What the Nose Knows: The Science of Scent in Everyday Life (26 page)

The power of smell to evoke a particular place gives the smell museum a unique opportunity for innovative exhibits. Perhaps something along the lines of a recent presentation by the designer Hilda Kozári and the perfumer Bertrand Duchafour. They linked scent and place in a 2006 artwork called
AIR—Urban Olfactory Installation.
Kozári suspended three translucent globes from the ceiling, each one large enough for a visitor to step into through a hole in the bottom. Around each globe’s equator, a thin layer of spongy material was moistened with a city-scent composed by Duchafour. Monochrome video images were projected onto the sphere’s surface. By standing inside, one could experience Budapest (Kozári’s hometown), Helsinki (where she works), or Paris (just because).

Great balls of smell is a very cool concept. The light, leafy-green scent in the Helsinki ball was pleasingly matched by the greentinted video. The smells of Budapest and Paris, however, were indistinct, and the three videos, shot from a moving car, made all the cities look the same—an endless loop of roads, bridges, and traffic. I entered the balls with high hopes, but left underwhelmed. I thought of Kipling’s poem and yearned for a Lichtenberg experience; I wanted to smell wattle and watch it rain in Australia on one side of the globe, and in South Africa on the other.

I
F WE’RE SERIOUS
about preserving scents of place, it’s not enough to capture random locations; we should survey an entire geographical area. I once accompanied a
New York Observer
reporter on a sniffing safari of Manhattan. It was midsummer and New York was ripe, but nailing down the actual source of the malodors wasn’t easy. The air in an upscale sports club was a tad stale but not too objectionable. Our most noxious find was a puddle of rancid sidewalk water at University Place and Thirteenth Street. Something terrible had happened there, and the ghost of it lingered in the late afternoon. The
Observer
reporters conducted walking tours with other nose experts and published the story along with a whimsical odor map of the city.

The guided odor tour has become a features-section standby. For example, a
Washington Post
reporter rides along in a limo with a perfumer and a retired sanitation worker as they make a haphazard tour of New York. They hit the usual tourist sites with predictable results: rancid pork fat in the Meatpacking District, hot frying oil in a Chinatown kitchen, and intense horse manure near the carriages in Central Park. All the while the French-born perfumer plugs her line of neighborhoods-of-New-York-themed perfumes. (Fair enough—it was her limo and driver after all.)

The New York–based gossip blog Gawker took a refreshingly egalitarian approach to urban odor mapping. It invited readers to e-mail in odor reports for every train station and subway platform in the city. The general outcome was not in doubt. (Even Paris Hilton knows the score; in her memoir she writes, “Yes, I admit I’ve taken the subway in New York—and it smells. It literally smells like pee. Why can’t they do anything about that?”) Gawker compiled the vox populi into an interactive New York City Subway Smell Map. Mouse over a particular station, and colorful icons pop up to tell you which of ten malodor categories is found there. Waiting for the A-C-E train at Thirty-fourth Street and Eighth Avenue? Gawker icons indicate the presence of body odor, feces, urine, sewage, and vomit. Need more detail? Just double-click for reader comments: “Something dead and decaying…Old outhouse poop…Fresh poop…Sewer water…Urine post–asparagus buffet…Breath of a hungry old lady…Stinks like puke.” According to the Subway Smell Map, stations on the Upper East Side are exceptionally nonodorous. This may be true, or the result of sample bias—hipsters who read Gawker may never venture that far uptown.

The ultimate objective for nasal surveyors is a navigational chart of the entire American smellscape. Is such a thing possible? Helen Keller thought so: “I can easily distinguish Southern towns by the odours of fried chicken, grits, yams and cornbread, while in Northern towns the predominating odours are of doughnuts, corn beef hash, fishballs, and baked beans.” American cities were so distinctive she had her own Olfactory Positioning System: “I used to be able to smell Duluth and St. Louis miles off by their breweries, and the fumes of the whiskey stills of Peoria, Illinois, used to wake me up at night if we passed within smelling distance of it.”

Landmark smells, even those of home, are not always pretty. The writer Celeste Bowman describes her experience in Texas: “My eyes flew open as my nose was assaulted by the acrid odor of saltwater, decomposing fish and seashells, a peculiar fragrance that I love. Sea smell is the smell of home. I was back in Corpus Christi, a guest in the city of my childhood.”

Commercial odors serve as locator beacons on the smellscape. For fifty-five years the Life Saver factory poured fruity sweetness over Port Chester, New York. The Mars candy plant keeps Hackettstown, New Jersey, smelling chocolaty, and the Maxwell House roasting operation periodically gives Hoboken a jolt of joe. A Snapple bottling plant fruitifies part of Baltimore, while a rendering plant, vinegar distillery, and giant bakery define other areas of town. McCormick & Co. blew a potpourri of spice across Baltimore for more than a century before relocating to Hunt Valley. A paper mill leaves a big, if unfavorable, impression on Muskegon, Michigan, and the Owens Country Sausage plant gives Sulphur Springs, Texas, a special yumminess.

