Chapter 2 — The Nose Knows

Chapter 2 — The Nose Knows

"Smell is a potent wizard that transports us across a thousand miles and all the years we have lived." — Helen Keller, The World I Live In, 1908

Reader, before we go a step further, I must ask you to perform a small act of self-experimentation, one that costs nothing but a moment's dignity. Procure a jelly bean — the gaudier the better, for colour is precisely the sense about to be exposed as a bystander — and pinch your nostrils firmly shut with finger and thumb. Now place the jelly bean upon your tongue and chew, keeping the nose sealed throughout. You will taste sweetness, certainly, and perhaps a modest sourness besides; you will register a pleasant yielding texture giving way to mush. But the particular thing the sweetness is — whether it is announcing itself as cherry, lime, liquorice, or buttered popcorn — will stubbornly refuse to appear. It is only when you release the pinch, somewhere in the midst of chewing, that the fruit or flavour concerned arrives all at once, vivid and entire, as if a curtain had been drawn back on a room that was there all along. This modest parlour trick, performed in classrooms and kitchens for generations, contains the whole argument of this chapter in miniature: that the thing we so confidently call "flavour" is, in the greater part, not a matter of the tongue at all, but of the nose — and specifically of a secret second route by which the nose does its work, one that has nothing whatsoever to do with sniffing.

It is a curious inheritance of our language that we speak of "tasting" a strawberry, a glass of claret, a well-reduced stock, when the tongue itself is contributing only the barest sketch of the experience — sweetness here, a whisper of acid there, perhaps a savoury depth besides — while the character, the identity, the very thing that makes a strawberry unmistakably a strawberry and not a raspberry wearing its colours, arrives by the nose. Harold McGee, that most scrupulous of modern kitchen naturalists, puts the matter bluntly: what we call flavour is assembled chiefly from aroma, with taste and touch furnishing only the frame around it. The tongue, as the previous chapter showed, can report perhaps five or six qualities — sweet, sour, salt, bitter, umami, and the still-disputed matter of fat. Everything else in the vast library of flavour — the greenness of a tomato leaf, the smoke in a whisky, the particular mustiness of a good Cheddar, the whole sprawling vocabulary of the spice rack — is smell, and smell alone supplies the words. This chapter is devoted to that supplying: to how the nose does its extraordinary work, to the individual molecules that furnish some of its most memorable messages, and to the strange, direct wire that connects a scent to the oldest and least rational parts of the feeling brain.

Two roads to the same city

The nose, it transpires, keeps two separate appointments with the world, and confuses the unwary because it uses precisely the same apparatus for both. The first appointment is the one everyone recognises: a rose lifted to the face, a wine glass swirled and inhaled, a loaf drawn hot from the oven and held beneath the nostrils in frank appreciation. Air is drawn in through the nostrils, carrying with it whatever volatile molecules the object has shed into the surrounding atmosphere, and arrives at a patch of specialised tissue high in the nasal vault. This is orthonasal smell — "ortho," straight or direct — the front door, the smell that announces a thing before it has even been touched, let alone eaten.

The second appointment is stranger, and it is the one this chapter is chiefly concerned with. When food is taken into the mouth and worked by tongue, teeth, and the warm, moist business of chewing, it releases its volatile compounds not into the room but into the mouth itself — a small, sealed chamber connected to the nasal cavity by a passage at the very back of the throat, behind the soft palate. With each swallow and each outward breath, a bellows-like action pumps a plume of these mouth-borne aromatics backward and upward through the nasopharynx, to arrive at the very same olfactory tissue, but from behind. This is retronasal smell — "retro," backward — so called because the air arrives at the nose travelling in the opposite direction to ordinary sniffing: outward-bound breath, not inward-bound. The psychologist Paul Rozin, who did more than anyone to give this duality its proper name in a 1982 paper, called it a strange and singular fact about olfaction: alone among our senses, smell perceives both the world beyond the body and the state of the body's own interior, using one and the same organ, which wears two different hats depending on which direction the air happens to be travelling.

