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Methylmercury in European rice varies by more than a factor of 50


Mercury is a global pollutant with adverse effects on all living organisms and ultimately on human beings. The awareness and urgency of this pollution problem is emphasized by the Minamata convention, a global treaty for the reduction of mercury emission to the environment. Once emitted to the environment, mercury can be methylated by microbes to methylmercury which can be accumulated in organisms and biomagnified along the food chain. In general the main exposure of humans to methylmercury is through the consumption of marine fish and especially predatory species such as tuna or swordfish.

In addition to seafood, rice was recently identified as a major exposure source to methylmercury for populations consuming high amounts of rice grown on contaminated soil. The bioavailability of inorganic mercury and methylmercury is very different: while 95% of MeHg is absorbed via the human gastrointestinal tract from seafood, only 7% of the inorganic mercury is taken up. Such important difference is the reason why the European Food Safety Authority (EFSA) has suggested a provisional tolerable weekly intake (PTWI) of 1.3 µg week-1 kg-1 b.w. for methylmercury and not for total mercury. Again, such differences in bioavailability and toxicity between different mercury species emphasize the importance of mercury speciation analysis of rice.

The new study:
While the mercury content and speciation in rice has been extensively studied for polluted areas in China, data on MeHg concentrations in rice from supermarkets in Europe is extremely rare. Most analytical methods for methylmercury have been developed for seafood, and cannot easily be used for rice, because of the much lower MeHg content and the totally different starch based matrix. Researchers from UK now have developed and validated a method for analysis of rice based on solid phase extraction and preconcentration  followed by online separation using high performance liquid chromatography coupled with cold vapor generation atomic fluorescence detection (SPE-HPLC-CVG-AFS).

Using this technique, the researchers analyzed 87 rice products sold by supermarkets and Asian Food suppliers in Europe. The products included mainly rice grains but also rice flour, pre-cooked baby-food rice products, and other processed rice products such as noodles or crackers.  Total Hg (determined by CV-AFS after open vessel digestion) varied from 0.53 to 11.1 µg/kg with the mean value of 3.04 +- 2.7 µg/kg. The concentration of MeHg in all 87 rice products varies from 0.11 to 6.45 µg/kg1 with an average concentration of 1.91 ± 1.07 µg/kg1. The variability of the MeHg concentration in the commercial market samples varies by a factor of 58, much higher variation than those observed for total mercury (a factor of 20).

It was further observed, that the MeHg content could not be estimated from the total Hg content with a mean percentage of 71 +-26%, whereby the MeHg part is reduced for increasing total Hg content. For this reason, risk assessment based on the neurotoxicity of MeHg can only be based on direct speciation data. Using a daily rice consumption of 400 g, the maximum MeHg concentration of 6.45 ng/g and a body weight of 65 kg, this would result in a weekly uptake of 0.28 µg MeHg kg -1 b.w. which amounts to 21% of the PTWI. Since infants are especially vulnerable to the neurotoxin MeHg, it is concerning that 30% of the tested rice reached more than 10% of the PTWI.

The cited study:

Christoph-Cornelius Brombach, Parinda Manorut, Piumi P.P. Kolambage-Dona, Mohammed Farouk Ezzeldin, Bin Chen, Warren T. Corns, Jörg Feldmann, Eva M. Krupp, Methylmercury varies more than one order of magnitude in commercial European rice, Food Chem., 214 (2017) 360–365. doi: /10.1016/j.foodchem.2016.07.064

Related studies (newest first):

Chuan Hong, Xiaodan Yub, Jihong Liu, Yue Cheng, Sarah E. Rothenberg, Low-level methylmercury exposure through rice ingestion in a cohort of pregnant mothers in rural China, Environ. Res., 150 (2016) 519–527. doi: 10.1016/j.envres.2016.06.038

C.-C. Brombach, M.F. Ezzeldin, B. Chen, W.T. Corns, J. Feldmann, E.M. KruppQuick and robust method for trace determination of MeHg in rice and rice products without derivatisation, Anal. Methods, 7 (2015) 8584-8589. doi: 10.1039/c5ay01640a

Sarah E. Rothenberg, Nomathamsanqa L. Mgutshini, Michael Bizimis, Sarah E. Johnson-Beebout, Alain Ramanantsoanirina, Retrospective study of methylmercury and other metal(loid)s in Madagascar unpolished rice (Oryza sativa L.), Environ. Pollut., 196 (2015) 125-133. doi: 10.1016/j.envpol.2014.10.002

Sarah E. Rothenberg, Lisamarie Windham-Myers, Joel E. Creswell, Rice methylmercury exposure and mitigation: A comprehensive review, Environ. Res., 133(2014)407–423. doi: 10.1016/j.envres.2014.03.001

X. Wang, Z.H. Ye, B. Li, L.N. Huang, M. Meng, J.B. Shi, G.B. Jiang, Growing rice aerobically markedly decreases mercury accumulation by reducing both Hg bioavailability and the production of MeHg, Environ. Sci. Technol., 48/3 (2014) 1878–1885. doi: 10.1021/es4038929

