Flavor

by Bob Holmes

  Over the years, Laska has used this method on everything from bats to mice to elephants to several species of monkey. Out of curiosity, he compared his results with what other researchers had reported for humans—and noticed that the animals weren’t necessarily any better smellers than we are. Intrigued, he started searching the literature for every study he could find that reported an olfactory threshold for a nonhuman animal, then looked to see if he could find a comparable threshold for humans.

  The results showed that his initial comparisons weren’t a fluke. Human noses are more sensitive than those of rats for thirty-one of the forty-one chemicals that have been tested on both species, for example. Humans even outperform dogs in detecting five of fifteen scents. “The traditional textbook view that humans have a poorly developed sense of smell is not warranted,” says Laska. “We are not that hopeless.”

  If so, why do customs agents use beagles instead of Bostonians to detect drug smugglers? Why don’t we track our dogs through the park as readily as they track us? Part of the difference may be that most of the time, we’re distracted by our senses of sight and sound. “Except for smell researchers such as me,” says Laska, “we are not constantly aware of the odor stimuli in our environment.” For one thing, it’s simply harder to pay attention to smells than to sights or sounds. If you’re looking for a friend’s face in a crowd, or scanning a bookshelf for a particular title, your vision is focused on a specific point in space. Similarly, when you’re trying to listen to one conversation amid a noisy cocktail party, you’ll turn to face the speaker and concentrate on that one spot. This tight spatial focus helps us notice what we see and hear.

  By contrast, we don’t ordinarily focus our smelling in the same way. Sure, you can stick your nose into a wine glass, or take a sniff at the back of a toddler’s diaper—both instances where we really do pay attention to odors. But that’s not the way we usually use our noses. For most of the day, our noses aren’t focused on any one particular thing. Instead, we smell an undifferentiated mix of everything that’s going on around us, the olfactory equivalent of peripheral vision with nothing in the center of focus. Even when we’re trying to pay attention to a particular odor—what’s that herb in this sauce?—studies show we don’t get any better at detecting that target.

  Subconsciously, we probably make a lot more use of smell than we think. For example, did you know that you tend to smell your hand shortly after shaking hands with someone? Well, you do. We all do. Sobel—him again—secretly filmed unsuspecting students who thought they were waiting idly to participate in a psychology experiment. The experimenter came in, introduced him- or herself—sometimes with a handshake, sometimes not—then left the room again. Within seconds, the students who had shaken hands would lift their hand to their nose and sniff it—especially if the experimenter was of the same sex as the student. “We would see people sniffing themselves just like rats,” Sobel told a reporter. Clearly, we’re taking in information of some sort, even though we’re not aware of it. (Knowing this may forever taint your experience of greeting people.)

  Sight and sound also come to us in a continuous stream, while smell comes in discrete sniffs, separated by several seconds of “olfactory silence.” That may not seem like an important difference, but it is. Continuity makes it much easier to notice changing sights and sounds—and when there’s a break in continuity, we often become “change blind.” In one famous experiment, an actor carrying a map approached an unsuspecting pedestrian and asked for directions. Before the person finished giving the directions, an annoyingly oblivious pair of “workers”—actually accomplices in cahoots with the experimenters—barged between the two carrying a large door. While the view was blocked, a second actor took the first one’s place. After the workers left, half the people simply resumed giving directions and never even noticed that they were now talking to a different person. They were blind to the change that happened during the visual gap.

  If change blindness affects even frontline senses like vision, it’s likely to be even more significant to our sense of smell, where the equivalent of a large door passes through after every breath. This change blindness makes the changing smellscape much harder to keep track of, and is another reason why we don’t notice smells the way we do sights and sounds, says Sobel.

  But there’s an even simpler reason why we humans don’t often pay as much attention to smells as our dogs do. Dogs’ noses are down there near the ground where most of the smells are, while ours are way up in the air. Except in unusual circumstances, like Sobel’s human chocolate hounds, we simply aren’t aware of the rich olfactory world of scent trails that we could be monitoring.

