What the Nose Knows: The Science of Scent in Everyday Life

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Odors in the Mind

It is very obvious that we have very many different kinds of smells, all the way from the odor of violets and roses up to asafetida. But until you can measure their likenesses and differences you can have no science of odor.

--ALEXANDER GRAHAM BELL, 1914

No satisfactory classification of odours can be given.

--Encyclopaedia Britannica, 1911

How many smells are there? It's an odd question, but give it some thought. Mentally flip through the pages of your personal smell catalog. You find burnt toast, shaving cream, Grandma's kitchen, and pine trees. There's the weird glue in the binding of that pocket-size Latin/English dictionary from high school. With a little effort you can come up with a lot of smells, but putting a number to them is difficult. How does one count the odors of a lifetime, much less all the odors in the world?

Some people aren't daunted by the task: they simply estimate. Better yet, they pass along estimates made by others. Journalists like to say that we can smell thirty thousand different odors. The New Age guru Michael Murphy cites this figure in The Future of the Body (1992): "According to the calculations of one [fragrance] manufacturer, an expert can distinguish more than 30,000 nuances of scent." Murphy, in turn, got the number from Vitus Droscher, a German pop science writer (1969): "A perfume manufacturer has worked out that a real expert must distinguish at least thirty thousand nuances of scent." Droscher doesn't provide a source. Perhaps it was in Science Digest (1966): "Industrial chemists have identified some 30,000 different smells." Unfortunately, the magazine didn't provide a source either. What this proves, I suppose, is that dubious facts thrived in the media long before the Internet.

One would like to think that smell scientists have a better grasp of the matter, and indeed they prefer to quote a different estimate. When Linda Buck and Richard Axel won the 2004 Nobel Prize for discovering the olfactory receptors, the Nobel Foundation issued a press release. It noted that people recognize and remember "about 10,000 different odours," a figure the Swedish publicists took from the prize winners themselves. Surely that's a number we can take to the bank. But the number 10,000 didn't originate with Buck and Axel: it had been tossed about for years by other scientists. Something about it had always bothered me--why such a nice fat round number? Why was there no date of discovery? And, strangest of all, why did nobody take credit for it?

If you try to track down the mysterious number 10,000 to its original source in the scientific literature, you are in for an adventure; like walking a maze, dead ends abound. For example, I began with a paper in Behavioral Ecology (2001), which I followed to another in Trends in Genetics (1999), which in turn didn't provide a source. I started again, this time with a prominent smell researcher, the Brown University psychologist Trygg Engen. In 1982 he wrote, "Some have claimed that an untrained person can identify by label at least 2,000 odors and an expert can identify as many as 10,000." Engen credits this claim to R. H. Wright, Canada's most famous smell scientist. Wright seemed a likely source, at least until I read what he actually wrote, back in 1964: "[I]t seems likely that the average person would have no trouble in distinguishing between several thousand odours, and an experienced authority in the field has claimed the ability to recognize well over ten thousand. Still another has simply said the number is apparently unlimited." Wright goes on to say, "It would be an interesting exercise to design an experiment to verify these estimates." Ooof! So Wright didn't discover any number at all--he just passed along what he had heard and Professor Engen repeated it. These eminent smell experts remind me of kids at summer camp passing along ghost stories.

I was beginning to think I'd never find the source of the magic number 10,000, when I found it once more in a 1999 food chemistry textbook. From there I followed it to a 1966 paper, and then to a paper published in 1954 by researchers from the Arthur D. Little, Inc., consulting company. At a scientific conference the previous year, the Little folks presented a paper titled "An information theory of olfaction." Their goal was to place numerical limits on odor perception. They said "There are experts who affirm that it is possible to recognize at least 10,000 odors," a figure they used in their mathematical subsequent analysis. The name of their expert was buried in a footnote: he was Ernest C. Crocker, a chemical engineer and 1914 MIT graduate who, not coincidently, was also an employee of Arthur D. Little, Inc.

