Flavor

by Bob Holmes

  When people talk about flavor, they usually focus on taste and smell, just as we have so far. But there’s a third major flavor sense, as well, one that’s often overlooked: the physical sensations of touch, temperature, and pain. The burn of chili peppers is the most familiar example here, but there are others. Wine mavens speak of a wine’s “mouthfeel,” a concept that includes the puckery astringency of tannins—something tea drinkers also notice—and the fullness of texture that gives body to a wine. Gum chewers and peppermint fans recognize the feeling of minty coolness they get from their confections. And everyone knows the fizzy bite of carbonated drinks.

  None of these sensations is a matter of smell or taste (though of course Coca-Cola also tastes sweet and smells of caramel, citrus, and other flavors). In fact, our third primary flavor sense flies so far under our radar that even flavor wonks haven’t agreed on a single name for it. Sensory scientists are apt to refer to it as “chemesthesis,” “somatosensation,” or “trigeminal sense,” each of which covers a slightly different subset of the sense, and none of which mean much at all to the rest of the world. The common theme, though, is that all of these sensations are really manifestations of our sense of touch, and they’re surprisingly vital to our experience of flavor. Taste, smell, touch—the flavor trinity.

  Sensory scientists have known for decades that chili burn is something different from taste and smell—something more like pain. But the real breakthrough in understanding chili burn came in 1997, when pharmacologist David Julius and his colleagues at the University of California, San Francisco, finally identified the receptor for capsaicin, the active ingredient in chili heat. The task demanded a lot of patience: Julius and his team took every gene active in sensory nerve cells, which respond to capsaicin, and swapped them into cultured kidney cells, which don’t. Eventually, they found a gene capable of making the kidney cells respond. The gene turned out to encode a receptor—eventually named TRPV1, and pronounced “trip-vee-one”—that is activated not just by capsaicin but also by dangerously hot temperatures. In other words, when you call a chili pepper “hot,” that’s not just an analogy—as far as your brain can tell, your mouth really is being burned. That’s a feel, not a smell or taste, and it passes to the brain through nerves that handle the sense of touch. Like other touch receptors, TRPV1 receptors are found all over the inner layer of your skin, where they warn you of burn risk from midsummer asphalt, baking dishes straight from the oven, and the like. But they can only pick up pepper burn where the protective outer skin is thin enough to let capsaicin enter—that is, in the mouth, eyes, and a few other places where the sun doesn’t shine. This explains the old Hungarian saying that “good paprika burns twice.”

  Further tests showed that TRPV1 responds not just to heat and capsaicin but to a variety of other “hot” foods, including black pepper and ginger. More recently, several more TRP receptors have turned up that give other food-related somatosensations. TRPA1, which Julius calls the “wasabi receptor,” causes the sensation of heat from wasabi, horseradish, and mustards, as well as onions, garlic, and cinnamon. TRPA1 is also responsible for the back-of-throat burn that aficionados value in their extra-virgin olive oil. A good oil delivers enough of a burn to cause a catch in your throat and often a cough. In fact, olive oil tasters rate oils as “one-cough” or “two-cough” oils, with the latter getting a higher rating. (One reason wasabi feels so different from olive oil is that the sulfur-containing chemicals in wasabi are volatile, so they deliver wasabi’s characteristic “nose hit,” while nonvolatile olive oil merely burns the throat. Olive oil may also trigger TRPV1 receptors to some extent.) Curiously, TRPA1 is also the heat receptor that rattlesnakes use to detect their prey on a dark night.

  TRPM8—which is triggered by cool temperatures, rather than warm ones—is responsible for the cooling sensation of menthol (from mint) and eucalyptol (from eucalyptus). Food companies love TRPM8, by the way, because there’s a huge market for the mouthfeel of coolness in, for example, gums and mouthwashes. In fact, the menthol that used to make chewing gum minty fresh has largely been replaced by other molecules that do a more effective job of triggering TRPM8, so that your gum holds its minty flavor longer.