  Our noses may be poorly positioned for following scent trails on the ground, but there’s another class of odors that they’re perfectly positioned to appreciate: those that contribute to the flavors of food and drink. In fact, we humans might be the virtuosos of the flavor world. To understand why, we need to recognize that what we think of as “the sense of smell” is really two different senses that share the same equipment, like taxi drivers sharing a car on alternate shifts.

  Until now, we’ve been talking mostly about smell as a process of sniffing air in through the nostrils to the olfactory epithelium. This kind of smelling tells you about what’s out there in the world: fragrant flowers, burning leaves, your nearby lover. Experts call it orthonasal olfaction, but it’s fine to think of it as just sniffing.

  But there’s another route that odor molecules can take to get to the olfactory epithelium: through the backdoor. This retronasal olfaction only happens when we eat or drink something. As we exhale, some of the odor compounds from the food or drink rise up the back of the throat and into the nasal cavity from the rear. In fact, the shape of our throat helps push food odors into our nasal cavity as we exhale. To show this, Gordon Shepherd and his colleagues used CAT scans to determine the precise shape of the nose, mouth, and throat of a fifty-eight-year-old volunteer, then used a 3-D printer to build a full-scale model of her anatomy. When they measured airflow through this model, they found that air inhaled through the nose forms an air curtain in the throat that effectively walls off the mouth, so that food particles and odor molecules from a mouthful of food aren’t swept into the lungs. (That’s a good, practical reason for chewing with your mouth closed: airflow through an open mouth disrupts the air curtain.) The curtain also ensures that our orthonasal sniffing isn’t contaminated with food aromas from the mouth. But when we exhale, this air curtain shuts off, so that odor molecules from the mouth can eddy up into the nasal cavity and reach the olfactory epithelium. Retronasal olfaction, in other words, is all about flavor.

  And according to Shepherd, retronasal olfaction is a skill that we humans are uniquely good at. Think about how the shape of a dog’s head compares with yours. The dog has a long snout and its head projects forward from the neck, so that its nasal cavity sits well forward of the back of the mouth. As a result, the retronasal path to the olfactory epithelium is a long journey down a narrow tube, and relatively few odor molecules are likely to make the trip. Dogs’ noses, in other words, are optimized for orthonasal smelling. In contrast, humans have relatively short noses. More importantly, our upright posture means that instead of projecting forward, our heads sit immediately above our necks, so that retronasal odor molecules just have to waft up a short way from the back of the mouth to the olfactory epithelium. It’s a much shorter, easier path, and it’s reasonable to think that our retronasal olfaction—and therefore, our appreciation of flavor—is correspondingly better. (We also have bigger brains to think about the flavors we taste, which further sharpens our appreciation. More on that in a later chapter.) The result is that when you sit back and appreciate the complex flavors of a soup or a glass of wine, you’re doing something that few other species—perhaps none—would be capable of. We should feel special!

  The existence of these two ways of smelling might explain one of the peculiarities in our experience of flavor. Most
of the time, sniffing a food tells us pretty much what flavor we’re going to get when we eat it—but not always. We can all think of foods—really stinky cheeses such as Limburger come to mind, and the notorious Asian fruit known as durian—that smell vile as you’re trying to work up the courage to eat them, yet “taste” divine once you actually put them in your mouth. Similarly, almost everyone loves the smell of freshly brewed coffee, but not everyone likes its flavor. Those differences—one professional flavorist told Mainland that they happen for about 15 percent of odors—would make sense if we respond differently to orthonasal smells than to retronasal ones.

  Confirming this scientifically is easier said than done, because it’s not easy to study retronasal olfaction. You can’t just squirt a dose of coffee in someone’s mouth, since that also delivers taste and touch signals that aren’t there when you wave a cup of the stuff under their nose. Instead, scientists have to go all techno and thread two plastic tubes into the nose so that one opens just inside the nostrils and the other at the top of the throat. Then they can use a computer to deliver precise doses of odorant to either the orthonasal or the retronasal tube, while flowing unscented air through the other tube to avoid any telltale puff-touch signals.

