What the Nose Knows: The Science of Scent in Everyday Life (12 page)

The psychologist Pamela Dalton and her colleagues took this result and pushed it much further: they showed that expectations alter the perception of actual odors. She had volunteers sit in a test chamber for twenty minutes while exposed to odors that were neither pleasant nor unpleasant. Some subjects were told nothing about the odor. Others were told it was a potentially harmful industrial chemical or, alternately, that is was a distilled, pure natural extract. To use the Clinton-era term for expectation management, the experimental conditions differed only in spin. By the end of the test, all three groups had higher detection thresholds—their noses had been dulled by adaptation to the real odor. However, their perception of odor intensity was spin-dependent. With positive spin or no spin at all, the odor seemed less intense as time went on; with negative spin it smelled as strong or stronger. In other words, odors we think are benign fade from awareness, while those we believe to be hazardous hold our attention and stay strong.

It may not even matter whether the actual smell is good or bad. Spin can alter these perceptions as well. Dalton tested odors that were pleasant (wintergreen), unpleasant (butyl alcohol, a solventlike smell), and neutral (isobornyl acetate, a balsamlike note). Negative spin made all three smell stronger. Information bias is very effective at distorting the clear evidence of our senses—the brain easily trumps the nose.

Biasing information doesn’t have to come from an authority figure in a lab coat. Dalton tested two people at a time in the environmental chamber. One was an unsuspecting volunteer, the other a carefully scripted actor pretending to be naive. The actor kept up an ongoing verbal and behavioral commentary about the odor in the air. This peer-to-peer kibitzing worked splendidly. When the spin was negative, 70 percent of volunteers reported health symptoms (everything from throat irritation to dizziness to stomachache); when it was positive, only 12 percent did so. Given a scent in the air—any scent—acquaintances can literally talk you into feeling sick.

The commonly acknowledged power of scent derives in large part from the power of suggestion. Negative placebo effects may exacerbate the symptoms of “sick building syndrome”—for example, if you believe that the musty smell in your office is from a toxic mold—while positive placebo effects explain the popularity of aromatherapy treatments. Beneficial mood change is one of the biggest claims made for aromatherapy. For example, lavender is usually extolled as relaxing and neroli as stimulating. A recent study showed that positive spin can completely reverse the aromatherapeutic effects of these two scents. When told the lavender they were smelling “has relaxing properties,” people did in fact relax, as measured by changes in heart rate and skin conductance. Yet when told it “has stimulating properties,” the same measures showed—presto change-o—that people were stimulated. The same reversal happened with neroli. It takes only the slightest waving of hands to create a positive placebo effect in aromatherapy.

The effects of spin often play out in everyday life. When the crew of a Norwegian air ambulance noted a cabbagelike smell in flight, they figured the patient they were transporting had passed gas and they ignored it. When the smell reappeared on another flight later that day, the crew was puzzled; it was unusual for two patients to be so extraordinarily gassy. Soon flames were shooting through the cockpit and the pilots were forced to make an emergency landing. The fartlike smell was smoldering insulation on electrical wires. The crew was in a medical mind-set, not a mechanical one, and their preexisting expectations led to a near-fatal misreading of what their noses were telling them.

Smells don’t happen to a passive nose alone. The brain actively regulates the physical and cognitive aspects of odor perception: it exerts moment-by-moment control of sniffing to govern how much scent enters the nose; it systematically dials down the intensity of one smell to prepare us for the next; it automatically makes a provisional interpretation of a smell, based on context cues, to prime us for a behavioral response. From sniff to spin, the nose and brain constantly reshape our awareness of the smellscape.

CHAPTER 5

A Nose for the Mouth

Blindfold a person and make him clasp his nose tightly, then put into his mouth successively small pieces of beef, mutton, veal, and pork, and it is safe to predict that he will not be able to tell one morsel from another. The same results will be obtained with chicken, turkey, and duck; with pieces of almond, walnut, and hazelnut….

