Modern haute cuisine is working so hard to add scents to our plates: pillows of vaporized fresh-mown grass that vent at Grant Achatz's Alinea, bowls of smoke that seep at José Andrés's Minibar, hay brulée. With a flourish, the chefs at these restaurants call it "avant-garde cuisine"—drama and novelty are also very important. But according to the FDA, drama can sometimes be deadly.
Enter the tonka bean, a flat, wrinkled legume from South America with an outsize flavor that the federal government has declared illegal. Nonetheless, it proliferates on elite American menus. The tiniest shavings erupt in a Broceliande of transporting, mystical aromas.
The aroma of the tonka bean shavings is so affecting that it seems like an actual taste in the way that opium, which has no taste in the traditional sense, "tastes" like its rich, flowery smoke.
The taste of the tonka bean is linked strongly to its scent. "Scents," I should say, as the tonka bean has many at once. I register the aromas of vanilla, cherry, almond, and something spicy—a bit like cinnamon. When served cold—say, in tonka bean ice cream—the taste is like a vanilla caramel with dark honey. When warm, perhaps shaved over scallops, it moves toward spiced vanilla. Additionally, the aroma of the tonka bean shavings (it's almost always shaved) is so affecting that it seems like an actual taste in the way that opium, which has no taste in the traditional sense, "tastes" like its rich, flowery smoke.
The French have had "fièvre tonka" ("tonka fever")—an overused food-mag pun on fève, the French word for "bean"— for years. The French version of Saveur magazine features recipes that call for the tonka bean without fanfare. But in the United States it's a different story. Here, all foods that contain the chemical compound coumarin are considered by the FDA to be "adulterated" and have technically been illegal since 1954. Tonka beans are a major source of coumarin.
Before the law, refined coumarin was commonly added to commercial foods like cream soda, and used in synthetic vanillin. Extreme concentrations caused liver problems in rats (how unappetizing), and a rather overreaching ban on even natural sources of the compound was put in place. Coumarin has since been found to occur naturally in cinnamon, lavender, licorice, and a host of other commonly eaten plants—all of which would seem to be illegal under the regulation. Coumarin also accounts for the particular smell of fresh-cut grass and of fresh-dried hay (both in Alinea's grass-gas scent-pillows, and on your front lawn).
The fear of coumarin in the U.S. stems from the oft-repeated saw that it is a blood thinner. It's not. Coumadin® is the blood thinner trademarked by Bristol-Meyers Squibb. To make matters more confusing, Coumadin is made, in part, by changing the chemical structure of coumarin. Doctors who spoke with me (and who were terrified of being quoted) said there they're aware of no anti-coagulant effect from naturally occurring coumarin in general, or tonka beans in particular. In nature, only certain rare decomposition fungi can convert coumarin to the anti-coagulant molecule. Cows grazing on (pounds of) such rotting sweet clover led to the discovery of the Coumadin drug.