W
E COULD FILL
an almanac with the site-specific scents of America. Because I grew up there, my nasal circuits are hopelessly imprinted on California. It’s the source of dozens of characteristic smells—all true and equally essential—enough to fill a wing of the smell museum. The Golden State overwhelmed the intrepid Helen Keller: “I think I could write a book about the rich, warm, varied aromas of California; but I shall not start on that subject. It would take too long.”

I’ll give it a try. Start with the redwoods and the Sierra foothills full of
kit-kit-dizze
and coyote mint. Leave space for the La Brea Tar Pits and the pleasant, clean, tarry note that hovers over them. Include the stinkpots of Mount Lassen in the far north, and the sulfury hot springs of Esalen, down near Big Sur. The Pacific Coast has its own special collection: heaps of rotting kelp and the rich funk of tidal mud inside the Golden Gate. Depending on the wind direction, there’s the stink of guano off Seal Rocks or the stench of the elephant seals at Point Año Neuvo.

The journalist and social observer Heather MacDonald grew up in the tony Bel Air section of Los Angeles. Living in a dense urban metropolis, she delighted in the nearby outdoors—a typical California contrast. “I spent a lot of time in the Santa Monica Mountains. The smell of the dry chaparral in the summer time and the eucalyptus and the wild mustard plants and the light…. There are so many smells that I associate with the land around here.”

Eucalyptus, that Australian import, is everywhere in California. Another Australian, the Victorian box tree, has become part of the Southern California smellscape. Its nighttime perfume—an intoxicating blend of orange and honey—blankets Los Angeles every February. The local columnist Mary McNamara writes, “Seeping in through open windows, under doors, the scent saturates the air, the bedclothes, so dense you can taste it. Ambrosia rising, within and without.”

The best way to sample California smells is by car. Drive down I-80 with the windows open as you pass the oil refineries in Pinole. Cruise past the Harris Ranch and the stockyards off I-5 in Coalinga and get the full blast of the cattle. Take US 101 through Gilroy and inhale the garlic. (And don’t forget that the famous Lockheed “Skunk Works” in Los Angeles was named for the obnoxious smell of a nearby plastics factory.)

Maybe Helen Keller was right—California demands a lot of the cataloger, and these are just the bigger features of the smellscape. Zoom in to the level of neighborhoods and the picture gets more detailed, and even more evocative. Odor mapping is an exhausting effort. Is it really necessary to capture and preserve all this stuff that’s just out there, floating around? Of course it is. The Hunt’s tomato cannery in Davis is shuttered; the garlic depot in Vacaville is gone; Cannery Row smells only on paper; and it’s a rare day when Fisherman’s Wharf smells of a fresh catch. The recent past—our very lifetimes—is evaporating day by day.

CHAPTER 12

Our Olfactory Destiny

They were, I now saw, the most unearthly creatures it is possible to conceive. They were huge round bodies—or, rather, heads—about four feet in diameter, each body having in front of it a face. This face had no nostrils—indeed, the Martians do not seem to have had any sense of smell.

—H. G. W
ELLS,
The War of the Worlds

I
N THE IMAGINATION OF
H. G. W
ELLS,
M
ARTIANS WERE
more advanced than humans: they didn’t need a primitive sensory system with nostril holes and wet mucous membranes. Martians were big-eyed, big-brained, and gutless, with squidlike tentacles instead of arms and legs. What these creatures lacked in biology they made up for with technology: they roamed the Earth in mechanical exoskeletons. Since
The War of the Worlds
appeared in 1898, science-fiction writers and alien abductees have insisted that space visitors are noseless. I remember an
Outer Limits
episode in which the hero, a radio station engineer, makes contact with a creature from the fourth dimension. The curious alien asks him about the function of those strange holes below his eyes.

Like the Freudians, futurists are quick to dismiss the sense of smell as an evolutionary dead end. They speculate that our noses will shrink and our smelling ability will devolve along with it. But is this really our fate? To peer into our olfactory future, we must look toward smelling machines and olfactory genes.

