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Introduction
Lentinula edodes belongs to the family omphalotaceae. It is an edible mushroom, similar to Sarcodon aspratus and Tricholoma matsutake (Jiang et al., 2013; Han et al., 2015). L. edodes is mainly produced in Korea, China, and southeast Asia, where it is reported to grow on pieces of wood and stubble, such as those of oak (Kim et al., 2003). L. edodes contains numerous nutrients, including proteins, vitamins, and minerals, as well as β-glucan, eritadenine, ergosterin, and lentinan (Hong et al., 1988).
The domestic production of L. edodes in Korea in 2016, 2017, and 2018 was 23,470 tons, 23,984 tons, and 22,255 tons, respectively. Developments in mushroom- cultivation technologies have resulted in increased mushroom production (Korean Forest Service, 2020; RDA, 2011). However, since the domestic production of this mushroom is unable to meet consumer demand in Korea, Chinese L. edodes are imported in large quantities (Bak et al., 2013; Kim et al., 2017), and these low-priced Chinese L. edodes samples are falsely sold as domestic products. L. edodes is reported as a product vulnerable to fraudulent sale, along with pork and red pepper powder (KREI, 2011). The origin of L. edodes is judged by the visual identification of several features, such as a hat-like shape and the condition of wrinkles (Lee et al., 2006); because such methods are not reliable, an accurate and objective method of identifying the country of origin of L. edodes is needed. However, thus far, studies have only performed the volatile fragrance component analysis and residual pesticide survey of L. edodes; very few studies have reported the identification of the country of origin of L. edodes (Hong et al., 1988; Kim et al., 2020).
Therefore, this study analyzed the contents of macro minerals, trace minerals, and hazardous heavy metals in domestic and Chinese L. edodes samples and compared the contents of these components to ascertain the differences observed according to the production country.
Material and methods
In total, 13 domestic and 17 Chinese L. edodes samples were used in this study. L. edodes samples were randomly purchased from Jangheung in Korea (domestic L. edodes) and Korea Songi Trading Co. Ltd., Chilgok, Korea (Chinese L. edodes).
The L. edodes samples were decomposed according to the microwave method recommended by the Korean Food Code (MFDS, 2020). Briefly, approximately 0.5 g of the dried mushroom sample was placed in the microwave digestion system, followed by decomposition using nitric acid and hydrogen peroxide (Dong Woo Fine Chem. Co., Ltd., Iksan, Korea); finally, the samples were added with distilled water. The nitric acid and hydrogen peroxide solutions used for the treatment were of extra pure grade.
To verify the analytical method, a calibration curve was prepared using a standard solution, and the correlation coefficient (R2) of the calibration curve was calculated. The limit of detection (LOD) and limit of quantification (LOQ) were calculated according to the International Conference on Harmonization for Registration of Pharmaceutical for Human Use (ICH) (Kim, 2020; ICH Steering Committee, 2014).
δ: Standard deviation of the response
S: Slope of the calibration curve
The standard solution prepared at each concentration was added to the sample, and the accuracy and precision were measured three times. According to the ICH criteria, the accuracy range should lie within 80-120%, and the precision range should be within 20% (ICH Steering Committee, 2014).
α: Spiked concentration
β: Measuring concentration
The contents of macro minerals such as Na, Mg, K, and Ca were analyzed in accordance with the atomic absorption spectrophotometric method specified by the Korean Food Code (MFDS, 2020). The conditions set for the analysis using an atomic absorption spectrophotometer (AAnalyst 400, Perkin Elmer, Norwalk, CT, USA) were fuel and oxidant flow at 2.0 L/min for C2H2 and 10.0 L/min for air. The analytical wavelengths for Na, Mg, K, and Ca were determined to be 589.00, 285.51, 766.49, and 422.67 nm, respectively.
Trace minerals (Cr and Cu) and hazardous heavy metals (Cd and Pb) were analyzed in accordance with the inductively coupled plasma mass spectrometer method specified by the Korea Food Code (MFDS, 2020). The conditions set for the analysis using an inductively coupled plasma mass spectrometer (NexION 300 D, Perkin Elmer, Norwalk, CT, USA) were as follows: RF power, 1,600 W and pulse stage voltage, 900 V. The flows of the Aux. and Neb. gases were 1.2 L/min and 1.02 L/min of argon, respectively. The mass spectra for Cr, Cu, Pb, and Cd were determined to be 51.941, 62.930, 207.977, and 110.904 m/z, respectively.
