Blommaert et al. 2022 — Cd translocation pathway in cacao using isotope fractionation and XAS
Blommaert et al. traced the cadmium pathway from soil through roots, branches, leaves, pod husk, and nibs of Theobroma cacao using stable Cd isotope fractionation combined with X-ray absorption spectroscopy (XAS) and laser ablation ICP-MS (LA-ICP-MS). The study found that Cd concentrations in cacao nibs were 10 to 28 times higher than those measured in the topsoil, demonstrating substantial bioconcentration. Isotope fractionation patterns indicated that roots are a key compartment for Cd retention and that the xylem loading step is a critical control point for how much Cd reaches the harvestable bean. XAS revealed that Cd in cacao tissues is primarily bound to organic acids and thiol ligands rather than to phosphate or sulfide minerals, consistent with an active chelation-based transport mechanism. This mechanistic understanding is directly relevant to identifying which agronomic and processing interventions can most effectively reduce Cd in cocoa-derived ingredients.
Key numbers
- Cd bioconcentration in nibs relative to topsoil: 10 to 28× (exact ratio varied by plant and soil treatment)
- Cd speciation in tissue: predominantly organic-acid and thiol-bound forms (XAS data); no significant phosphate or sulfide mineral species
- Isotope fractionation signature: lighter Cd isotopes preferentially retained in roots; heavier isotopes translocated toward shoots and nibs, consistent with membrane transporter selectivity
- LA-ICP-MS mapping confirmed Cd hotspots in vascular tissue of branches
- Pot experiment; Belgian greenhouse conditions; Cd-spiked soil at environmentally relevant and elevated concentrations
Methods (brief)
Greenhouse pot experiment with Theobroma cacao. Soil Cd amended to represent contaminated agricultural conditions. Stable Cd isotope analysis (MC-ICP-MS) on sequential extracts from roots, stems, branches, leaves, pod husk, and nibs. XAS (XANES and EXAFS) on lyophilized plant samples at synchrotron. LA-ICP-MS spatial mapping of Cd distribution in cross-sections of cacao branches. No LOQ for food-safety purposes reported; study aimed at mechanistic understanding rather than routine occurrence monitoring.
Implications
Certification: The 10–28× bioconcentration from soil to nib means that even moderately contaminated soils (40–100 mg Cd/kg typical of some South American cocoa-growing regions) can produce nibs well above EU maximum levels (0.3 mg/kg for chocolate with ≥50% cocoa solids). Soil screening and origin selection remain the highest-leverage mitigation levers; this paper supports that framing.
Courses: Illustrates why “the food is made from a crop, not from the soil” framing misleads when Cd bioconcentration is this strong; teaches the plant physiology basis for origin-of-supply risk.
App: Supports the model logic that cacao-derived ingredients (cocoa powder, cocoa butter, chocolate) carry systematically higher Cd than their soil Cd concentrations would naively imply; the bioconcentration factor range (10–28×) is a useful sensitivity parameter.