Unlike space aliens, electronic noses are already among us—the first commercial units were delivered around 1992, intended for use in quality control in the flavor and fragrance industries. An e-nose uses an array of chemical sensors to detect odor molecules, and pattern analysis software to distinguish between them. Early models were large boxes that sat in the laboratory; more-recent handheld versions resemble something the meter reader might carry. What sets the e-nose apart from other chemical detectors—like those that measure breath alcohol or warn of carbon monoxide—is that it responds to a broad range of molecules. (Smoke alarms that work on optical principles are even less specific, which is why they sometimes mistake steam or fine dust for smoke.) The chemical sensors of an e-nose can be made from all sorts of materials, with conducting polymers being a popular choice. A conducting polymer changes its electrical resistance in the presence of volatile molecules. Some versions respond to odor at concentrations near the limits of human perception. These polymers are sensitive but not sophisticated; they are basically chemical sponges with different absorbent qualities.

The usefulness of an e-nose depends on its software as much as its sensors. The software extracts a pattern from the sensor input using formidable statistical methods. Multiple sensors give the e-nose a big advantage over single-molecule detectors. In particular, they avoid the pitfall of cross-interference. Imagine a fart detector that works by responding to a single molecule, namely hydrogen sulfide. Embarrassingly, it would go off every time your mom makes some egg salad. In contrast, a broadband e-nose reads the hydrogen sulfide along with other molecules, and would be less likely to mistakenly insult the lady of the house.

How well does an e-nose actually perform? Does it have the potential to take jobs away from humans? Early models were intensely hyped by their manufacturers, and when the devices failed to live up to expectations, customers were left with a lingering negative impression of the technology. The hype hasn’t entirely disappeared. An informal test in 2006 concluded that one brand of consumer e-nose—a handheld, battery-operated model that detected spoiled meat using the amines released by contaminating bacteria—oversold both the accuracy and the benefits of the device.

In general, the practical skills of the e-nose are real but modest; they include telling whether two smells are the same or different. This simple talent is useful in quality control where a manufacturer needs to keep batch-to-batch variation within limits or reject tainted raw materials. An e-nose excels at same/different judgments, and unlike human sensory panelists, it doesn’t get tired or bored. (This doesn’t mean it’s maintenance-free; e-noses have to be recalibrated frequently owing to “sensor drift.”) E-noses are good for dirty and dangerous jobs that humans don’t want, such as monitoring emissions from animal feed lots and sewage treatment plants, or searching for land mines.

The e-nose also has a future in medicine. One device can detect diabetes from volatiles in the breath of a patient; another can find evidence of lung cancer. (Those cancer-sniffing dogs might be out of work before they know it.) An e-nose diagnostic scan would be quick and noninvasive. The main technical challenge is detecting a disease-related odor signal against a varying background of body odor.

Potential consumer applications could be in the offing, such as monitoring ambient fragrance levels—built-in scent systems for homes and offices will be more attractive if they include a feedback mechanism. A programmable olfactostat would maintain a pleasant level of scent in your environment; a wearable one could gauge the odor levels on your person.

Executives in the fragrance and flavor business dream of an e-nose that could stand in for a consumer test panelist. The device would be programmed with the exact preferences of urban preteens or suburban soccer moms in different zip codes. When presented with a test sample, it would respond “I like it” or “it’s too floral.” A roboconsumer has many advantages over human panelists: it’s always on time and you don’t have to pay it.

A surprising number of scientists are working on smell-capable robots; one of them published an entire book on the topic in 1999. Amy Loutfi, a researcher at Sweden’s University of Örebro, has attached an e-nose to an intelligent, mobile robotic system. Her prototype resembles a Roomba—it wanders around an apartment under its own control, locating and identifying smells in the air. Loutfi improved her nose-bot’s performance by adding psychological context to its decision-making process. The device identifies smells better when it knows it’s in the living room rather than the bathroom.

Will police departments deputize the e-nose for remote drug sniffing? The U.S. Supreme Court held that thermal imaging of a suspected marijuana grower’s home, because it relies on sense-enhancing technology that is not “in general public use,” is an unconstitutional invasion of privacy. Under this standard, waving an e-nose downwind of a suspected grow house would also violate the Fourth Amendment’s guarantee against unreasonable search and seizure. Until e-noses are available at Circuit City, police officers are going to have to rely on their own noses.

As with all technology, the law of unanticipated consequences will undoubtedly affect how the commercial e-nose market develops. For example, one near-term application is a pocket-size sniffer that tells from a woman’s breath whether she is ovulating. The Ovulatron 5000 certainly will be a boon to couples trying to conceive, but it might also become a must-have technology for single guys on the prowl.

Y
OU CAN’T EXPECT
an e-nose to work as soon as you take it out of the box. Training is essential, even to achieve competence at a simple same/different task. If its job is to pick out rotten apples, you must fill its database with examples of good apples and bad apples, so that it can create a statistical profile for each, and a decision-rule for telling them apart. An untrained e-nose would probably group wine samples according to alcohol content. It must be trained to distinguish Pinot Noir from Zinfandel. An e-nose is only as impressive as the training it gets. You can’t follow your e-nose—you have to lead it.

