and 109% for the aqueous extraction. The harvesting time had only a very slight influence on the Se concentration of the fruiting bodies, with slightly higher values for the first day. After 45 days, the fruiting bodies contained approx. 10% of all Se provided in the nutrient solution. Taking into account that the dietary reference value for Se is 55 μg day−1, it is evident that these fruiting bodies can be a significant source of Se.
The obtained results from speciation analysis revealed that Se-Methionine and Methionine are the predominant organic species in the P. pulmonarius mushrooms in all treatments, followed by noticeable amounts of cystine and specially sulfate. Other species detected were below quantification limits. There were also several unknown Se signals depending on sample type. Whether these species are degradation products calls for further investigation. The most striking result obtained in the present study was the fact that significant amounts of Se and S species accumulated in fruiting bodies (60.1 - 83.1 %) and extraction efficiencies were similar with water and enzymatic digestive extraction. This finding can be explained by the speciation, indicating that Se-Met and Met are largely present as free molecules rather than being incorporated in protein structures.
With respect to the use of cultivated mushrooms with fortified Se concentration as a food supplement, one should consider the recommended daily selenium intake of 55 μg Se/day, which would limit the consumption of such mushrooms to very small amounts.
The original study:
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Walter Goessler, Simultaneous selenium and sulfur speciation analysis in cultivated Pleurotus pulmonarius mushroom, Food chem., 279 (2019) 231-236.
DOI: 10.1016/j.foodchem.2018.12.009
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