  Now that scientists understand the receptors that account for these perceptions, we can begin to unpack some of the subtleties of chili pepper flavor. Chili aficionados get pretty passionate about their pods, choosing just the right kind of chili for each application from the dozens available. The difference among chili varieties is partly a matter of smell and taste: Some are sweeter, some are fruitier, some have a dusky depth to their flavor. But there are differences in the way they feel in your mouth, too.

  One difference is obvious: heat level. Chili experts measure a chili’s level of burn in Scoville heat units, a scale first derived by Wilbur Scoville, a pharmacist and pharmaceutical researcher, in 1912. Working in Detroit—not exactly a hotbed of chili cookery, especially back then—Scoville had the bright idea that he could measure a pepper’s hotness by diluting its extract until tasters could no longer detect the burn. The hotter the pepper was originally, the more you’d have to dilute it to wash out the burn. Pepper extract that had to be diluted just tenfold to quench the heat scores ten Scoville heat units; a much hotter one that has to be diluted one hundred thousandfold scores one hundred thousand Scovilles. Nowadays, researchers usually avoid the need for expensive panels of tasters by measuring the chili’s capsaicin content directly in the lab and converting that to Scoville units. The more capsaicin, the hotter the chili.

  However you measure it, chilis differ widely in their heat level. Anaheims and poblanos are quite mild, tipping the scale at about 500 and 1,000 Scovilles, respectively. Jalapeños come in around 5,000, serranos about 15,000, cayennes about 40,000, Thai bird’s eye chilis near 100,000, and the habanero on my table somewhere between 100,000 and 300,000 Scovilles. From there, intrepid souls can venture into the truly hot, topping out—as of this writing—with the Carolina Reaper at a staggering 2.2 million Scovilles, which approaches the potency of police-grade pepper spray. Check out the online videos of hardy souls eating these peppers. “Successful” consumers sometimes end up in an ambulance. “They’re in pain, they swallow, they retch for a couple of hours. I miss the point,” says Bruce Bryant, who studies hot peppers and other mouthfeel perceptions at the Monell Chemical Senses Center. “I’m not a big pepper eater,” he adds. “I was 30 years ago, but I don’t have anything to prove any more about how much pain I can stand.”

  For those with an extreme mind-set, it’s worth noting that pure, unadulterated capsaicin packs a whopping sixteen million Scovilles. If you’re looking for the ultimate in hot, though, a chemical called resiniferatoxin—found in a Moroccan spurge plant—delivers sixteen billion Scovilles in its purified form. That’s easily enough to cause lethal chemical burns. People don’t eat the stuff.

  Returning to the culinary realm, many chili heads claim that a pepper’s heat is defined by more than just intensity. If anyone would know about this, I figured, it would probably be Paul Bosland, the director of the Chile Pepper Institute at New Mexico State University. Bosland’s institute has its fingers in all things chili—and, as a plant breeder by trade, he has a keen professional interest in all the tiny details of how chili heat differs from one pod to the next.

  Bosland says he and his colleagues distinguish four other components to chili heat in addition to heat level. The first is how fast the heat starts. “Most people, when they bite the habanero, it maybe takes 20 to 30 seconds before they feel the heat, whereas an Asian chili is immediate,” he says. Chilis also differ in how long the burn lasts. Some, like jalapeños and many of the Asian varieties, fade relatively quickly; others, like habaneros, may linger for hours. Where the chili hits you also varies. “Usually, with a jalapeño, it’s the tip of your tongue and lips, with New Mexico pod types it’s in the middle of the mouth, and with a habanero it’s at the back,” says Bosland. And fourth, Bosland and his crew distinguis
h between “sharp” and “flat” qualities of burn. “Sharp is like pins sticking in your mouth, while flat is like a paintbrush,” he says. New Mexico chilis tend to be flat while Asian ones tend to be sharp—a quality I certainly noticed the last time I ate Thai food.

  Some of that difference no doubt arises because chili burn is about more than just capsaicin. In fact, there are at least twenty-two different chemicals in the capsaicin family, each of which tickles the TRPV1 receptors a little differently. Nordihydrocapsaicin, for example, seems to burn more at the front of the mouth, while homodihydrocapsaicin hits more at the back of the throat, explains Michael Mazourek, a chili breeder at Cornell University. But the difference matters less than you’d think. Nordihydrocapsaicin, for example, is only half as potent as capsaicin, and makes up at most 7 percent of the capsaicin content. “So you can see its vote doesn’t count for much, even though it would have some different properties,” says Mazourek. The two most common capsaicinoids—capsaicin itself and dihydrocapsaicin, which together make up 50–90 percent of the capsaicinoids in every pepper variety—are also by far the most potent, so they pretty much run the show.

