What the Nose Knows: The Science of Scent in Everyday Life (13 page)

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ORNELL
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NIVERSITY
evolutionary biologist Paul Sherman is another scientist rethinking the assumption that all variation in food habits is cultural. Sherman studies how spice use relates to human survival. He and his collaborator Jennifer Billing were intrigued by the fact that spices often have antimicrobial properties: they contain natural chemicals that kill bacteria and fungi. Could the point of cooking with spices be to reduce spoilage and food-related illness? To test their idea, Sherman and Billing assembled a collection of ninety-three cookbooks from thirty-six countries. From these, they selected 4,578 meat-based recipes and meticulously noted what spices were used in each.

On a worldwide basis, nearly every meat dish (93 percent) had one or more spices. The results varied, however, with a country’s climate: the number of spices per recipe increased with the average annual temperature. In Finland and Norway, for example, one-third of recipes used no spices at all. In contrast, in Ethiopia, Kenya, Greece, India, and Thailand, every recipe called for at least one spice. Sherman and Billing ran other statistical analyses and found that average annual temperature was correlated with the proportion of recipes containing spices, and the total number of spices used. Since unrefrigerated meat goes bad faster in a warm climate, more spices might mean better protection against spoilage. Sherman and Billing examined the antibacterial power of the various spices, and found that the hotter a country, the more bacteria species are inhibited by the local selection of seasonings. They conclude that while spice use is something we do because it tastes good, it also rids food of pathogens and therefore provides a biological advantage in keeping people healthy. (They briefly considered whether spices might be used to mask the bad taste of spoiled food, but dismissed the idea as a nonstarter: there would be little benefit to survival in encouraging people to ingest toxins.)

In their tally of thousands of meat-based recipes, Sherman and Billing found that the most commonly used spices are onion (in 65 percent of all recipes) and pepper (63 percent), followed by garlic (35 percent), hot peppers (24 percent), lemon and lime juice (23 percent), parsley (22 percent), ginger (16 percent), and bay leaf (13 percent). Another thirty-five spices appear only occasionally (in 10 percent or fewer of all recipes). They found that the vast majority of the world’s recipes could be made with about four dozen spices—a number remarkably close to the length of Elisabeth Rozin’s world cuisine shopping list. Further, the average meat recipe calls for 3.9 spices, a number that is consistent with Rozin’s flavor-principle concept.

Sherman returned to his cookbook collection and analyzed another 2,129 recipes, this time looking at only vegetable dishes. Compared with meat recipes, these use fewer spices (2.4 per recipe on average). Still, the results supported the antimicrobial hypothesis: the hotter the climate, the more spices, though this relationship proved somewhat weaker for vegetable dishes. Why? Fruits and vegetables come prepackaged with physical and chemical defenses against microorganisms, which makes the health benefit of adding spices correspondingly smaller.

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OOKING AND SPICING
are behavioral adaptations with biological consequences. They have shaped our face and made mouth-based smelling a defining human trait. Outlandish as it sounds, spicy cooking may even have altered the core of our biological identity—our DNA.

It is often said that a species’s DNA code can be read like a book. If so, some biologists read it like the sports pages—they add up the number of odor receptor genes and rank us against other species according to the results. Rats lead the Mammalian League with the most functioning receptor genes; dogs and mice are a few games behind, while chimpanzees and humans are looking for a wild-card berth; and dolphins—an aquatic expansion team—own the cellar.

Among primates, humans have the highest proportion of nonfunctional receptor genes; we keep a lot of obsolete junk in our genetic attic. Superficially, it looks like the human nose is weak (relatively few receptors) and getting weaker (losing receptor genes at four times the evolutionary rate of other higher primates). Some, such as the science writer Nicholas Wade, see this as a case of use it or lose it. He says that “the price of civilization is that the faculty of smell is inexorably being degraded.” Wade’s gloomy conclusion may not be justified. Humans continue to evolve, and geneticists have identified hot spots in our genome—areas of biological function in which new genes are being born. Olfaction is one such hot spot. In the last 5,000 to 10,000 years, genes for smell receptors, along with genes related to diet and metabolism, have been evolving faster than those in any other physiological system. One new study finds that “many changes in the human olfactory repertoire may have occurred very recently,” the changes in this case being beneficial genetic mutations that have become fixed traits throughout the population.

