Mitigating the level of cadmium in cacao products: reviewing the transfer of cadmium from soil to chocolate bar

The new EU regulation on cadmium (Cd) in cacao-derived products affects the cacao market worldwide. Here, we reviewed the journey of Cd from soil to chocolate bar and collated current data on the topic, giving due attention to data quality. Cacao bean Cd concentrations are typically about a factor two larger compared to the soil on which the cacao tree grows, this is high but not unusual and, therefore, the cacao plant is not classified as a Cd hyperaccumulator. Mean Cd concentrations in cacao beans range 0.02-12 mg Cd kg-1 and are markedly higher in Latin America, where more than half of cacao bean samples exceed the commonly applied threshold for export to the EU (0.60 mg kg-1). This regional enrichment is related to relatively high soil Cd concentrations in the young soils of Latin America. The source of Cd is, in general, likely geogenic rather than derived from phosphate fertilizers or contamination. A meta-analysis of 780 soil-plant paired data shows that soil Cd, soil pH and soil organic carbon largely explain cacao bean Cd concentrations. Detection of effects of cultivars, soil treatments, or agronomic practices are strongly hampered by the spatial variability in phytoavailable soil Cd concentrations. Application of lime or biochar has the potential to lower bean Cd in acid soils. In the long-term, breeding low Cd cultivars likely provides the highest potential for mitigation but genetics and breeding research is currently limited by the lack of understanding of how Cd is loaded into the developing cacao fruit of this cauliflorous tree. Postharvest practices such as fermentation can slightly lower Cd concentrations in the final product but also play a large role in product quality. In the short term, mixing of cacao from different origins may be the most feasible strategy to meet the EU limits.