Cadmium is a naturally occurring heavy metal element that finds its way into the cocoa beans used to produce chocolate. Toxic in high amounts with chronic exposure, its presence presents a challenge for the global chocolate industry and the farmers who supply it. The presence of cadmium is generally not due to industrial pollution but is instead a complex issue rooted in geology, plant biology, and farming practices. Understanding the full pathway—from the earth’s crust into the cocoa tree and finally into the chocolate bar—explains why the concentration of this metal varies widely across different regions and products.
The Geological Origin of Cadmium
The primary source of cadmium contamination is the soil, where the element is naturally present in the Earth’s crust. Certain cocoa-growing regions, particularly those in Latin America, have soils enriched with cadmium due to specific geological processes. This includes areas with young, volcanic bedrock, as well as alluvial plains formed by rivers that have deposited mineral-rich sediments from mountain ranges like the Andes.
The cadmium found in these soils is considered geogenic, meaning it originates from the weathering of the underlying parent rock material. This leads to localized “hotspots” where the concentration of cadmium in the soil is significantly higher than in other cocoa-producing regions, such as those in West Africa.
How the Cocoa Plant Absorbs Cadmium
The cocoa tree, Theobroma cacao, readily absorbs cadmium from the soil through its root system. The plant’s root transporters, designed to take up essential micronutrients like zinc and calcium, often mistake the cadmium ion for these beneficial elements, a process known as ionic mimicry. Specific transporter proteins are believed to play a role in this indiscriminate uptake.
Once absorbed by the roots, the cadmium is translocated upward into the aerial parts of the tree, which is an efficient process in cocoa compared to many other crops. Research indicates that the metal tends to accumulate significantly in perennial woody structures, such as the stems and branches. From these storage areas, the cadmium is remobilized and transferred through the plant’s vascular system, making its way into the developing cocoa fruit.
The final destination of the metal is the cocoa bean, specifically the nib, which is the part used to make chocolate. Within the bean, the cadmium binds to specific organic molecules, such as phytate, to form stable complexes. Evidence suggests it is transferred via the phloem from the branches rather than being primarily remobilized from the leaves.
Farming Practices and Soil Conditions That Affect Cadmium Levels
The amount of cadmium a cocoa tree absorbs is heavily influenced by the soil chemistry and the farmer’s agricultural practices. Soil acidity is a major factor, as cadmium is significantly more soluble and available for plant uptake in acidic soils, typically with a pH below 6.5. Conversely, increasing the soil pH through the application of lime can chemically immobilize the cadmium, making it less accessible to the cocoa roots.
The presence of organic matter and clay in the soil also affects cadmium availability by binding the metal and reducing its solubility. Farming practices that increase soil organic carbon, such as certain agroforestry systems or the addition of compost, can help mitigate uptake. Furthermore, not all cocoa trees accumulate the metal equally, as there is a large genetic variation among different cocoa cultivars; some varieties are genetically predisposed to absorb and translocate far less cadmium into the beans than others grown in the same soil.
Cadmium Concentration in Finished Chocolate Products
The concentration of cadmium in the final chocolate product is directly related to the amount of non-fat cocoa solids it contains. Cadmium is concentrated in the cocoa nib, the core of the bean, which is ground to make cocoa liquor and cocoa powder. Since the metal is contained within these solids, post-harvest processing steps like fermentation, drying, and roasting do not remove significant amounts of cadmium from the bean.
This link explains the difference between various types of chocolate. Dark chocolate, made with a high percentage of cocoa solids, consequently has a higher average concentration of cadmium than milk chocolate. Testing has shown that dark chocolate samples can have a cadmium range up to 0.937 mg/kg, while milk chocolate samples often fall much lower, ranging from 0.009 mg/kg to 0.092 mg/kg. Consumers of dark chocolate carry a higher potential burden of the heavy metal due to the product’s ingredient profile.