—H
ENRY
T
HEOPHILUS
F
INCK (1886)

W
HEN IT COMES TO FOOD
, I’
M A SMELL CHAUVINIST
: taste is boring. The tongue supplies just five channels of information: bitter, sweet, sour, salty, and umami. (My Japanese colleagues insisted for years that monosodium glutamate delivered more than a salty impression. The discovery in 1996 of glutamate receptors on the tongue finally proved their case. The savory taste of umami is now in the official pantheon.) While five taste channels are nothing to sneeze at, they’re rudimentary compared with the 350 different receptors and two dozen perceptual categories available to olfaction.

There is another reason why I think taste is overrated. We are accustomed to experiencing flavor as a singular sensation in the mouth. As a result, we use the words “taste” and “flavor” interchangeably in casual conversation. This makes it easy to forget that flavor is actually a fusion of taste and smell, and that the apparent simplicity of flavor is just an illusion, one that is somtimes reinforced by language. For example, there is only one word for taste and flavor in Spanish (
sabor
), German (
geschmack
), and Chinese (
wei
). I think the tongue gets more credit that it deserves.

That smell makes the far greater contribution to flavor becomes obvious once it is taken out of play. Pinch shut the nostrils, and flavor disappears. What’s left, as the American philosopher and critic Henry T. Finck noted 120 years ago, is bland texture. Caviar tastes like salty oatmeal, and coffee is merely bitter water. This simple, powerful truth is ignored by those who claim the sense of smell is weak and of little importance to modern humans. For example, the pop-science icon Carl Sagan once said “it is clear that smell plays a very minor role in our everyday lives.”
Science Digest
claimed, “Modern man seldom uses the sense of smell except to detect a burning roast in the oven, or to enjoy a rose bush.” The pioneer sexologist Havelock Ellis had such contempt for smell that he tried to minimize its role in flavor: “If the sense of smell were abolished altogether the life of mankind would continue as before, with little or no sensible modification, though the pleasures of life, and especially of eating and drinking, would be to some extent diminished.” One hesitates to imagine what sort of cramped, joyless inner life could lead a person to write such things, for the reality, made clear by Finck’s demonstration, is that the sense of smell contributes mightily to our enjoyment of food and for this alone deserves to be celebrated.

In his essay on “The Gastronomic Value of Odours,” Finck described a particular type of smelling we use to savor food. He pointed out that aromas released from food in the mouth reach the nasal passages via the back of the throat, and are exhaled through the nostrils. The act of swallowing drives aromas along this reverse path. In effect, we smell our food from the inside out. Today this is known as retronasal olfaction, but I prefer Henry Finck’s name for it: a “second way of smelling,” a phrase that sets it apart from the usual nostrils-first mode. Retronasal olfaction has become a hot topic among sensory scientists, and recent findings confirm Finck’s intuition: the second way of smelling operates by its own set of sensory rules.

T
HE TWO PHYSICAL
paths to the nose—one from the outside world and the other from the mouth—have parallels in the psychology of odor perception. The apparent location of a smell—inside or outside of our body—determines how we perceive it. The psychologist Paul Rozin demonstrated this in a simple experiment. He taught people to recognize the smell of four unusual fruit juices. They sniffed the samples while blindfolded, and quickly learned to tell the them apart with perfect accuracy. When Rozin squirted the same juice samples into their mouths with a syringe, they could not identify them reliably. A smell well-learned when sniffed by the nose is poorly recognized in the mouth. This suggested to Rozin that location has consequences: a food smells one way “out there” and a different way “in here.” The psychological difference between outside-in and inside-out smelling, when combined with taste sensation from the tongue, produces strange contrasts. It makes for foods that smell good but taste bad (coffee, for example), and others that smell bad but taste good (blue cheese).

The psychologist Debra Zellner studies a peculiar sensory illusion involving sight and smell. She pours a clear, scented liquid into two glasses and adds color to one. To a blindfolded person the two samples smell equally strong; with the blindfold removed, the colored version smells stronger. In the classical version of this colorodor illusion, the liquid is sniffed by nose. Zellner wondered what would happen if the smell were delivered by mouth. She had people sip the samples through a straw—the liquid was visible under a clear plastic lid, which prevented through-the-nostril smelling. Under these conditions the illusion was reversed: adding color reduced perceived odor strength.