Statistical analysis was performed using the SPSS statistics program (Statistical Package for the Social Science, Version 26.0, SPSS Inc., Chicago, IL, USA) to calculate the means and standard deviations. Significant differences were analyzed using independent sample t-tests or one-way ANOVA and Duncan’s multiple range test.
Results and discussion
The accuracy, precision, correlation coefficient, LOD, and LOQ of the analyses of the contents of minerals and hazardous heavy metals by atomic absorption spectrophotometry (AAS) and inductively coupled plasma mass spectrometry (ICP/MS) are presented in Table 1. For the AAS analysis, the accuracy obtained was 94- 109%, the precision was within 20%, and the correlation coefficient was over 0.995. For the ICP/MS analysis, the accuracy was 98-101%, the precision was within 20%, and the correlation coefficient was over 0.999. These results satisfied the ICH criteria, which require an accuracy of 80-120% and precision of <20% (ICH Steering Committee, 2014).
The macro mineral contents of domestic and Chinese L. edodes, as analyzed using AAS, are presented in Table 2. The contents of Na, Mg, K, and Ca in domestic L. edodes were determined to be 161.33±44.34 mg/kg, 746.22±197.55 mg/kg, 17,676.84±3,279.58 mg/kg, and 50.50±26.98 mg/kg, respectively, whereas those in Chinese L. edodes were 310.69±84.80 mg/kg, 318.49±45.06 mg/kg, 11,182.01± 2,446.44 mg/kg, and 33.36±6.08 mg/kg, respectively. Upon comparing the macro mineral contents of domestic and Chinese L. edodes, it was found that the Na content was significantly higher in the Chinese L. edodes samples (p<0.001), whereas significantly higher levels of Mg and K were present in the domestic L. edodes samples (p<0.001).
L. edodes is a cultivable mushroom that grows on broad-leaf trees, such as oak trees. The mineral content of L. edodes is reported to be mainly affected by the acidity and organic content of the soil, as well as the growing environment (Gast et al., 1988; Kim et al., 2003). Our results revealed significant differences between the Na, Mg, and K contents in domestic and Chinese L. edodes (p<0.001). We believe that examining the Na, Mg, and K contents in many samples of L. edodes, whose exact cultivation conditions have been investigated, will help determine the country of origin of the mushroom samples.
The contents of trace minerals and hazardous heavy metals in domestic and Chinese L. edodes were analyzed using ICP/MS (Tables 2 and 3). The Cr and Cu contents in domestic L. edodes samples were determined to be 2.82± 2.61 mg/kg and 77.13±45.88 mg/kg, respectively, while those in Chinese L. edodes samples were 12.95±10.05 mg/kg and 101.19±43.78 mg/kg, respectively. Upon comparing the contents of trace minerals in L. edodes according to the production country, no significant difference was observed for the Cu contents, but the Cr contents were significantly higher in the Chinese L. edodes samples (p<0.01). The mineral content of L. edodes is reportedly influenced by the cultivation environment (Gast et al., 1988; Kim et al., 2003). Our results showed significant differences between the Cr contents of domestic and Chinese L. edodes (p<0.01). Thus, we propose that the Cr content can be used to distinguish between domestic and Chinese L. edodes samples.
The hazardous heavy metal contents of domestic and Chinese L. edodes were as follows: 1.80±0.84 μg/kg and 5.44±4.43 μg/kg for Cd, respectively, and 1.94±4.52 μg/kg and 2.41±3.44 μg/kg for Pb, respectively. Only a few studies have reported the hazardous heavy metal contents in L. edodes; hence, comparisons of such results with those of the current study are not possible. According to the Korea Food Code (MFDS, 2020a), the hazardous heavy metal content of mushrooms is defined as less than 300 μg/kg for Cd and Pb. Thus, the results of the current study demonstrate that the heavy metal contents in the L. edodes samples analyzed herein are within the safety limits specified by the Ministry of Food and Drug Safety standards in Korea. In conclusion, our results revealed significant differences between the Na, Mg, and K contents (p<0.001) and Cr contents (p<0.01) of domestic and Chinese L. edodes samples. These results showed that the Na, Mg, K, and Cr contents can be used as basic data to determine the origin of domestic and Chinese L. edodes. Therefore, there is a requirement for further studies that analyze the Na, Mg, K, and Cr contents in many samples of domestic and Chinese L. edodes, the cultivation conditions for which have been investigated, for the accurate identification of the country origin of L. edodes mushrooms.
