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 MeHg which can be accumulated in organisms and biomagnified along the food chain. In general the main exposure of humans to MeHg 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 MeHg for populations consuming high amounts of rice grown on contaminated soil. The bioavailability of inorganic mercury and MeHg 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 MeHg 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 MeHg 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 vapour generation atomic fluorescence detection (SPE-HPLC-CVG-AFS).

Using this technique, the researchers analysed 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.