Because smell and taste are inextricably linked in flavor perception, experience in one modality can affect the other. For example, some odors are commonly described in terms of taste: honey smells “sweet” and vinegar smells “sour.” The Australian psychologist R. J. Stevenson and others have shown that odors acquire taste qualities through associative learning. After a novel odor is paired a few times with the sweet taste of sucrose, the odor is perceived as smelling sweet. If paired with citric acid, it seems to smell sour. This cross-sensory link works in the other direction as well: smells can alter tastes. Strawberry odor, for example makes a weak sugar solution taste sweeter, and a whiff of soy sauce boosts the perceived saltiness of a saline solution. Sensory researchers have just begun to understand the psychological interplay between smell and taste. They are now looking at how these senses are neurologically cross-wired in the brain. To a smell-centric guy like me, the study of taste is about to become much more interesting.

The Pleistocene Barbecue

Carnivores rarely savor their food: they rip, chomp, and swallow. Herbivores chew for hours on end, not for sensory pleasure but to make tough, fibrous plant matter digestible. Humans, in contrast, anticipate, savor, and linger over the aroma of food. We go to great lengths to increase the appeal of food by cooking it and adding spices. The second way of smelling not only provides the pleasure we take in eating, but also may be the key to how the human sense of smell has evolved over time.

Traditionally, researchers in cultural anthropology and sociology have treated food preparation as an expression of culture, as a collection of behaviors driven by custom and creativity only. A new generation of behaviorally oriented evolutionists is now challenging this profoundly unbiological point of view. The Harvard University anthropologist Richard Wrangham, for example, sees cooking not as an optional behavior—a cultural frill—but as a biological requirement for human survival. Surveying the evidence, he finds that “no human populations are known to have lived without regular access to cooked food.” Even the Inuit hunters of the Arctic, famous for their raw diet, occasionally cooked their whale blubber.

Hominids—the near-human species that link us to our common ancestor with the chimps—were definitely cooking with fire 250,000 years ago. Wrangham finds evidence of cooking as far back as 790,000 years, and speculates that it may have begun as far back as 1.7 million years ago. In any case, cooking with fire was well established when our first anatomically modern ancestors emerged in Africa some 100,000 years ago. We’ve grilled a lot of mastodon steaks through the ages.

The invention of cooking had profound consequences for diet and social behavior. Cooking releases nutrients and makes vegetables faster to eat and easier to digest. Wrangham calculates that for a 120-pound woman to take in 2,000 calories a day, she would have to eat eleven pounds of raw fruits and vegetables. That’s a lot of time at the salad bar. Clinical studies show that raw-food cultists in Germany struggle to keep up nutritionally with their countrymen: they suffer from chronic energy deficiency and the women fail to menstruate. If European sophisticates with desk jobs and handy supermarkets can’t thrive on a raw-vegetable diet, how well would a band of hunter-gatherers do?

Adding meat greatly enhances the diet. Chimpanzees in the wild are big fans of monkey meat, but even with their powerful jaws they take hours to gnaw the raw flesh from a bone. Given the effort involved, raw meat isn’t a routine source of nutrition for chimps. Nor would it have been for early hominids. A
Homo erectus
female (our evolutionary cousin) would have needed six hours a day to get all her calories from raw meat, according to Wrangham’s calculations. Cooked meat, however, is a different story: it is nutrient-dense, easily chewed, and rapidly consumed. The time saved by cooking changes our behavior patterns. Where all other large primates snack throughout the day on raw fruits and leaves, we eat a few discrete meals, leaving more time for other activities. The widespread popularity of cooking among protohumans meant that powerful jaw muscles and large teeth were no longer essential, and as their evolutionary advantage shrank, so did they. In the last 100,000 years our teeth and jaw muscles have become even smaller, making possible finely controlled chewing movements of the tongue and jaw. The more nimble modern mouth makes an easy-to-swallow “bolus” of food and releases more aroma in the process. In the long run, cooking has literally changed the shape of our face.

