measure for measure Some like it hot Working in a Mexican restaurant during his teenage years, Mark Buchanan discovered his love for jalapeños. Since then he has climbed higher and higher on the Scoville scale.

eautiful, dark-green jalapeño peppers The recipe for an extract is simple: will make your tongue burn — immerse pulverized chilli in very pure Bwonderful! But as I later discovered, alcohol, and let the alcohol draw capsaicin- the jalapeño is a mild creature compared containing oils into the solution. Later, to the bird’s eye chilli, which gives an evaporate off the alcohol. I have an extract aggressive fire to Asian curries and soups. in my kitchen — a nearly black oil with And that heat, I now know, bears no relation frightening tinges of exotic orange, which to the savage pain produced by a Naga or The Scoville scale measures around 1.2 million on the Scoville Bhut jolokia chilli, or the trauma of the Jalapeño: 3,500–8,000 scale. This is useful for ramping up the heat pointy-tailed Carolina Reaper, currently Bird’s eye: 50,000–100,000 of any salsa. But other extracts are just a the world’s hottest pepper. Bred as a hybrid Bhut jolokia: 800,000–1,000,000 click away: Psycho Serum is far hotter than in 2013 by ‘Smokin’ Ed Currie in South Naga Viper: 900,000–1,400,000 any natural chilli, scoring 6.4 million on Carolina, the Reaper reaches 2.2 million Carolina Reaper: 400,000–2,200,000 the Scoville scale. Or there’s Mad Dog 37 Pure capsaicin: 16,000,000 Scoville units on the scientific scale used to Plutonium coming in at 9 million Scoville measure the capacity of an edible substance units, or the elegantly named Death Cap to produce heat and pain. at a ludicrous 13 million, close to the That’s not too hot for thrill-seeking theoretical maximum of 16 million for enthusiasts who are creating new hybridized capsaicin-based extracts. peppers and chemically extracted chilli That number is attained only by a pure products, some of which soar far beyond crystalline extract of capsaicin — obviously the Reaper on the Scoville scale. Invented too dangerous to be ingested by any by American pharmacist Wilbur Scoville human being. Well, not really, as you can in 1912, the Scoville unit helps the food plants — binds to a key temperature see in a delightful video (https://go.nature. industry achieve chilli sauces with precise receptor, TRPV1, present on nerve fibres. com/2DK7T79) self-recorded by chilli levels of heat. For chilli extremists, it helps The result is an urgent message sent to extremist Darth Naga nearly a decade ago. distinguish the devoted chilli-head from the brain — “You’re on fire!” Forty-five seconds after swallowing some the dilettante. This mechanism reflects an evolutionary pure capsaicin extract, in agony and suffering Scoville’s original method — the Scoville adaptation by chilli plants, which spread from tremors and hiccups, Darth bravely organoleptic test — employed a panel of their seeds more effectively if birds, rather manages to put his experience into words: human tasters to determine how many than mammals, eat their fruit. Mammals “The heat is unfathomable!” before shutting times a substance needed to be diluted in chew food with teeth, destroying ingested off the video. Darth survived, fortunately. neutral solution until people tasted no seeds. In contrast, seeds pass easily through For now, 16 million Scoville units is the heat at all. Under this test, the heat of an the digestive tract of birds, emerging intact upper limit to the chilli experience — there’s extract of the mild jalapeño disappears after to be scattered far and wide. The burn nothing hotter, at least with the biomolecule being diluted about 5,000 times, yielding a of capsaicin makes most level-headed capsaicin. But other biomolecules carry Scoville rating of roughly 5,000, although mammals avoid chilli pepper plants, helping more punch than capsaicin. Resiniferatoxin, individual chillies fluctuate between 3,500 those plants thrive. a chemical produced by the cactus-like plant and 8,000. The hotter bird’s eye scores The exception is one species of mammal, Euphorbia resinifera, is the hottest chemical between 50,000 and 100,000. Homo sapiens, which seems hell-bent on currently known, 1,000 times hotter even Today, the multi-billion-dollar seeking the most intense chilli pain possible. than capsaicin. A pure extract of this stuff hot sauce industry measures Scoville The creator of the Carolina Reaper has scores 16 billion Scoville units. Eating just ratings with a faster and more objective recently claimed to have bred a new chilli, a gram or two would cause serious internal scientific method based on high-pressure named Pepper X, with a Scoville rating burns, and possibly death. ❐ liquid chromotography of pulverized over 3 million, well beyond the Reaper. chilli samples in solution. This method Pepper X awaits confirmation as the world’s Mark Buchanan detects the concentration of capsaicin, new hottest chilli by the Guinness Book of Science writer, based in Europe. the chemical responsible for chilli heat. Records. But those seeking insane levels of e-mail: [email protected] Chomp down on a Carolina Reaper capsaicin agony aren’t waiting for selective and your tongue burns because capsaicin — breeding, and have used chemistry to take Published online: 8 January 2020 a phytoalkaloid produced by chilli chilli extracts into new territory. https://doi.org/10.1038/s41567-019-0766-3

112 Nature Physics | VOL 16 | January 2020 | 112 | www.nature.com/naturephysics