Dark chocolate can sometimes contain surprisingly high levels of lead and cadmium, two heavy metals that have been linked to health problems, as Consumer Reports’ recent investigation into dark chocolate shows.
While that investigation raised a number of questions, one the most important ones—which also helps show what might be done to get metals out of dark chocolate—is how lead and cadmium end up in chocolate in the first place.
To answer that, researchers have studied how metals get into the cacao plant itself, says Michael J. DiBartolomeis, PhD, a toxicologist and former official at the California Department of Public Health. Between 2019 and 2022, he and other researchers studied how metals might contaminate chocolate, as part of a settlement to a lawsuit against chocolate manufacturers brought by As You Sow, an organization that pushes for corporate accountability. As You Sow had previously found high levels of lead and cadmium in some chocolates.
The metals in chocolate are primarily found in cocoa solids. To make chocolate, cacao beans are crushed to separate the nonfat cocoa solids from the cocoa butter.(The cocoa solids are also where the cocoa beans’ heart-healthy antioxidants are found.)Cocoa solid levels tend to be higher in dark chocolate than in milk chocolate, which is why heavy metal contamination is considered more of a dark chocolate problem.
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But DiBartolomeis and colleagues found that lead and cadmium appear to get into the cacao bean in different ways, at different times in the growing and production process. That means that different strategies will be required to get the lead and cadmium out.
To make chocolate, growers harvest cacao pods from the plants. Wet, sticky beans are extracted from the pods, then covered and left to ferment and develop flavor for several days. The beans are then dried—typically in the sun—before being transported to a manufacturing facility where they are sorted, cleaned, and roasted.
While studying the source of contamination, DiBartolomeis and other researchers discovered that the cadmium is typically in the beans already when they are harvested. This happens because the cacao plants take up cadmium that’s in the soil, which then accumulates in the beans as the trees grow, similar to how heavy metals contaminate some other crops.
Lead, however, appears to get into cacao after beans are harvested. The researchers studying contamination found that lead levels were low when beans were first picked, but increased as beans were left to dry for days out in the sun, when lead-filled dust and dirt could get stuck to the outside of the beans. There’s lead in dust and dirt near industrial sites and in places where leaded gasoline has been used.
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A third of the brownie mixes, chocolate chips, hot cocoa, and otherchocolate products CR tested contained concerning levels of lead or cadmium.
To lower cadmium levels, growers will need to take a number of steps that may take years, DiBartolomeis says. Chocolate makers could start by surveying soil in places where cacao is grown, and favoring plants from regions with lower levels. When needed, beans from regions with more cadmium could be blended with beans from other regions—something some manufacturers told CR they already do. Since cadmium levels tend to rise with tree age, older trees could be replaced with younger ones. In some places, contaminated soil could be treated or replaced. And eventually, it may be possible to breed or genetically engineer cacao plants that are less likely to absorb cadmium in the first place.
Fixing lead could be quicker, says Danielle Fugere, president of As You Sow, since doing so primarily requires changing harvesting and manufacturing processes. Growers and chocolate makers could take steps including minimizing soil contact with drying beans and drying them on tables or tarps away from roads and with protective covers to keep contaminated dust off. And in some cases, it may be possible to remove more lead during the cleaning process after beans reach a factory.
“Lead reductions can be expected within the first year of implementing new handling practices,” according to the National Confectioners Association, which funded the research into heavy metals in chocolate as part of the As You Sow settlement. Lower cadmium levels may take longer, but should still be possible, according to both the NCA and DiBartolomeis.
Kevin Loria
Kevin Loria is a senior reporter covering health and science at Consumer Reports. He has been with CR since 2018, covering environmental health, food safety, infectious disease, fitness, and more. Previously, Kevin was a correspondent covering health, science, and the environment at Business Insider. Kevin lives in Washington, D.C., with his wife and children. Follow him on Twitter @kevloria.
