Given these facts, I’m unwrapping my online delivery with a level of suspicion usually reserved for bomb disposal. Inside is a jar of wrinkled black beans, each resembling an elongated raisin. These are ‘tonka beans’ – the aromatic seed of a giant tree from deep in the Amazon rainforest.
When grated into desserts or infused into syrups, they impart a flavour so transcendent, tonka has been dubbed the most delicious ingredient you’ve never heard of.
Notes of freshly cut grass mingle with vanilla, liquorice, caramel and clove, topped off with a suggestion of warmth and a hint of magnolia – that is, according to the internet. I unscrew the lid and take a whiff. They smell faintly like furniture polish.
“As long as you don’t use a copious amount of it – obviously a copious amount could cause death – it really is delicious,” says Thomas Raquel, head pastry chef at the Michelin-starred Le Bernardin in New York, not particularly reassuringly.
Selling tonka beans to eat has been illegal in the US since 1954. Foods containing tonka are considered to be ‘adulterated’, though that hasn’t stopped them appearing on the menus of Michelin-starred restaurants, from New York to California. In fact, the United States is the biggest importer of tonka on the planet.
Tonka beans contain unusually high levels of the chemical coumarin, which gives them their flavour and is found naturally in hundreds of plants, including grass, lavender and cherries. Even if you’ve never seen a tonka bean in your life, there’s a good chance you know what they smell like without realising it.
Coumarin was first isolated from tonka beans in 1820 – the name comes from the Caribbean term for the tonka tree, ‘coumarou’ – shortly afterwards, an English chemist better known for inventing the first synthetic dye worked out how to make it in the lab.
By the 1940s, artificial coumarin was really taking off. As one of the first synthetic additives, it was dirt cheap. It was widely used in place of natural vanilla, added to chocolate, sweets and cocktail bitters, vanilla essence and even soft drinks. It swiftly became a staple ingredient in tobacco and lent its complex aroma to the perfume industry.
But there was a problem. Studies in dogs and rats had revealed it to be toxic, with relatively low levels causing considerable damage to the liver in just a few weeks. In sheep, just 5g (around two teaspoons) is fatal. Both tonka and coumarin were outlawed.
Fast-forward to 2017 and they’ve never quite disappeared. “Let’s just say I know where to get em’, it’s not a problem to get them,” says Paul Liebrandt, the former co-owner of the Corton in New York.
This is despite a government crackdown nearly a decade ago, including raids on several gourmet restaurants. Grant Achatz, who is head chef at Chicago restaurant Alinea, later told The Atlantic “They [the supplier] said, ‘Don’t be surprised if the FDA shows up soon’…. Two days later, they walked in: ‘Can we look at your spice cabinet?’”.
Cinnamon rolls were nearly banned in Denmark because nearly half those tested were found to contain high levels of coumarin.
Tonka and coumarin both still regularly turn up in Mexican vanilla flavouring, where they’re used to mask a low quality product. “I was talking to a vanilla purveyor recently and he offered me tonka bean paste,” says Raquel. “I was like ‘If I want to use tonka bean, I’ll use tonka bean.’”
Even if fancy restaurants aren’t your scene, there’s a good chance you’re being exposed from other sources. It’s still perfectly legal to add coumarin to tobacco and cosmetics, though it’s easily absorbed through the skin and the fragile lining of the lungs. The chemical is used copiously in detergents, shower gels, hand soaps and deodorants and blockbuster scents such as Coco Mademoiselle (Chanel) and Joop! Homme. It’s even found its way into e-cigarettes.
In fact, there’s a good chance you’ve got some coumarin lurking in your kitchen cupboards. True cinnamon is made from the bark of the plant Cinnamomum Zeylanicum and is native to Sri Lanka. This type naturally has extremely low levels of coumarin and proven medicinal properties, but that’s probably not what you’ve got in your spice rack. That’s because what we think of as cinnamon isn’t really cinnamon at all, but a Southeast Asian imposter made from the bark of the cassia tree.