Making chocolate safer

07-07-2022

Scientists from the Université Grenoble Alpes and the ESRF are investigating ways of decreasing the amount of cadmium in cocoa beans, with the goal of making chocolate safer.

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Cadmium is a heavy metal present in small concentrations in many soils where cocoa is being cultivated particularly in Latin America and the Caribbean (LAC) region (concentrations between 0.22 and 10.8 mg Cd kg-1).  The cocoa tree bioaccumulates cadmium and stores a portion of it into the cocoa beans, the raw material used to produce chocolates and cocoa-based confectionaries. ., The EU legislation restricts the  maximum allowable limit of cadmium in cocoa based products as  it deems it unsafe for consumers.  This has the effect of impacting on cocoa exports to Europe by farmers from the LAC region.

“We are trying to find ways of mitigating this cadmium transfer to the beans, and for this we need to understand and control how cadmium transfers from soil to the plant and then within the tree”, explains Geraldine Sarret, director of research at CNRS and leader of this study. “This is a completely new subject of research, there is very little literature on how cadmium is managed/detoxified by cocoa plants”, she adds.

With this aim, Sarret and her PhD student, Hester Blommaert, travelled to the Cocoa Research Centre in Trinidad, where cocoa cultivars with many different genotypes are planted. “We took samples of different parts of the plants in order to get a complete picture of what happens to cadmium when it enters inside the crop”, explains Blommaert.

The team then came to the ESRF’s ID21 and ID16B beamlines to unveil the speciation and location of cadmium in the plant tissues. The samples need to be frozen hydrated and for this the researchers use the cryostage at ID21: “If we want to preserve the native state of the cadmium we need to freeze the samples so that there is no change in speciation of the metal”, says Hiram Castillo, researcher at the ESRF involved in the study.

The big challenge of this research is the low concentration of cadmium, around 1mg per kg found in cocoa tissues and sometimes even less. “We really need high sensitivity to detect cadmium, and the new EBS at the ESRF is perfect for our needs”, says Sarret. The team will combine the techniques of bulk X-ray absorption spectroscopy (on BM30B at ESRF) to find the speciation of cadmium and micro and nano imaging to locate it in the different parts of the plant.

Preliminary results have already provided some hypotheses on the mechanisms developed by the plant to detoxify cadmium. “We still don’t know how it does it though, but we hope that our research at the ESRF will provide us some answers, and with them we could try to define mitigation strategies”, explains Sarret.

Text by Montserrat Capellas Espuny. Video by Mark McGee and Montserrat Capellas Espuny.

Top image: Cocoa beans in a tree. Credits: Geraldine Sarret.