What the Nose Knows: The Science of Scent in Everyday Life (6 page)

In Longstaff’s field notes, one finds an astonishing range of butterfly odors. Some are like confections (vanilla, chocolate, burnt sugar), others like flowers (freesia, jasmine, heliotrope, mango flower, honeysuckle, sweetbriar). Yet others are like herbs and spices (cinnamon, lemon verbena, orris root, sandalwood, musk). Longstaff also found a spectrum of unpleasant scents, some reminiscent of cockroach or muskrat, others of rancid butter, butyric acid, vinegar, acetylene, musty straw, cow dung, horse stable, horse urine, and ammonia.

We now know that the lemony body odor of the Green-veined White contains alpha-pinene, beta-pinene, myrcene, limonene, linalool, p-cymene, neral, and citral. (The first five ingredients are also found in cannabis oil. Why should a psychoactive hemp plant and a butterfly share odors? Nature is wonderfully strange.) Males of the Green-veined White have another scent, which they hold in reserve for special occasions. It is methyl salicylate, easily recognized as the odor of wintergreen (or Pepto-Bismol). The male uses it as an antiaphrodisiac: he transfers the scent into the female at mating and it discourages other males from copulating with her afterward. Related species have their own versions of this turn-off tactic: the Small White uses a blend of methyl salicylate and indole; the Large White uses benzyl cyanide. These chemical countermeasures can backfire, as when the Large White’s antiaphrodisiac aroma draws unwelcome attention of a tiny parasitic wasp called
Trichogramma brassicae.
When a female wasp smells a recently mated Large White female, she grabs on and hitches a ride. As the butterfly lays her eggs, the wasp parasitizes them by laying her own eggs inside them, and her young later use the butterfly’s eggs for food. In the end, the male Large White who tried to defend his genetic investment ended up sacrificing some of his potential offspring.

N
ATURAL BOTANICAL
scents have a soft-focus, flower-child ambience about them. They are perceived as innocuous and innocent, a gift from Earth Mother Gaia to aromatherapists everywhere. In reality, they are biological communication systems, a way for plants and animals to talk to each other. This also makes them instruments of deception and treachery. Once a smell is used as a signal, other organisms can turn it to their selfish advantage. (Ask a female Large White how she feels about the parasitic wasp on her back.) A Mediterranean plant called the dead-horse arum fakes the stench of rotting meat. It attracts blowflies looking to lay eggs on a nice ripe carcass. The blowfly gets fooled into pollinating the plant for free, traveling from one stinky plant to the next carrying pollen on its legs, in what has been called “a striking example of evolutionary cunning that exploits insects for pollination purposes.” Other examples are more sinister and almost perverse. An Australian orchid emits a smelly molecule called 2-ethyl-5-propylcyclohexan-1,3-dione, which happens to be the exact molecule produced as a sex attractant by females of the wasp species
Neozeleboria cryptoides.
When the orchid joins the action, the result is an aroma-based, cross-species sexual deception in which hapless male wasps attempt to copulate with the orchids. In the end, the orchid is pollinated and the male wasp is frustrated. Sex and exploitation are never far apart.

In nature, smells also serve defensive purposes. Essential oils, cherished as healing elixirs by aromatherapists, are really weapons in the ongoing battle between a plant and its predators. Take the orange tree as an example. It provides three different materials used in perfumery: neroli oil from its flower, orange peel oil from its fruit, and pettigrain from its leaves. Orange trees didn’t evolve for the perfumer’s convenience. Flowers smell good to attract pollinators; fruits smell and taste good to attract seed dispersers. A leaf releases volatile aromatic compounds as soon as an herbivore bites into it. This makes the leaf unpalatable or even toxic to the attacker (a caterpillar, say) and simultaneously alerts predators (such as wasps) that food is available. To an aromatherapist the orange tree is a repository of healing oils; to a caterpillar it looks like a weapons depot ringed with alarms and booby traps.

