By Lauren Quinn
Committed chocoholics, be warned. A health-robbing heavy metal, cadmium, lurks in the velvety recesses of your favorite indulgence.
Researchers have chased the source of cacao's cadmium contamination for years, but an array of distinct sampling methods and sites led to mixed results. In a new analysis, University of Illinois scientists consider the soil factors influencing cadmium's ride into cacao beans, with the aim of recommending feasible and cost-effective ways farmers can minimize uptake.
"Instead of trying to glean universal drivers of cadmium uptake from single studies conducted here or there, we said, 'let's look at this across the entire Cacao Belt,'" says Jordon Wade, who coordinated the effort as a postdoctoral researcher at U of I. Wade is now an assistant professor at the University of Missouri.
Wade worked with graduate students in the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois, as well as crop sciences assistant professor Andrew Margenot. Together, they re-analyzed thousands of data points reported in dozens of published studies from cacao cropping systems around the world. Their synthesis appears in PLOS ONE.
"We found it was the total amount of soil cadmium and pH that explained the amount of cadmium that ends up in the bean," Margenot says. "It seems a little too simplistic, but it is consistent with soil chemistry theory. When you get into more acidic pH values, cadmium is more soluble and more available to the plant. That was the major takeaway."
Acidic pH values also contributed to bioaccumulation of cadmium in leaves and beans, Wade says.
Understanding the routes of cadmium into the bean is the first step to mitigating its uptake, the researchers say. And there's more reason than ever to keep cadmium levels low.
Recent EU regulations cap cadmium at 0.1 to 0.8 milligrams per kilogram, depending on the cocoa product. Margenot says the standard is forcing many companies to limit imports from cacao-producing regions in the global south where soils are naturally high in the heavy metal.
While cadmium in cocoa products is a legitimate health concern—chocolate is a leading source of dietary cadmium in non-smokers—the regulation is bad news for 8 million smallholder farmers for whom cacao is an important cash crop.
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