Mei Meng, Bing Li, Jun-juan Shao, Thanh Wang, Bin He, Jian-bo Shi, Zhi-hong Ye, Gui-bin Jiang, Accumulation of total mercury and methylmercury in rice plants collected from different mining areas in China, Environ. Pollut., 184 (2014) 179-186. doi: 10.1016/j.envpol.2013.08.030

Bo Meng, Xinbin Feng, Guangle Qiu, Christopher W. N. Anderson, Jianxu Wang, and Lei Zhao, Localization and Speciation of Mercury in Brown Rice with Implications for Pan-Asian Public Health, Environ. Sci. Technol., 48 (2014) 7974-7981. doi: 10.1021/es502000d

B. Li, J.B. Shi, X. Wang, M. Meng, L. Huang, X.L. Qi, B. He, Z.H. Ye, Variations and constancy of mercury and methylmercury accumulation in rice grown at contaminated paddy field sites in three Provinces of China, Environ. Pollut., 181 (2013) 91-97. doi:  10.1016/j.envpol.2013.06.021

Ping Li, Xinbin Feng, Xiaobo Yuan, Hing Man Chan, Guangle Qiu, Guo-Xin Sun, Yong-Guan Zhu, Rice consumption contributes to low level methylmercury exposure in southern China, Environ. Int., 49 (2012) 18–23. doi: 10.1016/j.envint.2012.08.006

Bo Meng, Xinbin Feng, Guangle Qiu, Peng Liang, Ping Li, Chunxiao Chen, Lihai Shang, The Process of Methylmercury Accumulation in Rice (Oryza sativa L.),  Environ. Sci. Technol., 45 (2011) 2711–2717. doi: 10.1021/es103384v

Sarah E. Rothenberg, Xinbin Feng, Bin Dong, Lihai Shang, Runsheng Yin, Xiaobo Yuan, Characterization of mercury species in brown and white rice (Oryza sativa L.) grown in water-saving paddies, Environ. Pollut., 159 (2011) 1283-1289. doi: 10.1016/j.envpol.2011.01.027

Hua Zhang, Xinbin Feng, Thorjřrn Larssen, Guangle Qiu, Rolf D. Vogt, In Inland China, Rice, Rather than Fish, Is the Major Pathway for Methylmercury Exposure, Environ. Health Perspect., 118 (2010) 1183–1188. doi: 10.1289/ehp.1001915

Hua Zhang, Xinbin Feng, Thorjorn Larssen, Lihai Shang, Ping Li, Bioaccumulation of Methylmercury versus Inorganic Mercury in Rice (Oryza sativa L.) Grain, Environ. Sci. Technol., 44 (2010) 4499–4504. doi: 10.1021/es903565t

Lu Li, Feiyue Wang, Bo Meng, Marcos Lemes, Xinbin Feng, Guibin Jiang, Speciation of methylmercury in rice grown from a mercury mining area, Environ. Pollut., 158 (2010) 3103-3107. doi: 10.1016/j.envpol.2010.06.028

Bo Meng, Xinbin Feng, Guangle Qiu, Yong Cai, Dingyong Wang, Ping Li, Lihai Shang, Jonas Sommar, Distribution Patterns of Inorganic Mercury and Methylmercury in Tissues of Rice (Oryza sativa L.) Plants and Possible Bioaccumulation Pathways, J. Agric. Food Chem., 58 (2010) 4951–4958. doi: 10.1021/jf904557x

Guangle Qiu, Xinbin Feng, Ping Li, Shaofeng Wang, Guanghui Li, Lihai Shang, Xuewu Fu, Methylmercury Accumulation in Rice (Oryza sativa L.) Grown at Abandoned Mercury Mines in Guizhou, China, J. Agric. Food Chem., 56 (2008) 2465–2468. doi: 10.1021/jf073391a

Xinbin Feng, Ping Li, Guangle Qiu, Shaofeng Wang, Guanghui Li, Lihai Shang, Bo Meng, Hongmei Jiang, Weiyang Bai, Zhonggen Li, and Xuewu Fu,  Human Exposure To Methylmercury through Rice Intake in Mercury Mining Areas, Guizhou Province, China, Environ. Sci. Technol., 42/1 (2008) 326-332. doi:  10.1021/es071948x

Used Instrumentation:

PS Analytical - 10.025 Millennium Merlin System

Related EVISA Resources

Brief summary: Atomic Fluorescence Spectrometry as a Detection System for Speciation Analysis
Brief summary: Chemical speciation analysis for nutrition and food science
Brief summary: Speciation and Toxicity
Link database: Toxicity of Organic mercury compounds
Link database: Human exposure to methylmercury via the diet
Link page: All about Atomic Spectrometry

Related EVISA News

September 14, 2013: Toxic Methylmercury-Producing Microbes More Widespread Than Realized

September 16, 2010: Rice is the Major Pathway for Methylmercury Exposure in Inland China

last time modified: July 20, 2016


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