In college I lived for a time near the eucalyptus grove at the West Gate of the Berkeley campus. I loved to walk through its aromatic shade on the way home from class. The fresh astringency of the trees, like the fog that sometimes shrouded them, was to me a key element of Bay Area aesthetics. Back then I took a simple pleasure in that smellscape, and I still do. But today I also see it another way: as the lingering haze of biological warfare. Eucalyptol, chief among the fragrant molecules wafting about the West Gate, wards off leaf-eating bugs and suppresses the growth of seedlings of competing tree species.

The Web of Nature

Near Guaraqueçaba in southern Brazil is a remnant of the rain forest that until recently covered all 4,650 miles of the country’s Atlantic coast. While prospecting there one spring for unusual smells, Roman Kaiser found the forest suffused with a strong fruity-floral scent. He tracked it to a tree with white bottle-brush flowers. Nearer the tree the scent took on a blackcurrant quality; close to the flower itself the smell resembled cat pee. With chemical analysis, Kaiser was able to trace both odors back to a single molecule: 4-mercapto-4-methylpentan-2-one, or MMP. (It is one of many molecules whose odor character depends on airborne concentration.) For most people, that would be the end of it: Odd Molecule Found in Exotic Locale. But Kaiser—a man with a chemist’s brain and a perfumer’s heart—has probably sniffed more GC samples than any living human. For him, MMP isn’t a singularity—it’s one node on a web of connections. Follow this molecule through the web and you’ll find yourself transported all over the world. MMP is a key aroma in Japanese green tea, grapefruit, basil leaves, tomato leaves, box tree, cabernet sauvignon wine, and
Paeonia lutea
(the Tibetan peony). Is this a fluke? Or is 4-mercapto-4-methylpentan-2-one the clue to a hidden pattern in nature?

Since the advent of GC-O studies in the 1980s, chemists have analyzed everything from tomato paste to parsley, boiled beef to baby farts. In each substance they find many volatile molecules, yet only a few that are responsible for its characteristic aroma. Scientific journals are loaded with such studies, which are all cross-referenced in print. Imagine that this information is digitally organized and can be accessed as coolly and smoothly as Chloe calls up building diagrams for Jack Bauer on
24.
Each natural substance has its own web page listing key odorants—one can hyperlink from molecule to substance in any direction. Start, for example, with the home page for fresh oysters from the coast of Brittany. They contain 1-octen-3-one, which produces a mushroomy citrus note fancied by oyster lovers. Click on 1-octen-3-one, and you find yourself on the home page for Moroccan sardines, which they express this molecule after sitting on ice for a couple of days. In browsing the sardine page you find that fresh ones have a pleasant seaweedy scent traceable in part to (
E,Z
)-2,6-nonadienal. Click on that molecule and you are returned to the Brittany oyster home page. Why? Because (
E,Z
)-2,6-nonadienal is a characteristic odor molecule in fresh oysters.

Let’s play the game again, this time starting with dimethyl sulfide, another key oyster odorant. It shows up in tomato paste, spoiled refrigerated chicken, and pinto-bean farts. Jump to the spoiled chicken page and click on methyl mercaptan; this will take you back to farts, or on to feces and french fries. From feces we can transfer to dimethyl trisulfide, which leads to Asian fish sauces and Gewürztraminer wine. Another key to the varietal character of Gewürztraminer is
cis
-rose oxide. Follow the link to
cis
-rose oxide and you see that this molecule is also responsible for the floral quality of fresh lychee fruit. On the lychee fruit home page you find that another potent odor is 1-octen-3-ol; clicking on it takes you to the Brittany oyster home page. Why? Because 1-octen-3-ol lends an earthy odor to both French oysters and lychee.

Is there a profound meaning in the hyperlink path from oysters to spoiled chicken to feces to Gewürztraminer to lychee and back? I doubt it. It’s just Six Degrees of Kevin Bacon played with molecules. The olfactory web of 4-mercapto-4-methylpentan-2-one that links green tea to peony is not unusual. A given odor molecule turns up time and again; nature is economical and uses the same molecule different ways in different organisms.

B
Y 1974, ROUGHLY
2,600 volatiles had been identified in food. By 1997 the estimate had swelled to 8,000 and was predicted to climb eventually to 10,000. These are large numbers. They would be even larger if we included volatiles from nonfood items like airplane glue, dirty socks, and that crust of dried vomit under the backseat of the family minivan. Add them all up and the numbers are overwhelming. When it comes to potential smells, nature’s bounty seems infinite.