  These studies show that retronasal odors are indeed handled differently from orthonasal ones. For one thing, olfactory thresholds tend to be lower for smells that arrive orthonasally. That makes sense: Orthonasal delivers early warnings of changes in the environment, which would need the most sensitive detector available; retronasal, on the other hand, perceives the flavor of foods that are already in the mouth—there’s plenty of stimulus there, and it only needs to pick out the distinctive features so you can identify what you’re eating. And in keeping with that division of labor, retronasal odors turn out to be more effective at stimulating the brain regions responsible for processing flavor.

  There’s likely to be a physical reason, too, why the same food might yield a different experience orthonasally and retronasally, and that has to do with the direction of airflow. Researchers haven’t worked out the details yet, but it’s becoming clear that our four hundred or so odor receptors aren’t scattered randomly across the olfactory epithelium, but instead are sorted into several zones with a different mix of receptors in each. In particular, our most ancient odor receptors—inherited from our fish ancestors, and tuned to water-soluble odorants, the only kind that fish could experience—are clustered right at the front of the olfactory epithelium. That means they get first crack at orthonasal odors, but are last in line for retronasal ones. By the time retronasal airflow reaches these fishlike receptors, many of the water-soluble odorants may have already dropped out, mired fruitlessly in the watery nasal passages farther back. Sure enough, Sobel (again!) has found evidence that the nose is actually sorting odors from front to back in the nose. The world smells different to each nostril, he finds, with the higher-airflow nostril more attuned to non-water-soluble odors, which the orthonasal air current carries farther back to the relevant receptors. For the same reason, odorants should sort differently when inhaling orthonasal smells than when exhaling retronasal ones. The real clincher would be if someone could show that the wonderful aromas of fresh coffee, and the obnoxious aromas of ripe cheeses and durian, tended to be water soluble so that they’re more accessible orthonasally than retronasally. Unfortunately, no one has done that yet, as far as I know.

  The same day that I’d talked with Shepherd in Florida about retronasal olfaction, I unexpectedly put my newfound knowledge to use. I ate dinner that night in a cheap-but-excellent Mexican restaurant not too far from my cheap-but-adequate motel. I ordered a Negra Modelo, my favorite Mexican beer, and the waiter set the bottle on the table. I was about to ask for a glass—I’ve always been a bit of a snob about drinking my beer out of a glass “to appreciate the flavor better”—when I recalled something Shepherd had told me that afternoon. We forget what we know about retronasal smell, he said, as soon as we sit down to eat. “Think about it. Most of the flavor is when you’re breathing out.” Aha, I thought. The glass won’t do anything for the flavor of the beer—it will only enrich my orthonasal experience, which is different. I drank my beer from the bottle—and sure enough, the flavor was all there.

  But what flavor was it? Let’s pause that scene—me with beer bottle to mouth, enjoying the chocolate and caramel flavors of the Negra Modelo—and ask whether someone else would have the same flavor experience. We already know that people differ in their taste receptors, so that your experience of the beer’s hoppy bitterness could be different from mine. And we already know that even people who taste a lot of bitter—like me—sometimes learn to love it in their beer. But since the lion’s share of flavor comes through retronasal olfaction (remember the jelly bean test!), it’s also worth looking at how people differ in their sense of smell.

  We’ve already seen that people have about four hundred odor receptors, more or less. Here’s where things get interesting. Of those four hundred, about half work in everyone, so all of us can smell the molecules they target. The other half work in some people and not others, which means there’s a huge range of stuff that some of us can smell and others can’t. To further complicate things, even the working receptors often have small genetic differences from person to person, so that you might be more sensitive to certain odors than I am, and vice versa. In fact, the sample of one thousand genomes showed that you and I are likely to have meaningful differences—that is, differences that affect odor detection—in about 30 percent of our odor receptors. That means your flavor world is different from mine, and from your best friend’s, and even from your parents’. Chances are that no two people (except, perhaps, identical twins) share exactly the same sense of smell. Every one of us lives in their own unique flavor world.

