Warnings about arsenic in rice or rice cakes have been circulating for some time. Since 2016, the EU has set limit values for this. These relate to inorganic arsenic (iAs), which can be reliably analyzed and toxic in sufficient doses: it is held responsible for an increase in the risk of cancer. Researchers at the University of Bayreuth have now also discovered the organic arsenic compound dimethylmonothioarsenate (DMMTA) in a worldwide screening of rice and rice cakes.

The results appeared last year in the "Journal of Agricultural and Food Chemistry" and in "Environmental Science & Technology". DMMTA belongs to the group of sulfur-containing arsenic compounds, the so-called thioarsenates, and has been shown to be more toxic than the inorganic arsenic compounds in studies on various cell lines. In order to be able to estimate the hazard potential of DMMTA, the authors call for further toxicological studies and advocate a regulation of the total content of arsenic as a precaution.

Arsenic is a natural component of the earth's crust and thus appears in rocks as well as by leaching in the soil and groundwater. Thus, arsenic can be absorbed by crops such as rice. At the beginning of March, the European Commission announced a further tightening of the permissible iAs concentration in food as part of the "European Plan to Fight Cancer". For example, the limit value for iAs in white, polished rice is to be reduced from 0.20 to 0.15 milligrams per kilogram.

However, organic arsenic species, especially DMMTA, are still not being tested, as the authors of the DMMTA studies Britta Planer-Friedrich and Stephan Clemens emphasize. According to Planer-Friedrich, the relevance of organic arsenic has long been misunderstood, because dimethylarsenate (DMA) has been found in nature so far. However, this was only classified as "potentially carcinogenic" and therefore not regulated.

Higher arsenic concentrations than rice itself

For a long time, therefore, it remained unnoticed that DMMTA can form from DMA in the presence of reduced sulfur in rice soils, since standard measurement methods wrongly co-determine DMMTA as DMA. Only after the development of suitable stabilization and analysis methods did Planer-Friedrich succeed in 2007 in detecting thioarsenates in highly sulfurous geothermal waters of Yellowstone National Park. In 2020, it was discovered that even low sulphur concentrations are sufficient to form thioarsenates in peatlands, groundwater and rice soils. Two years later, the scientists succeeded in detecting the substance in rice grains and rice products.

Using a method that simulates the digestion of rice in the gastrointestinal tract, the Bayreuth researchers have now demonstrated that DMMTA can be released from rice after consumption and thus represents a potential health hazard. In addition, they observed, initially on a small amount of samples, that rice cakes have higher DMMTA concentrations than rice itself.

"In rice cakes, there seem to be conditions during production that lead to low levels of DMMTA in the rice accumulating more strongly in the wafer or even more DMMTA being formed from DMA," Planer-Friedrich suspects. This seems to apply to a broad spectrum of rice cakes, says Stephan Clemens: "Since the first findings, we have examined more than 70 commercially available rice wafer products and found DMMTA concentrations that alone exceed the valid iAs limit of 0.30 milligrams per kilo."

The author has also created a film report on the subject.