The human genome responds rapidly to cultural changes. For example, in ancient populations the gene for lactose absorption ceased to function shortly after weaning. With the emergence of dairy farming, natural selection favored individuals in whom the gene stayed active into adulthood. The selective advantage of being able to eat milk products was so great that adult lactose absorption became a widespread trait within 5,000 years, a mere blink of the eye in evolutionary time. I suspect there has been ample time for the aromas of cooked food to influence our odor receptor repertoire in a similar way. If our gut evolved to digest dairy products, why wouldn’t our nose evolve to appreciate the smells of cheese, butter, and yogurt?

In the recent evolutionary past we have evolved entire subfamiles of odor receptors not shared by the chimpanzee—our closest living relative. An intriguing possibility is that these new receptors are tuned to detect new smells—ones that only recently became important to human survival. It’s speculation on my part, but I’d bet these receptors pick up the nuances of grilled meat—salmon filets and mastodon steaks—along with the volatiles of fermentation: not only milk products, but alcoholic drinks from beer to wine. On a daily basis we season food to please our palate, but over the long run our palate is evolving to match our menu.

I also suspect that dogs are part of the whole story. Dogs were first domesticated by man somewhere in Siberia about 15,000 years ago, just as human populations were shifting from a hunter-gatherer existence to sedentary village life. Increasingly preoccupied with the complex man-made aromas of the cooking pot, our ancestors began to rely on hunting dogs to locate the telltale scent of game. Having co-opted the canine nose, our own scent-tracking ability began to fade. Dogs became, in effect, our long-distance noses, while we specialized in the close-in smelling of food in the mouth.

Dog and humans have complementary nose skills: dogs have little retronasal ability but great distance detection; humans vice versa. (I’m unable to find a single scientific paper on canine retronasal smell. According to pet-food manufacturers, dogs sniff first and gulp later; they don’t spend a whole lot of time savoring food in the mouth.) The Yale University neurobiologist Gordon Shepherd suggests that retronasal smelling “has delivered a richer repertoire of smells in humans than in nonhuman primates and other mammals.” I would go further and claim that humans are a retronasal species; our best olfactory skills are reserved for appreciating food aromas at the point of eating. Our talent is smelling food in the mouth, not food on the hoof. When it comes to tracking the scent of a gazelle on the savannah, we can’t compete with our hounds; but once we drag it back to the campfire we can sure season the hell out of it.

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ULTURES ALL OVER
the world may choose from the same selection of spices, but that doesn’t guarantee that we all find each other’s cuisines equally appealing. Aromas mark differences between cultures, along with all the moral baggage that entails. On a field trip to Costa Rica, when Miss Stevens admonishes him to “respect other cultures this instant!” Eric Cartman replies, “I wasn’t saying anything about their culture, I’m just saying their city smells like ass.” Offhand dismissals of cultural differences aren’t limited to the fourth graders of
South Park.
Before he became the president of France, Jacques Chirac was mayor of Paris, and made himself notorious for observing that “the noise and the odor” of freeloading immigrant families would reasonably push a hardworking Frenchman over the edge. He hastened to add, in Cartman fashion, “It is not racist to say this.”

Smell prejudice is not just a Eurocentric trait. Wang Lung, the fictional hero of Pearl Buck’s
The Good Earth
, moves to another region of China where his scent marks him as a outsider: “[W]hen an honest man came by smelling of yesterday’s garlic, they lifted their noses and cried out, ‘Now here is a reeking, pig-tailed northerner.’ The smell of the garlic would make the very shopkeepers in the cloth shops raise the price of blue cotton cloth as they might raise the price for a foreigner.”

Anthropologists tell us that olfactory stereotyping is central to tribal identity. The Desana people of Colombia’s Amazonian rain forest, for example, believe each tribe has a characteristic odor due partly to heredity and partly to what it eats: “Thus, the Desana, who are hunters, are said to exude the musky smell of the game which they eat. Their neighbours, the Tapuya, on the other hand, live by fishing and are thought to smell of fish. The nearby Tukano are agriculturalists and they, in turn, are said to smell of the roots, tubers and vegetables which they grow in their fields.” Traditional Scottish clans put a different spin on it. Before the invention of woven tartans, each clan was associated with a plant, worn by its members as an aromatic badge of identity. An enterprising smell scientist is attempting to reintroduce the concept by marketing clan-based perfumes. Eau de Whortleberry, anyone?