  A bigger reason why some peppers feel different from others, Mazourek thinks, may be something that food scientists call “matrix effects.” In order to perceive a pepper’s heat, the capsaicin has to get out of the pepper’s cells and onto your tongue, lips, or palate. Pepper varieties with tougher cells would release their capsaicin more slowly as you chew, giving you a burn that builds more slowly, and perhaps hits you farther back in the mouth. Oil content might affect how quickly the capsaicin washes away, thus affecting the burn’s duration.

  None of this explains the difference between “sharp” and “flat” chili heat, though. In fact, no one I talked to had a good explanation for this phenomenon. Mazourek and Julius both punted. Bosland suggested that different capsaicinoids could be involved, but he admitted he had no data to back up that conjecture. And Bryant wondered whether everyone is just fooling themselves into thinking there’s a difference. “All you need to do is tell somebody that it’s going to be different,” he says. “I’m really skeptical about some of these reports.”

  Okay, enough theory. I’ve put off my chili tasting as long as I could, and now it’s time to take the plunge. First up, the jalapeño. As you’d expect from its relatively wimpy ranking in the hot pepper standings, it gives only a mild burn, which builds gently and mostly at the front of the mouth. (Score one point for Bosland.) Confronted with such a tame burn, I have plenty of attention left to focus on its thick, crisp flesh and sweet, almost bell-peppery flavor.

  The Thai bird’s-eye chili, second on my list, is much smaller, and its flesh proves to be much thinner and tougher. Despite that, though, it almost immediately lets loose a blast of heat that explodes to fill my mouth from front to back, making me gasp for breath. No gradual build to this one—it’s a sledgehammer blow. If I think hard, I might imagine that the chili heat is a little bit sharper, pricklier, than the jalapeño. But I could just be fooling myself.

  Finally, the one I’ve been dreading, the habanero. I cut a tiny slice (call me a coward—I never signed up for the full three-hundred-thousand-Scoville experience) and start chewing. The first thing that strikes me is how different the flavor is. Instead of a vegetal, bell pepper flavor, the habanero gives me a much sweeter, fruitier impression that’s surprisingly pleasant. For about fifteen or twenty seconds, anyway—and then, slowly but inexorably, the heat builds. And builds. And builds, long after I’ve swallowed the slice of pepper itself, until I can’t think of much else besides the fire that fills my mouth. It definitely hits farther back in the mouth than the Thai chili, though there’s a late-breaking flare-up on my tongue as well. The whole experience lasts five or ten minutes, and even now—a good half hour later—it’s as though coals are gently banked in my mouth. Wow.

  Having set my mouth afire, I’d now like to quench the burn. Surprisingly, scientists can’t offer a whole lot of help in this regard. A cold drink certainly helps, because the coolness calms the heat-sensing TRPV1 receptors that capsaicin excites. The only problem—as you’ve no doubt noticed if you’ve tried to cope with a chili burn this way—is that the effect goes away in just a few seconds, as your mouth returns to normal body temperature.

  You’ve probably heard, too, that sugar and fat help douse the fire, but the researchers themselves aren’t entirely convinced. “The best thing out there is probably cold, whole milk,” says John Hayes of the University of Pennsylvania. “The cold is going to help mask the burn, the viscosity is going to mask the burn, and the fat is going to pull the capsaicin off the receptor.” When pressed, though, he notes that there’s not a lot of data to back that up. Making a food more viscous has been shown to damp down taste—probably just because it provides a competing sensation to distract our attention, he notes, but he can’t think of anyone who’s tested whether it also reduces chili burn. And he’s not entirely sure that sugar really helps, either. “I’m not convinced that it actually knocks the heat down, or whether it just makes it more pleasant,” he says. Even the value of fats or oils—which sounds like they ought to help wash capsaicin, which is fat soluble, off the receptors—is in dispute. If you’re feeling the burn, says Bryant, the capsaicin has already penetrated your tissue, so a superficial rinse of whole milk or olive oil isn’t going to help much. Instead, Bryant has another suggestion: “Go kick a brick wall or take a hammer to your thumb. You’ll forget all about your tongue,” he says.