Back in 1927, Crocker and another Little chemist named Lloyd F. Henderson were struggling for an objective way to classify odors. They settled on a method in which an odor was rated by how strongly (on a scale of 0 to 8) it resembled each of four elementary odor sensations. Given the mathematics of their rating system, it was theoretically possible to discriminate 94 or 6,561 different odors. The math is watertight, but the outcome is highly dependent on the initial assumptions. Had Crocker and Henderson used, say, five elementary sensations and a 0-to-10 rating scale, the estimate would have been 115 or 161,051 different odors. (Harvard psychologist Edwin Boring was a fan of the new system, but he believed the rating scale should have fewer steps. He did some calculations and decided that the number of distinguishable smells was somewhere between 2,016 and 4,410.) Discussing this work years later, Ernest Crocker generously rounded up the estimate to 10,000 odors. His colleagues took the number and ran with it.

In the end, it appears that no one has ever attempted to count how many smells there are in the world. Estimates of odor diversity lead either to a dead end or to Ernest C. Crocker. The comfortable, often-cited figure of 10,000 smells is, from a scientific perspective, utterly worthless.

Why does it matter exactly how many smells there are? Suppose we want to build a device that can reproduce every possible odor. (This is a popular fantasy. As a kid you may have scratch-and-sniffed your way through Mickey Mouse and the Marvelous Smell Machine.) A pair of industrial engineers once looked into how many distinct odors it would take to create a lifelike smellscape in virtual reality. They settled on a figure of 400,000. (This number has no more basis in fact than 10,000 or 30,000; its ultimate source is an obscure Japanese technical publication). Four hundred thousand is a staggeringly large number, but it sounds reasonable to engineers who use 16.7 million colors per pixel in visual displays for VR goggles. The trouble is that an engineer's solution doesn't always correspond to how the brain solves the problem.

The human eye detects tiny differences in color; across the visible spectrum we are capable of millions of such discriminations. Yet when it comes to naming categories of color, there is nearly universal agreement that only a half-dozen are needed to cover the range of human perception. People in all cultures get by very well with white, black, red, green, yellow, and blue. (Esoteric hues such as ecru and mauve occur mainly in clothing catalogs.) The physical spectrum of visible light is continuous; the stripes of the rainbow are created in our head. We give color names to these few categories.

This simplification of sensory input is a general feature of the brain, a phenomenon psychologists call categorical perception. In hearing, categorical perception helps us carve the continuous dimension of pitch into the individual notes of the musical scale, or the sonic blur of vowel sounds into a distinct a or e. Perhaps we shouldn't obsess about the number of odors. Our question should be, How many natural odor categories are there, and how do our nose and brain simplify the world?

The Art of the Achievable

I grew up in Davis, California, amid the smells of agriculture. Our house, when we first moved there in 1962, was near vast tomato fields; walking through them brought up the sharp, funky smell of the vines. The approach of a new school year was signaled by the heavy, stewed smell of tomatoes being cooked into ketchup at the Hunt-Wesson plant a mile upwind. My buddies and I played on mountains of newly baled alfalfa, stacked high and smelling grassy-sweet. The playground at the Valley Oak Elementary School offered the hot-metal smell of monkey bars and the dusty, sour resin of tanbark underfoot. The water from the sprinklers in the town park had a musty tinge to it. The office of the Davis Enterprise, where I rolled copies for my afternoon paper route, was saturated with the smell of fresh ink, newsprint, and rubber bands. In grade school my class toured the Spreckles refinery, where truckloads of sugar beets were turned into pure white sugar, a magical transformation dimmed by the suffocating scent of dark molasses that hung over the place.

We moved to Davis because my father joined the faculty of the philosophy department at the university there. Davis was originally the agricultural field station for the main university campus at Berkeley. Set in the hot, flat, and fertile Sacramento Valley, but near the cooler hills of Napa and Sonoma, UC Davis came into its own in the 1960s, when it added a law school and a medical school. At the same time, researchers in the Department of Viticulture and Enology got the ball rolling for what became the California wine-making revolution. They measured microclimates and soil composition, developed new grape varieties, and invented cold fermentation and other wine production techniques. Davis graduates who took courses in wine tasting and wine-making are now among the world's leading vintners. As part of this effort, UCD researchers took up the sensory analysis of wine. Their challenge was to apply objective methods to one of the more rarefied arenas of subjective opinion: wine tasting.

One of this group was Professor Ann Noble, a chemist and sensory specialist. Among her interests was identifying volatile chemical compounds in wine. These substances create the characteristic aroma of grape varieties such as Cabernet Sauvignon or Riesling, and also the off-odors found in poorly made wines. Noble hoped to link aroma chemistry to broader grape-growing and wine-making factors.

