People have been harvesting the beans of the cocoa bush for at least 5000 years. They have learned to ferment, roast, grind and process the beans with sugar and fat to make delicious chocolates. Today, around five million tonnes of beans are on the market every year, coming from only a few growing areas in tropical regions.
Chocolate has a number of benefits: amino acids such as tryptophan brighten the mood. Cocoa beans also contain anti-inflammatory compounds and valuable trace elements. However, cocoa plants also absorb toxic heavy metals if the soils are polluted, for example by mining, which can gradually poison groundwater and soils.
An important question is where exactly the heavy metals accumulate in the bean, whether in the shell or rather in the endosperm inside the bean. From the harvest to the raw material for chocolate, the beans undergo many steps of different treatments, which could possibly reduce the contamination. And ideally the treatment could be optimised in order to make sure that the heavy metals are reduced but the desirable trace elements are retained.
A team led by Dr Ioanna Mantouvalou (HZB) and Dr Claudia Keil (TU Berlin/Toxicology) has now combined various imaging methods at the BAMline of BESSY II to precisely map the heavy metal concentrations in cocoa beans. They examined cocoa samples from a cultivation region in Colombia, which were contaminated with an average of 4.2 mg kg‒1 cadmium. This is well above the European limits of 0.1‒0.8 mg cadmium kg‒1 in cocoa products.
The team worked with three different X-ray fluorescence techniques to examine the cocoa beans. Among other things, they developed a new analytical method for absorption correction when imaging with an X-ray colour camera. “There has been little understanding of how cadmium migrates from the soil through roots into the plant and where the element accumulates in the beans. Especially because it was not possible to precisely localise the cadmium content non-invasively”, says Mantouvalou. PhD students Frank Förste (TU Berlin) and Leona Bauer (TU Berlin and HZB) carried out the experiments.
Cadmium is particularly difficult to detect, explains Mantouvalou. This is because the cadmium signal, which produces the excitation of the outer electrons, lies exactly below the much stronger fluorescence signal of the element potassium, which occurs in higher concentrations in cocoa. “We therefore excite a deeper electron shell of the cadmium atom, which is only possible with hard X-rays at the BAMLine”, says Frank Förste. “This enabled us to map the cross-sections of cocoa beans with high resolution, and show that cadmium predominantly accumulates in the outer shell”, says Leona Bauer.
They also discovered interesting 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. The combination of the different experimental methods allows for the first time to precisely measure the accumulation of cadmium. Further investigations could systematically explore how to improve the processing steps in order to minimise the exposure.