  Bryant’s tongue is firmly in his cheek there. At least I think so, since I don’t know of any hot pepper fans who are actually tempted to smash their thumb with a hammer to cope with the burn. In a way, that highlights what might be the most fascinating aspect of chili peppers, wasabi, and their ilk. Millions of people actively seek out the pain of hot chilis as a form of pleasure. The burn features prominently in more than a few of the world’s great cuisines, with more than a quarter of the world’s population eating hot peppers daily. More than three quarters of Americans in one recent survey expressed an interest in eating hot chilis, and the British spend £17 million (approximately $22 million, at the time of this writing) annually on hot sauce.

  And yet, only the oddest among us makes a similar fetish of inflicting other sorts of pain. We don’t take pleasure in eating food that’s still searingly hot from the oven, even though that delivers exactly the same sensation we get from chilis: same receptors, same nerves. We don’t choose to chemically burn our tongues with strong acids. We don’t hammer our thumbs for fun. So why do we happily, even eagerly, inflict pain by chilis? Whatever the secret is, it seems to be unique to humans. No other mammal on the planet has a similar taste for chilis. (Birds eat them enthusiastically, but only because they lack receptors that respond to capsaicin. To a parakeet, the hottest habanero is as bland as a bell pepper.)

  One possible explanation is that chili lovers simply don’t feel the pain as intensely as those who shun hot peppers. In the lab, it’s certainly true that people who are repeatedly exposed to capsaicin become less sensitive to it. In fact, liniments such as Heet and Icy Hot contain capsaicin for its painkilling properties. It’s easy to imagine that something similar might be happening to chili eaters, too, because people who eat hot peppers more often report less burn from a given test dose of capsaicin. But when you look more closely at that result, it gets less convincing. For one thing, an inexperienced chili eater might think a sample is the hottest thing they’ve ever eaten and rate it nine out of ten for burn (“I could light a cigarette from my mouth!” a shocked Romanian friend exclaimed once during an Indian dinner), while an old hand might say, “Hah, I’ve had hotter,” and give the same sample a five. Alert researchers can avoid this problem by following Linda Bartoshuk’s lead and pegging the upper end of the scale as “the most intense sensation of any kind you’ve ever experienced,” but not every study does so.

  Genetics may play some part, too. Studies of identical twins (who sh
are all their genes) and fraternal twins (who share only half) suggest that genes account for between 18 and 58 percent of our liking for chili peppers. Some people may have more sensitive TRPV1 receptors, for example—though Hayes, who’s looking into that now, says, “The jury is really still out on whether there is meaningful TRPV1 variation.” Similarly, some (but not all) studies have found that chilis cause more pain in supertasters or those with more fungiform papillae (and therefore, presumably, more pain nerve endings) on their tongue.

  It’s abundantly clear, though, that chili lovers aren’t immune to the pain. Just ask one. “I like it so all my pores open up and tears are rolling down my face,” says Hayes. “But with two young kids in the house, I don’t get that very often.” For now, Hayes gets his pain mostly from a handy bottle of Sriracha hot sauce. “My kids refer to it as Daddy’s ketchup,” he says.

  It’s clear from listening to Hayes that he—and probably most other chili eaters—actively enjoys the pain. That paradox has drawn the attention of psychologists for several decades now. Back in the 1980s, pioneering chili researcher Paul Rozin of the University of Pennsylvania proposed that chili eating is a form of “benign masochism,” like watching a scary movie or riding a roller coaster. After all, most forms of pain are warnings of imminent harm. That baked potato still steaming from the oven is hot enough to kill the cells lining your mouth, potentially causing permanent damage. The hammer landing on your thumb can break bones. But chili burn—except at its uppermost, million-Scoville extreme—is a false alarm: a way to get the thrill of living on the edge without the risk of exposing yourself to real danger.