How Do They Actually Measure the Heat of Hot Peppers?

In September 2013, food critic Matt Gross sat down at the Second Annual New York City Hot Sauce Expo and prepared to face a daunting challenge. Before him on the table lay three small, dark red, shrivelled peppers, which he was to eat as quickly as he could. Gross, a veteran spice connoisseur or “chilihead”, completed the challenge with ease, downing the three peppers in a little over 21 seconds. While at first the peppers seemed to have little effect, around a minute later they finally began to kick in:

“Dots of sweat started breaking out under my eyes, and then on my forehead. The burning began—not in the front of my mouth, where the capsaicin would have been had I really, really chewed the [peppers]—but in my throat, where the fiery flurry had flowed. I danced a little jig. I waved at everyone. I theatrically brushed my brow.

[But] this was pain I could take, pain I could savour. I could get through this—all I had to do was wait. And really, all I had to do was wait five or six minutes for the heat to reach a peak. After that, the pain and swelling began to subside, and the endorphin rush began to kick in. I felt great. No: I FELT GREAT! I was bouncing around, full of energy, talking to anyone and everyone I could.”

But this endorphin rush – the chilihead’s equivalent of the so-called “runner’s high” – came at a steep price, for later, as Gross was returning home from the expo:

“Sometime after 5:30, I began to notice a burning sensation just beneath my sternum. Strange, I thought: Why did it take so long for this to begin? Slowly, it began to grow in intensity, but still I thought nothing of it…But then, as I…descended into Penn Station to catch the subway home to Brooklyn, it got worse. Much worse. My breathing became laboured. I broke out in a cold sweat. I was hunched over, taking baby steps through the station. Finally, I had to pause, and sat on a ledge. To my right, a homeless man in a similar posture of misery. To my left, the same thing. The smell was not good. And yet all I wanted to do was curl up there on the ground and wait it out…It was like a white hot ball of nickel implanted just above my stomach—and again it made my want to stop in my tracks and lie down on the rain-drenched sidewalk till it passed.”

Gross had just experienced the searing wrath of the Carolina Reaper, which at 2 million Scoville Units holds the Guinness Record for the world’s hottest pepper. But what on earth is a Scoville Unit? How is the spiciness of peppers actually measured? And just what causes that burning sensation in the first place? Well, stock up on milk, antacid, and triple-ply toilet paper as we dive into the fiery and fascinating world of hot peppers.

What we commonly call “chili peppers” are the fruits of plants belonging to the genus capsicum, originally native to the Americas but now cultivated worldwide. What gives chilis their spicy kick is the compound 8-methyl-N-vanillyl-6-nonenamide – better known as capsaicin. A white, waxy solid in its pure form, capsaicin is a powerful tissue irritant and mild neurotoxin which directly stimulates the TPRV1 pain receptor channel in mammals. This means that spiciness is not actually a flavour but rather the sensation of pain. It also means that hardcore chiliheads are technically engaging in a form of masochism. Fun. So potent is capsaicin that even small amounts can overload nerve endings to the point of inducing numbness, making the compound useful as a topical analgesic for muscle pain, arthritis, and other mild aches.

Capsicum plants evolved to produce capsaicin in order to deter mammals from eating their fruits, for herbivorous mammals tend to have grinding molar teeth that can destroy the seeds before they can pass through the digestive system. Instead, capsicum seeds are largely dispersed by birds, whose nervous systems are impervious to the effects of capsaicin.

While capsicum plants have been cultivated and consumed for centuries, it was not until 1912 that a method for quantitatively measuring their spiciness or piquancy was developed by American pharmacist Wilbur Scoville. Though remembered today for his spiciness scale, during his lifetime Scoville was best known for his 1895 treatise The Art of Compounding, which remained a standard pharmacological textbook well into the 1960s. In 1912, Scoville was working for Detroit-based pharmaceutical giant Parke-Davis, which was developing a capsaicin-based topical ointment known as Heet Liniment. In order to determine the exact dose of capsaicin required for this ointment, Scoville first had to develop a calibrated scale for spiciness and an experimental method for gauging the spiciness of various peppers. In this method, known as the “Scoville organoleptic test,” chilis are dried, ground into a fine powder, and mixed into a solution of alcohol, water, and sugar. This mixture is then given to a panel of 5 tasters to consume. As the perceived spiciness of any given food varies widely from person to person, this panel allows an average value to be obtained. The pepper solution is then progressively diluted with water and given back to the panel until all members can no longer detect any spiciness. The number of dilutions needed to cross the sensory threshold, rounded to the nearest multiple of 100, thus becomes the heat rating of the pepper, expressed in Scoville Heat Units or SHU. For example, if a saturated pepper solution had to be diluted 50,000 times before the tasters can no longer detect it, that pepper is given a Scoville number of 50,000 SHU.