Noble's approach to aroma was practical and effective. Not for her the pretensions of the wine snob, epitomized by Paul Giamatti's character in the movie Sideways, who sticks his nose in a wineglass and says, "I'm getting strawberries, some citrus . . . passionfruit, just the faintest soupcon of asparagus, and, like, a nutty Edam cheese." To better understand California Cabernets, Noble and her colleague Hildegard Heymann selected wines from seven regions around the state (Napa, Sonoma, Alexander Valley, Santa Ynez, etc.). The wines were rated by enology students who came up with their own simple descriptive terms: berry, bell pepper, eucalyptus, and so on. Reference samples were created by doctoring a neutral "base" wine. To represent berry, for example, one-half teaspoon of raspberry jam and one half of a frozen blackberry were added to a half-cup of wine; after soaking for ten minutes, the blackberry was removed. For the soy/prune standard, the basic wine was spiked with a quarter-cup of canned prune juice and seventeen drops of Kikkoman soy sauce.

Armed only with their noses and a nine-point rating scale, the students sniffed and sipped their way to an enormous pile of data. (It filled a metaphorical filing cabinet thirteen descriptors wide, twenty-one Cabernets tall, and thirteen judges deep.) With a computer program, Noble and Heymann extracted a small number of sensory dimensions and placed each wine at a precise location on them. They could now visualize the smell and taste relationships between the samples. Their conclusions: "Younger vines and/or vines from cooler areas tend to produce more intensely vegetative wines. Conversely, wines from older vines and/or warmer areas tend to have higher ratings for berry aroma, fruit flavor by mouth, and vanilla aroma." By quantifying wine aromas with a system of practical description, they discovered the vineyard conditions that produce them. Alexander Graham Bell would have been pleased.

What earned Ann Noble and her colleagues a place in the annals of smell classification was their Wine Aroma Wheel, published in 1984. The wheel was a visually pleasing presentation of a standardized wine aroma vocabulary. With twelve categories and ninety-four descriptive terms, it covers the aroma of any wine, regardless of grape or geographic origin. What makes the wheel just as useful to novices as to connoisseurs are the do-it-yourself reference standards. Simple kitchen chemistry lets anyone create and experience the standards for himself. Noble is critical of commercial wine-tasting kits; she believes that vials of flavor essence are chemically unstable and tend to degrade quickly. This spurred her to create reference standards that can be "prepared using foodstuffs available throughout the world during most seasons."

The Wine Aroma Wheel resembles a dartboard: three concentric circles divided pizzalike into a dozen slices of varying width. On the innermost circle, the pointed end of each wedge is an aroma category, such as fruity. In the middle circle, the wedge may be split into subcategories such as citrus, berry, or tree fruit. On the outer circle are specific materials, examples of each aroma subcategory. Thus you can follow the fruity wedge through the berry subslice to the outer circle; there you'll find blackberry, raspberry, strawberry, and blackcurrant. The beauty of Noble's wheel is that it links sensory concepts to actual everyday stuff--it connects Riesling to raspberries. Wheel in hand, it is possible to sniff your way to sensory enlightenment. This commonsense approach lets anyone grasp the esoteric inner-wedge category microbiological and its arcane subdivision lactic. It's only baffling until you sniff the examples: yogurt and sauerkraut. Then it clicks. The wheel even demystifies the wine-snob term wet dog: it's an example of sulfur aromas in the chemical category (along with skunk, cabbage, and burnt match).

There is no place on the wheel for the wine critic's gaseous adjectives. You will find "orange blossom" and "black olive" and the less flattering "soapy" and "cooked cabbage." But you will not find "an impertinent little Pinot Noir" or a "flabby, overripe Cabernet Franc," a la Miles Raymond. These free-form prose poems say more about a wine lover's pretensions than about the character of the wine. To use the wheel, all you need is a glass and grocery store.

A practical smell classification for beer was created in the 1970s by a Danish flavor chemist named Morten Meilgaard. His Beer Flavor Wheel has now been adopted worldwide. It uses fourteen categories and forty-four sensory terms to describe the smell and taste of any style of beer--lager, ale, or stout. Most of the descriptors deal with aroma; others involve taste (bitter for hops; sweet for malt) and sensory factors like carbonation.