An electronic sensing device appeals to hard-boiled process engineers because it is “objective.” It frees them from discussions with sensory experts, and from dealing with emotional consumer panelists, at least in theory. But wait until the e-nose in Manufacturing gives a different reading than the one in Quality Control. Who does the engineer believe then? Good luck finding an objective way to settle that argument.

One thing our brain does very well is separate signal from noise. We can, for example, follow a single conversation at a cocktail party full of chattering voices. Similarly a perfumer can work in an office reeking of background smells that change from day to day. But tracking a target against ever-changing background odors is hard for an e-nose. Even harder is following a moving target against such a background: a ripening peach in a farmer’s market, for example. Until it solves the cocktail-party problem, the e-nose will not be serious competition for the human nose.

A
S TECHNOLOGY ADVANCES,
the line between biology and hardware starts to blur. A group in Britain has developed what it calls “a truly biomimetic olfactory microsystem” by creating an artificial olfactory mucosa. In other words, they embedded electronic sensors in synthetic snot—a 10-micron-thick layer of an odor-retentive polymer called Parylene C. By delaying the detection of incoming odor molecules, the polymer slows the response time of the artificial nose, making it perform more like a biological one.

At the leading edge of technology, biological tissue is used as the odor sensor. For example, researchers can insert a mammalian odor receptor gene into yeast cells, which then manufacture the receptor and install it on their own cell surface. A tiny shred of the yeast cell membrane—including an intact, functioning receptor—is cut out and anchored to a chip that produces an electronic signal whenever the receptor is activated.

In a different approach, researchers use bacteria cells to produce odor receptors and then paint receptor-laden cell membrane fragments onto a tiny quartz crystal. The vibrational frequency of the crystal changes along with the weight of the layer coating it; this setup—known as a quartz crystal microbalance—is so sensitive it can tell when the receptors in the layer of bioslime have latched on to odor molecules, increasing its weight. An English company is using this technology to detect explosives. Another group has gone further and integrated entire rat olfactory cells into a semiconductor chip. They call this setup an olfactory neurochip, but it’s really a rat-machine hybrid.

University-based scientists in France have pushed hybridism a step further: they have inserted a human odor receptor gene into yeast cells, which then express functioning human receptors for the odor molecule helional. The modified yeast cells become biosensors for helional. This is a technologically elegant but somewhat disturbing achievement: a combination of human DNA controlled by a foreign organism, which in turn is enslaved to a machine. Is this really a direction we want to pursue?

At some point in the development of these fusions of silicon and biology, the question becomes not whether the e-nose can replace the human nose, but whether we want it to. Would I let an e-nose sniff-scan me for lung cancer? Sure. Would I use a robotic odor sentinel? Maybe, especially if I had a BO problem. But do I really want my refrigerator to tell me, “I’m sorry, Avery, I can’t let you eat those cold cuts”?

Genes of Scent

Supermarket tomatoes have no flavor. It’s a common complaint, and a valid one. Commercial tomato varieties have less sugar, acid, and aroma than the wild type. On the other hand, they have better color, yield, disease resistance, and physical toughness, or what growers like to call “shippability.” (Tomatoes are picked while still hard and green, to help them survive the trip to the store.) The guiding principle for tomato breeders is that it is better to look good than to taste good.

Help may be on the way: as scientists decipher the genetics of flavor chemical production in plants, they open the door to bioengineered flavor enhancement. One research group has discovered genes for the enzymes that are the first step in the biochemical production of phenylethyl alcohol, a key ingredient in tomato aroma. When overexpressed in transgenic tomato plants, these genes give the fruit ten times more rose alcohol, making it more fragrant than the ordinary variety. Another scientific team recently created a tastier tomato by altering the gene controlling a key enzyme involved in aroma production. They took the enzyme gene from lemon basil and inserted it into a tomato plant, where it modified biochemical activity to produce higher levels of key aroma molecules. This is cool science, but the proof of the pudding is in the eating, and here the new transgenic tomato is a winner: It was preferred by panelists in taste tests.

Rather than import genes from other plant species, genetic engineers may decide to pluck useful ones from so-called heirloom tomatoes, the distinctive-looking and interesting-tasting varieties prized in farmers markets across the country. Heirloom tomatoes—with names like Marvel Striped and Purple Cherokee—existed before the breeding programs that created the standardized, disease-resistant, high-yield, shippable kind that dominate today’s supermarket shelves. Ann Noble, the UC Davis wine expert and creator of the Wine Aroma Wheel, has been lured out of retirement by Central Valley tomato growers looking to promote their heirloom business. They hope she will do for tomatoes what she did for wine—encourage sensory analysis to help consumers understand and appreciate all their varied aromatic qualities.