It was Brillat-Savarin — no stranger to these pages, nor indeed to any pages on the subject of eating well — who anticipated the physiology by the better part of two centuries with an aphorism as tidy as it is true: "Smell and taste are in fact but a single composite sense, whose laboratory is the mouth and its chimney the nose."[^1] He had the mechanism slightly wrong, in that he imagined one sense where modern science finds two allied ones cooperating; but the image of the mouth as laboratory and the nose as its chimney, drawing the smoke of cookery upward and outward, is so apt that flavour science has never much bettered it. The mouth compounds and warms and macerates; the "chimney" — that retronasal passage behind the palate — draws the result upward to be read.

The two roads lead to the same city, in other words, but they arrive by entirely different gates, and — this is the important part — the brain does not experience them as identical. Sniff a wedge of ripe Époisses cheese across the table and it may strike you as frankly offensive, redolent of the farmyard and worse. Put a forkful in your mouth, chew, and swallow, and by the retronasal road the very same volatile molecules — many of them the products of bacterial ripening, sharing chemistry with the smell of unwashed feet — arrive instead as something rich, savoury, and delicious. Rozin used exactly this trick of perception, with Limburger cheese as his example, to demonstrate that orthonasal and retronasal smell, though built from identical molecules detected by identical receptors, are treated by the brain as though they were two different senses entirely: one referred outward to "the world" and judged accordingly, the other referred inward to "the self" and the business of eating, and marked as pleasant even when its orthonasal twin would be marked foul. The nose, we might say, keeps two different account books for the very same currency.

The professional taster exploits this duality quite deliberately, though rarely with the vocabulary to explain what she is doing. A sommelier who draws air noisily across a mouthful of wine — a habit that strikes onlookers as faintly absurd — is not merely aerating the liquid to release its orthonasal bouquet a second time; she is actively pumping air across the wine while it sits in the warm cavity of the mouth, forcing a richer plume up the retronasal passage than passive chewing would ever produce, and reading a second, quite different report from the one her first sniff over the glass provided. Practised tasters will tell you the retronasal register often reveals notes the orthonasal nose missed entirely — a fact this chapter's physiology explains rather better than instinct alone ever could.

