Tonka beans, prized for their intense flavour, contain coumarin, a chemical that can be lethal in large doses. The beans are used by chefs and food manufacturers, but their sale for consumption has been illegal in the US since 1954. Despite this, they appear in Michelin-starred restaurants and are smuggled into the country.
Coumarin occurs naturally in many plants, including tonka beans, and was first isolated in 1820. It became a cheap synthetic additive in the 1940s, used in vanilla substitutes, sweets, and tobacco. However, studies revealed its toxicity, causing liver damage in animals and fatal in sheep at just 5g.
In the UK, coumarin is still legal in tobacco and cosmetics, and is found in detergents, perfumes, and e-cigarettes. It also appears in some cinnamon products, particularly cassia cinnamon, which has higher coumarin levels than true cinnamon. Consumers may be unknowingly exposed to the chemical through everyday items.
While the risk from small amounts is low, experts advise caution. The European Food Safety Authority has set a tolerable daily intake, but some products may exceed this. Consumers are urged to check labels and limit intake of high-coumarin foods.
