Cocoa plants also absorb toxic heavy metals if the soils are appropriately polluted, for example by mining overburden, which can gradually poison groundwater and soils. Cultivation areas such as those in South America are sometimes heavily polluted with these heavy metals. However, it also depends on where the heavy metals accumulate in the bean, whether rather in the shell or rather in the endosperm inside the bean. because the beans go through many treatment steps from the harvest to the raw material for chocolate, which could possibly reduce the contamination – and ideally in such a way that the heavy metals are reduced but the desired trace elements are retained.

Through the interaction of various X-ray fluorescence techniques, it has now been possible for the first time to non-invasively determine where cadmium accumulates in cocoa beans: less inside the bean, but mainly in the shell.

A team led by Dr Ioanna Mantouvalou (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH) and Dr Claudia Keil (TU Berlin) used various methods to precisely map the heavy metal concentrations. For example, they examined cocoa samples from Colombia that were contaminated with an average of 4.2 mg/kg of cadmium. This is well above the European limits of 0.1–0.8 mg cadmium/kg in cocoa products. "Until now, there has been little knowledge about how cadmium migrates from the soil through roots into the plant and where the element accumulates in the beans. Particularly because it was not possible to precisely localise the cadmium content non-invasively," says Mantouvalou.

The researchers discovered differences between beans before and after the roasting process: "We were able to prove that roasting changes the element distribution in the beans," says Mantouvalou. Since the interaction of the methods used makes it possible to measure the accumulation of cadmium at a precise location, further investigations could systematically explore which improved processing steps minimise the exposure.