To give an idea of just how this scale works, a Bell Pepper, which contains no capsaicin, has a rating of 0 SHU, while the average Jalapeño clocks in at 3,500-8,000 SHU. The Scotch Bonnet, Red Savina habanero, and Bhut Jolokia or “Ghost Pepper” – previously among the hottest in the world – top out at 580,000 SHU, while police-grade pepper spray and the Carolina Reaper reach 1.5-2 million SHU. Chemically pure capsaicin, meanwhile, sits at a whopping 16 million SHU.

In the end, however, Parke-Davis never managed to perfect the use of capsaicin as a topical analgesic, and while Heet Liniment is still sold today, its active ingredient is methyl salicylate, a relative of acetylsalicylic acid or Aspirin. And while Wilbur Scoville’s scale is still used today, his method for measuring spiciness is not. The major flaw with Scoville’s organoleptic test was its dependence on human perception, which is extremely subjective and varies widely according to a number of factors, including a person’s age, the number of pain receptors in their mouths, their particular palate, and their experience with eating spicy foods. Sensory fatigue can also affect the test results, causing tasters’ palates to become less sensitive over time. Heat ratings for the same pepper can thus vary by as much as 50% between different laboratories. Consequently, since the 1980s high-performance liquid chromatography or HPLC – which measures the rate at which different compounds in a mixture migrate along a column of porous material – to determine the precise capsaicin content of different peppers. This method yields results in what the American Spice Trade Association refers to as “pungency units” – equivalent to one part capsaicin per million parts dried pepper mass. These units can easily be converted into Scoville Heat Units by multiplying them by 16. But while this method is far more accurate than the old organoleptic test, measuring spiciness is still not an exact science. The spiciness of any given pepper of the same species can vary widely due to seed lineage, growing conditions and other factors, while the spiciness of a sauce made from a given pepper depends on its water content. For this reason, the Scoville rating of a particular pepper variety is based on an average of multiple samples, while the heat of sauces is adjusted according to something called the “D-value”, defined as the total mass of the sauce divided by the mass of its dry chili component.

Indeed, the tendency of peppers of the same variety to vary widely in spiciness is the reason the Carolina Reaper still retains the Guinness World Record, despite several ostensibly spicier peppers having been developed since. For many years, the world’s hottest pepper was the 577,000 SHU Red Savina Habanero, discovered in 1994 by Frank Garcia of GNS Spices Inc. in Walnut, California. In 2007, however, the Red Savina was dethroned by a variety of the Indian Bhut Jolokia or “Ghost” pepper grown by New Mexico State University, which clocked in at 1,001,304 SHU. But even this was outdone in 2011 by the Carolina Reaper, created by Ed Currie of the delightfully named Puckerbutt Pepper Company of Fort Mill, South Carolina. Created by crossing a Pakistani Naga with a Red Habanero from St. Vincents in the Caribbean, the Carolina Reaper was officially certified at 2 million SHU. Currie has since bred a new pepper called “Pepper X” which he claims reaches 3.18 million SHU, but unfortunately the spiciness of each individual pepper varies so widely that the average SHU falls below that of the Carolina Reaper. Similarly, the “Dragon’s Breath” pepper created by English chili farmer Neal Price and Nottingham Trent University has been measured at 2.48 million SHU, but once again its high variability has prevented it from dethroning the venerable Carolina Reaper.

While the Scoville scale was originally developed to measure the effects of capsaicin, other plant compounds can also stimulate the same pain receptors. These include piperine, the active ingredient in black pepper, and gingerol, found in – you guessed it – ginger – which register at 100,000 and 60,000 SHU, respectively. But by far the most potent non-capsaicin compounds are Resiniferatoxin and Tinyatoxin produced by Euphorbia resinifera and Euphorbia poissonii, cactus-like plants native to northern Nigeria. These substances, also known as RTX and TTX, stimulate the same TPRV1 nerve channels as capsaicin, but are 1,000 times more potent, meaning that their pure forms would have a truly incomprehensible Scoville heat rating of 16 Billion SHU. Thankfully, these toxins haven’t found their way into any hot sauces, though they are used by the people of northern Nigeria as natural pesticides. Furthermore, due to the ability of RTX and TTX to selectively target pain receptors while leaving other nerve cells untouched, these compounds are currently being studied as possible treatments for chronic pain.

All of this brings us to an important question: since capsaicin is a neurotoxin, could you actually die from eating too many hot peppers? The answer, interestingly, is yes. A 1980 study by researchers at Mahidol University in Thailand determined that, extrapolating from experiments conducted on rats, around 170 grams of pure capsaicin is enough to kill an average 150 pound or 68 kilogram person. Death in these experiments resulted from the powerful inflammatory response triggered by the capsaicin, which caused widespread dilation of the subject’s blood vessels and a catastrophic drop in blood pressure. Thankfully, however, consuming this much capsaicin would require you to eat 3 pounds or 1.4 kilograms of dried and powdered Carolina Reaper in a single sitting – a daunting ordeal that is likely all but impossible for the average person to endure. So for all you diehard chiliheads out there: you are safe – though the same likely can’t be said of your bathroom….