  Not only does each person have their own distinctive set of working and broken odor receptors, but every person’s nose probably mixes its receptors in different proportions. The evidence for this comes from Darren Logan, a virtuoso molecular geneticist at the Sanger Institute in Cambridge, England. Logan is a slender, compact bundle of energy with trendy glasses, dark hair cut in a short buzz, and a fascination with olfactory receptors. In particular, he’s used gene-sequencing technologies to measure the abundance of each of the hundreds of olfactory receptors in the nose. There’s a catch, though: To properly census an individual’s complete repertoire of receptors, he needs to study entire noses—or, more precisely, entire olfactory epithelia. It’s hard to convince a living person to sacrifice their sense of smell for science, and tissue from cadavers, even fresh ones, hasn’t been good enough. So Logan works on mice instead.

  Mice use all 1,099 of their working odor receptors in their nose—but not in equal proportion, Logan finds. Instead, a few of the receptors are very common, a slightly larger number are moderately common, and most are rare. And that pattern seems to be dictated by genes. One of the advantages of working with mice is that you can pull out a catalog from a mouse-supply company and buy as many genetically identical animals as you want, from any of several very different strains. Sure enough, when Logan compares two genetically identical mice, they have exactly the same pattern of odor receptor frequencies. In other words, when it comes to the mix of odor receptors in a nose, genes rule. Pick a different mouse strain, and the pattern is much different. Take a mouse from a different subspecies, and the differences are bigger still, with half the receptors differing in abundance by as much as a hundredfold. “That means one strain is, in theory, a hundredfold more sensitive to whatever that receptor is detecting,” says Logan.

  We have to be cautious about extrapolating from mice to people—plenty of researchers have ended up with egg on their face from doing that too glibly—but if people are like mice in this respect, then not only do you and I have slightly different sets of working odor receptors but we’re probably genetically programmed to mix them in different proportions. If so, then the olfactory chord that coffee sounds in your brain might be
richer in the horns, while mine is richer in the strings. That would help to make your flavor world even more different from mine. As I write this, Logan is trying hard to secure fresher, better human olfactory epithelia so that he can test this idea directly. He’s got nine so far, donated by living people who were about to lose them anyway as a result of treatment for a rare cancer, but he needs a lot more. Stay tuned.

  All this talk of genetics makes it easy to assume that where the sense of smell is concerned, you’re stuck with the hand nature dealt you. To some extent that’s true, of course; if you only have broken copies of a particular odor receptor gene, you’re never going to be able to make use of that receptor. But the reality is a bit more complex than that. Just ask Charles Wysocki.

  Wysocki has been at Monell since the 1970s, making him one of the center’s longest-serving researchers, and right from the beginning he’s been fascinated by individual differences in the sense of smell. (For what it’s worth, early in his career, he also published a paper on how to tell male newborn mice from females.) It was Wysocki, together with Gary Beauchamp, who showed more than thirty years ago that a person’s genes help determine whether they can smell androstenone, a musky, urinous-smelling compound that male boars use to signal their virility and is also a key flavor component in truffles. Their study was one of the first clear proofs that genes affect the sense of smell. But along the way, they learned something else, too.

  Now semiretired, Wysocki is a small man with a slight stoop, thick gray hair, and an extravagant organ-grinder’s mustache. “I started working with the compound in 1978,” he recalls. “I could not smell it at all—was totally oblivious to it. I just had to trust the scales, the balances, that I was making the right stuff.” After a few months of working with the compound daily, he started noticing a new odor around the lab. To his surprise, the culprit turned out to be androstenone. Somehow, he had acquired the ability to smell it. And he wasn’t the only one—some of his technicians reported the same thing. Intrigued, he tested a larger sample of people. Sure enough, half of the nonsmellers became much more sensitive after a few weeks’ exposure to the compound. “These people went from a nonsmeller to a pretty sensitive smeller,” he says—though they never got to be as good as the best natural smellers, who can detect androstenone at just a few parts per trillion.