At cultural boundaries the smell of food become an invisible, fragrant fence. One study went to the trouble to prove that bonito flakes smell like food to Japanese people but not to Germans; the opposite is true for marzipan. You eat what you were raised on. The most unsettling result of this study was that nearly 40 percent of the German ladies interviewed found the smell of Vicks VapoRub to be edible.

Does the fragrant fence limit us to the food aromas of our birth culture? Not necessarily; but there are hazards in jumping the fence. These are nicely depicted in Radhika Jha’s novel
Smell.
Leela, a young Indian woman born in Kenya, is sent to live with relatives who run an Indian grocery in Paris. Aromatic crosscurrents are present from the opening sentence: “When the wind blew hard, as it did very often that spring, the smell of fresh baguette would fight its way into the Epicerie Madras to do battle with the prickly smell of pickles and masalas.” Leela has a fine awareness of scent and is skilled at cooking with traditional Indian spices. As she learns the ways of Paris she improvises new dishes and creates new possibilities for her love life and career. (She takes a French lover and becomes the darling of the Parisian fusion cuisine scene.) Eventually, Leela realizes that the scents that make her exotic and attractive also make her an outsider. As an author, Radhika Jha has an extraordinary feel for the boundary-creating power of scent, perhaps because she herself lived in Paris as an exchange student. By showing how one woman used scent to redefine her relation to two cultures, she proves it is possible to cross the fragrant fence.

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OME FOOD AROMAS
raise the fence to unscalable heights. For example, if you are not Swedish it is unlikely that you can be persuaded to try Surströmming. Surströmming is fermented herring, and is horrifically foul-smelling even to those who consider it a national delicacy. Another Scandinavian specialty is lutefisk. To make it, one soaks air-dried codfish in water for several days, then in a solution of caustic lye for another couple of days, and ends with a few more days in plain water. The result is a swollen, jellylike mass of smelly fish flesh that is popular in Norway and the Norwegian-heavy precincts of Minnesota and Wisconsin. Garrison Keillor recalls lutefisk as “a repulsive gelatinous fishlike dish that tasted of soap and gave off an odor that would gag a goat.” But people who consider themselves true Sons of Knut eat it at least once a year. Norwegians are not insane; they know lutefisk smells bad. But they have carved out a special exemption for it—they’ve made it a badge of belonging.

The psychologist Donald E. Brown compiled a list of cultural universals that includes things like music, proverbs, incest avoidance, and death rituals. I would like to propose an addition to the list of universals: every culture has a foul-smelling food for membership. You are not really Taiwanese unless you eat “stinky tofu” (chunks of fermented soybean curd). You are not really Icelandic unless you eat harkarl (rotten shark meat). Real Japanese eat natto (a gluey mass of fermented soybeans that smells like creosote). Then there is the fabulously stinky durian, or jackfruit, of southeast Asia. Singapore being Singapore, one is allowed to eat its sweet, custardy innards, but it is illegal to carry it on public transportation. I’m personally a big fan of kimchi, the national condiment of Korea. It’s made from fermented Chinese cabbage, vinegar, garlic, fish sauce, and lots of red pepper. It packs a punch—a bottle of it once exploded in my refrigerator. Its postingestive consequences are spectacular: the humorist P. J. O’Rourke described them as “a miasma of eyeglass-fogging kimchi breath, throat-searing kimchi burps, and terrible, pants-splitting kimchi farts.”

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MERICA IS IN
the midst of a great sensory reawakening; we are more open to new foods and flavors and smells than at any point in our history. In a country where quiche was once considered exotic, we are no longer surprised to find pad thai in Peoria or moussaka in Muskogee. Kraft Foods, an outfit best known for serving up millions of pounds of macaroni and cheese, recently introduced a Mango Chipotle seafood marinade. Yet in contrast to this growing abundance of sensory options, the regional differences that once characterized the national smellscape are fading. In 1947
The Saturday Evening Post
asserted confidently that “West Coast doughnut flour has a predominant lemon flavor, whereas in New England, doughnuts have a strong nutmeg flavoring, with little lemon.” Traces of these regional preferences linger on the contemporary American scene, as evidenced by variation in air-freshener sales. Food-inspired scents such as vanilla and cinnamon have a 39-percent market share in the North Central states, compared to only 28 to 29 percent in the Northeastern, Western, and Southern regions. Citrus and fruity scents (lemon, orange, grapefruit, mandarin, and green apple) show the reverse pattern: they are only 16 percent of sales in the North Central states, but 22 to 23 percent elsewhere.