C
OOKING HAS ALSO
changed our sensory world: it introduced novel aroma molecules and whole new classes of smells. The savory notes of roasted meat, toasted nuts, and carmelized vegetables were rare accidents before we fired up the Pleistocene barbecue. More new smells—baked bread and boiled mush—arose with the cultivation of wheat and other grains about 12,500 years ago. Sheep, goats, pigs, and cattle were all domesticated roughly 10,000 years ago. With them came the smell of butter and the fermented bouquets of yogurt and cheese. As early villagers mastered the art of fermentation, the heady aromas of beer and wine joined the mix.

We are a cooking species, and the smell of an impending meal is woven into our biology. Food aroma is an invitation and a spur to action. Even before the first bite, it triggers an elaborate sequence of physiological events: salivation, insulin release by the pancreas, and the secretion of various digestive juices. The aroma of bacon, at a level so faint it can’t be consciously identified, has been shown to trigger the flow of saliva. This would not have surprised cookbook author James Beard, who once said, “Nothing is quite as intoxicating as the smell of bacon frying in the morning, save perhaps the smell of coffee brewing.” We expect to be stimulated en route to a meal—the anticipatory smells of cooking have become almost a biological requirement. This is a big headache for manufacturers of prepared foods. The physics of microwave heating doesn’t create the toasted, roasted, and caramelized notes that signal impending “doneness.” Food companies spend a lot of time and money on technological work-arounds to restore these missing scents.

I
N ADDITION TO
cooking food, we spice it. Spice use is a universal human habit, though there are significant regional differences in the spices that are used and how they are combined. What qualifies as a spice? In one definition, it’s “any dried, fragrant, aromatic, or pungent vegetable or plant substance, in the whole, broken, or ground form, that contributes to flavor, whose primary function in food is seasoning rather than nutrition, and that may contribute relish or piquancy to foods or beverages.” Roots, seeds, dried leaves, even aromatic lichens fit this definition; including fresh herbs adds still more materials. There are a lot of spices, and yet, like the huge number of possible smells in the world, the closer one looks, the more this apparent diversity can be simplified. At the core of each of the world’s great culinary traditions is a small group of spices and flavorings. A perfumer would think of these combinations as an accord, the key ingredients that define a style of perfume. The late food expert Elisabeth Rozin called these combinations “flavor principles”: “Every culture tends to combine a small number of flavoring ingredients so frequently and so consistently that they become definitive of that particular cuisine.” Rozin could conjure up an entire culture using two or three key flavorings. She rarely had to use more than four. For example, soy sauce, rice wine, and gingerroot form the Chinese flavor principle, while the Hungarian one consists of paprika, lard, and onions. A beloved and easily recognized flavor principle gives ethnic authenticity to whatever is cooked in it. In the future, Hungarian deep-space explorers eating processed algae paste will find it quite palatable as long as it is seasoned with paprika, lard, and onions.

Some spices are used by many different cultures. What makes a flavor principle distinctive is its
specific combination
of seasonings. Consider lemon, a widely used flavor source. Add cinnamon, oregano, and tomato and you’ve got a Greek principle. Add fish sauce and chili and you’ve got Vietnamese. The extensive overlap in ingredients across flavor principles means that every traditional cuisine on the planet can be prepared from a very short shopping list. The thirty or so principles Rozin describes in her book require about four dozen ingredients. All the flavors of world food culture can fit into a single grocery bag.

Liz Rozin’s theory of food aroma strikes some people as too minimalist to account for the richness of human cuisine. What they fail to appreciate is the power of combinatorics, which makes it possible to generate huge numbers of flavor variations from a few basic odorous elements. The Chicago chef and restaurateur Charlie Trotter understands this. “You can prepare forty dishes from six ingredients,” says Trotter. He likens creative cooking to jazz improvisation. A chef who has mastered the basic repertoire—the classical flavor combinations—can improvise endless new dishes with only a handful of spices. Thus the cook and the chemist have arrived at the same fundamental truth: sensory diversity is achieved with relatively few ingredients. The chemist can re-create the aroma of any foodstuff with fewer than a thousand odor molecules, and the chef can build any global cuisine with a few dozen spices. The amazing variety of human cuisine, at the chemical as well as the aesthetic level, is a matter of basic themes and endless variations.