What does all this molecular variety mean for the sense of smell? If the same chemicals turn up repeatedly as key smell ingredients, what impact does the rest of nature’s chemical diversity have on the human nose? One answer is that we are missing most of it: we read the olfactory headlines and ignore the fine print. The field of sensory analysis confirms that only a fraction of the chemicals entering our noses from a given source make a difference to our perception of its odor. In most foods, for example, only a few of the volatiles detected by chemical analysis are present at nose-perceptible concentrations. Of the 400 or more volatiles found in a tomato, for example, only sixteen reach the threshold of human perception. One expert figures that fewer than 5 percent of the volatiles in a food actually contribute to its aroma. Perhaps odorants aren’t as numerous as they seem.

So-called aroma models take this insight even further. To create an aroma model for french fries, for example, scientists run a batch through the GC/MS and generate a complete list of all the volatiles. Their goal is to create a fully realistic french-fry aroma using as few of the volatiles as possible. They begin by selecting odorants present at concentrations well above our sensory threshold. If a blend of those doesn’t match the original aroma, they extend the list to include odorants at or below the sensory threshold. Once a blend closely matches the full aroma, it is tested further. One by one, odorants are subtracted from the formula. If the resulting formula smells less realistic, the subtracted odorant is restored. If the subtraction makes no difference, that odorant is dropped. The final aroma model is one of irreducible simplicity—a stripped-down formula that smells complete to the nose. An authentic french-fry smell, for example, can be made from nineteen ingredients. This includes a trace of stinky methyl mercaptan—without it, the formula lacks the necessary boiled-potato character.

Aroma models have been developed for Swiss cheese, Camembert, basil, olive oil, and baguette crust, among other things. These whittled-down formulas all point to the same conclusion—most volatiles in a food add nothing to its smell. A high-fidelity odor replica can be created from one or two dozen ingredients. A classic example is the cup of coffee. Chemists have been analyzing coffee aroma for more than 100 years and have found more than 800 different molecules. Using aroma models, German scientists found a mere twenty-seven high-impact molecules in medium-roasted Arabica coffee; they made a high-fidelity model using only sixteen of them.

The sensory logic of aroma models can be extended to nonfood areas, and may even have applications for environmental issues. Livestock feeding operations, for example, generate a big, messy stink that can annoy nearby residents. A typical Iowa swine barn contains more than 300 different volatiles, which sounds like a lot of bad news for the downwind neighbors. Yet a recent study found that four molecules account for about 85 percent of the piggy odor. One of these—para cresol—has a smell that by itself closely resembles the overall barnyard odor. This discovery may turn an overwhelming odor problem into a manageable project. Instead of going after all 300 suspect chemicals in the swine barn, one might suppress a handful of character-defining molecules. Pinpoint sensory targeting could produce bigger benefits at less cost.

T
HE SUCCESS OF
aroma models—those minimalist imposters—casts nature’s abundance in a new light. Lifelike smells can be made from a handful of molecules, and the same molecules turn up in smell after smell. Is nature’s chemical cornucopia really so impressive if only a tiny portion of it matters? And what does it say about our sensory abilities if there is so much more out there than meets the nose?

Terry Acree, the Cornell scientist who helped develop GC-olfactometry, has the numbers to back this up. He searched through hundreds of food-aroma studies and made a list of volatiles present at smellable concentrations. The first edition of the FlavorNet list was posted online in 1997. It contained three hundred chemicals. Today he has posted about eight hundred. Acree expects the list to top out at fewer than one thousand. In other words, all the smells in nature are built from fewer than one thousand smellable chemicals. What are those other thousands of volatiles doing? They may subtly round out a scent, give it shading and complexity. Acree speculates that many of them are intended for the noses of creatures other than ourselves; the scents of nature are largely a chemical conversation between plants and animals, and humans merely eavesdrop. Just as we are blind to certain patterns on the wing of a butterfly or the petals of a flower because we cannot see in the ultraviolet, our mammalian noses are not tuned to certain olfactory broadcasts.