What the Nose Knows: The Science of Scent in Everyday Life by Avery Gilbert Page B

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