What the Nose Knows: The Science of Scent in Everyday Life Read Online Free PDF Page B

component. Laing made the test easier: he gave people a target odor and asked them whether they could smell it in the mixture. Even then, they could rarely find the target in a mix of more than three odors. Could the problem be lack of skill? Laing tested perfumers and flavorists. The professionals were better than amateurs at identifying two and three items in a mixture, but even with their training and experience, they failed to pick more than three odors from the mix. Laing reasoned that mixtures of simple, single-chemical smells are somehow unnatural and hard to pick apart. So he repeated the experiments using as mixture components such complex odors as cheese and chocolate. The results were the same: no one could bust the four-odor limit. Were the individual smells not distinctive enough? Some odors, such as orange, almond, and cinnamon, blend together easily; perhaps those that blend poorly, such as mushroom, cut grass, and mandarin, are easier to pick out of a mixture. Laing found this was true to a point, yet the four-item barrier held firm.
    Why are we so feeble at smelling our way through a bouquet? Our ability to gather olfactory information is formidable: the human nose detects single smells at extraordinarily low concentrations. We do a better job of collecting smells than we do of tracking them in a complex mixture. The Laing Limit suggests that the problem is not in the nose but in the brain. We have limited ability to think about smells analytically.
    In the end, the question “How many smells are there?” may not be as relevant as “How many odor categories do we need to make sense of the world?” The answer to that question will reveal much more about how the brain handles the information that the nose provides.

CHAPTER 2
    The Molecules That Matter
    You cannot suppose that atoms of the same shape are entering our nostrils when stinking corpses are roasting as when the stage is freshly sprinkled with saffron of Cilicia and a nearby altar exhales the perfumes of the Orient.
    —L UCRETIUS
    S TRICTLY SPEAKING, SMELLS EXIST ONLY IN OUR HEADS. Molecules exist in the air, but we can only register some of them as “smells.” Odors are perceptions, not things in the world. The fact that a molecule of phenylethyl alcohol smells like rose is a function of our brain, not a property of the molecule. A tree burning in the forest does not smell if no one is there to smell it. The planet Mars has no atmosphere and is too cold for human life, yet the chemical composition of its surface suggests that if we could sniff it, it would reek of sulfur. Perhaps someday we will have the opportunity. Apollo moon-mission astronauts noticed that the lunar dust they tracked back into their craft smelled like wet ashes in a fireplace, or burned powder from a shotgun shell. Humans flying back from Mars may need to hang a little pine tree in the cockpit window.
    Semantics aside, an odor perception is usually caused by a physical substance—molecules light enough to evaporate and be carried on air currents to our nose. (There are strange exceptions: some observers of the early aboveground nuclear bomb tests experienced a metallic smell within moments of the blast, and in the rare condition known as phantosmia, patients perceive a smell in the absence of any external stimulus.) The sensory cells in our nose convert a chemical signal (the molecule) into an electrical signal (a nerve impulse) that travels up the olfactory nerves to the brain for interpretation. Since airborne molecules trigger odor perceptions, we should, in principle, be able to match a molecule to every odor. Hydrogen sulfide smells like rotten eggs and amyl acetate smells like banana—how hard can it be to complete the list? Very hard, it turns out. Most aromas in nature are elaborate bouquets, mixtures of dozens if not hundreds of different molecules.
    Prior to 1955, complete chemical analysis of the aroma from a cup of coffee was beyond the reach of routine