What the Nose Knows - Avery Gilbert [21]
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.
BY 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,