Earlier this year, non-profit Consumer Reports tested 28 different dark chocolate bars in the US and detected “concerning levels” of cadmium and lead in all of them.
Frequent exposure to lead in adults can lead to nervous system problems, hypertension, immune system suppression, kidney damage, and reproductive issues while cadmium can cause developmental problems, which is particularly concerning for children and pregnant women.
These toxic elements are not introduced during chocolate manufacturing but during the cultivation and processing of cocoa.
Cadmium is a heavy metal naturally present in the Earth’s crust but it becomes soluble in acidic soil and is absorbed by the roots of plants. Lead is also present in the soil and dust, mainly due to the use of leaded fuels that create particulates that settle in the environment.
Solutions require collaboration across the supply chain
“There are several ways to reduce lead and cadmium in the cocoa or chocolate supply chain,” said Kantha Shelke, PhD, principal at food science and research firm Corvus Blue and senior food safety lecturer at Johns Hopkins University.
“Commercial viability is imperative for adoption and usually requires participation and support from every node across the value chain to participate and support.”
Carefully controlling how cocoa beans are processed can also reduce levels. Cocoa beans are generally fermented to improve their flavour, for instance, and one study found that when they are fermented for longer, they become acidic. This acidity means that the cadmium migrates away from the cocoa nib – the part that is used to make chocolate – to the outer husk, which is discarded.
This can decrease cadmium concentrations in the nib by a factor of 1.3, making it a useful technique to reduce levels. However, processors must control the fermentation process carefully or the cocoa nibs – and final product – will have an acidic flavour.
“Ensuring that the levels of cadmium and lead are at their lowest requires staying close to the source and being knowledgeable of the processing methods and their implications,” said Shelke. “Manufacturers getting their cocoa supply on spec can be reasonably assured of consistent quality while those shopping for bargains will not.”
Another strategy is to blend cocoa sourced from different regions. Latin American producer countries tend to have greater cadmium levels than other countries due to the volcanic nature of their soil.
The Democratic Republic of Congo and Sierra Leone currently attract greater interest from European buyers as alternative sources of organic cocoa beans because of the lower cadmium content compared to Latin American sources, according to the Centre for the Promotion of Imports from developing countries (CBI), part of the Netherlands’ Ministry of Foreign Affairs.
Harnessing the soil microbiome
Several reduction strategies can also be used at the farmer level. One effective method is to add limestone and zinc to the soil, which reduces cadmium uptake by increasing the soil’s pH level. However, these inputs are expensive for many smallholder farmers and they can only be added to the topsoil, which does not prevent the cocoa tree’s deep roots from absorbing cadmium in the bottom soil layer.
Other agronomy methods include growing certain crops around the cocoa trees to selectively absorb cadmium as well as coppicing and heavy pruning.
An emerging approach is bioaugmentation, which involves adding cultured microorganisms to enhance the prevailing microbial community.
“Phytoremediation with degradative aerobic bacteria such as Pseudomonas, Alcaligenes, Sphingomonas, Rhodococcus, and Mycobacterium is promising for the treatment of contaminated biomass,” Shelke said.
One 2022 study isolated 12 native Colombian bacteria strains and identified two distinct ways they captured cadmium: biosorption and cadmium cytoplasm precipitation.
Genome-edited cocoa seedlings absorb less cadmium
Finally, farmers can use seeds and germplasm that have been genetically engineered or bred to absorb fewer toxic elements and in smaller quantities.
As part of the US-funded Cacao for Peace project, scientists from the Alliance of Bioversity International and the International Center for Tropical Agriculture (CIAT) developed nine gene-edited cacao lines to absorb less cadmium from the soil.
In February 2023, the scientists transplanted the genome-edited cacao plants from greenhouses into cadmium-containing soils from Colombia, testing the plants’ ability to grow in field conditions while maintaining low- or no-levels of cadmium.
The Cacao for Peace results have not yet been communicated although, according to Shelke, gene-editing can be an effective and viable method. She noted one limitation, however, namely that farmers must wait for the seedlings to grow and reach maturity before being able to harvest the pods.
What can consumers do to reduce lead and cadmium intake?
The bitter irony for consumers who choose to eat dark chocolate rather than milk or white chocolate for health reasons is that, while dark chocolate is lower in sugar, it contains greater levels of toxic elements because of the higher cocoa nib content.
“Choosing milk over dark may help those who indulge on chocolate,” said Shelke. “In the US, where people tend to favour milk over dark chocolate, they are getting chocolate with lower cocoa content, and therefore much lower cadmium content.”
Nevertheless, Shelke said that people should not hesitate about eating dark chocolate. “It takes a lot of chocolate to get to dangerous levels for adults,” she said.
Having a healthy and diverse diet with high amounts of fresh fruit and vegetables can also help mitigate the negative health effects of toxic elements such as trace heavy metals.
“It’s not what we eat; it’s what we absorb. Fibre and phytates in vegetables, whole grains, nuts, and especially legumes can significantly lower cadmium and lead bioavailability, ie what we absorb,” she said.
Shelke referred to an in vitro study, which found that cadmium bioavailability from cacao-2 cells exposed to animal-based foods (pork and clams) was higher than that from cocoa cells exposed to vegetable-based foods (kale).
“Antioxidants in plants have been shown to help inhibit the harmful effects of higher free radical production caused by cadmium exposure,” she added.