<figure> <svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 640 500" role="img"> <title>The Two Roads of Smell — a head in profile showing orthonasal and retronasal air paths converging on the olfactory epithelium</title> <defs> <pattern id="olf-hatch" width="5" height="5" patternUnits="userSpaceOnUse" patternTransform="rotate(20)"><line x1="0" y1="0" x2="0" y2="5" stroke="#9c5a2c" stroke-width="0.6"/></pattern> </defs> <rect x="12" y="12" width="616" height="476" fill="none" stroke="#43331f" stroke-width="1"/> <rect x="15.5" y="15.5" width="609" height="469" fill="none" stroke="#43331f" stroke-width="0.5"/> <g fill="#43331f"> <path d="M12,7 L17,12 L12,17 L7,12 Z"/><path d="M628,7 L633,12 L628,17 L623,12 Z"/> <path d="M12,483 L17,488 L12,493 L7,488 Z"/><path d="M628,483 L633,488 L628,493 L623,488 Z"/> </g> <text x="320" y="41" text-anchor="middle" font-family="Georgia, 'Times New Roman', serif" font-size="14" letter-spacing="2" fill="#43331f">THE TWO ROADS OF SMELL</text> <line x1="150" y1="56" x2="490" y2="56" stroke="#6f5a40" stroke-width="0.5"/> <path d="M250,95 C195,100 165,140 165,190 C165,230 175,270 180,310 C185,340 195,365 210,390 L300,390 C305,375 300,355 305,335 C310,320 320,308 322,298 C328,290 332,285 330,278 C332,270 330,262 335,255 C340,248 348,240 352,228 C356,218 358,208 352,196 C348,175 338,155 322,140 C310,125 290,108 250,95 Z" fill="none" stroke="#43331f" stroke-width="1"/> <path d="M186,178 Q200,172 200,192 Q198,210 186,206" fill="none" stroke="#6f5a40" stroke-width="0.6"/> <g fill="none" stroke="#6f5a40" stroke-width="0.7"> <path d="M345,207 C330,180 300,155 258,150"/> <path d="M340,215 C320,222 295,222 275,235"/> <path d="M275,240 C268,230 260,190 258,150"/> <path d="M280,268 C300,268 322,266 332,262"/> <path d="M275,240 C255,255 245,262 238,268"/> </g> <g fill="none" stroke="#6f5a40" stroke-width="0.6"> <path d="M275,240 C255,270 240,300 232,338"/> <path d="M255,232 C230,262 212,296 202,336"/> </g> <path d="M240,142 Q258,132 276,144 Q268,155 254,155 Q242,153 240,142 Z" fill="url(#olf-hatch)" stroke="#9c5a2c" stroke-width="0.7"/> <g fill="none" stroke="#43331f" stroke-width="0.8" stroke-dasharray="3,3"> <path d="M372,201 L347,207 C331,181 300,156 260,149"/> <path d="M268,266 C266,232 260,190 258,151"/> </g> <g fill="#43331f"> <path d="M260,148 L253,153 L258,157"/> <path d="M259,153 L251,155 L255,161"/> </g> <g stroke="#6f5a40" stroke-width="0.5" fill="none"> <line x1="390" y1="126" x2="266" y2="146"/> <line x1="428" y1="188" x2="373" y2="200"/> <line x1="158" y1="258" x2="266" y2="266"/> <line x1="392" y1="268" x2="333" y2="260"/> <line x1="148" y1="328" x2="218" y2="322"/> </g> <g font-family="Georgia, 'Times New Roman', serif" font-style="italic" font-size="12" fill="#43331f"> <text x="395" y="122">olfactory epithelium</text> <text x="430" y="184">orthonasal</text> <text x="88" y="256">retronasal</text> <text x="396" y="276">mouth</text> <text x="118" y="332">throat</text> </g> </svg> <figcaption><em>Plate 3. The two roads of smell: the direct road inward through the nostrils (orthonasal), and the secret road that climbs from the back of the mouth to the olfactory vault (retronasal), by which the greater part of flavour is truly perceived.</em></figcaption> </figure>

The proof is in the pinching

I return to our jelly bean, because it deserves a fuller accounting than a mere demonstration in passing. With the nostrils pinched shut, no air can travel either road: the front door is barred to incoming sniffs, and — this is the detail most people miss — the chimney is also, in effect, sealed, because a firmly pinched nose prevents the outward plume of retronasal air from escaping through the nostrils at all; it dead-ends in a closed nasal chamber and never reaches the receptors in useful quantity. Sweetness survives the pinch because sugar is detected by the tongue directly, no nose required. Colour survives because the eyes remain, inconveniently for the purposes of a clean experiment, open. But the strawberry-ness, the vanilla-ness, the identity of the confection — all of that is retronasal aroma, and all of it vanishes the instant the nostrils are sealed.

This is not merely a parlour trick; it is a diagnosis that anyone who has struggled through a head cold has already lived, without perhaps understanding what was happening to them. The common cold does not damage the tongue, nor does it touch the eyes; what it does is choke the nasal passages with mucus and inflame the tissue within, blocking both the orthonasal route — hence food no longer "smells of" anything when sniffed directly — and, more consequentially for the miserable sufferer at the dinner table, the retronasal route as well. The result is the familiar complaint that food has "no taste" during a cold, when what has actually happened is more precise, and in its way more remarkable: taste is entirely intact, the sweetness of honey, the saltiness of broth, the bitterness of black tea all present and correct, but flavour — the richer confederation of taste and aroma that gives food its particular identity — has collapsed to almost nothing. Reader, if you wish to know how much of what you habitually call "taste" is truly taste, and how much is a nose working quietly backstage, you need only wait for your next cold and take notes at the table. Clinically, a permanent loss of the sense of smell is called anosmia, and those who suffer it — through injury, infection, or the neurological aftermath of certain illnesses — report exactly this experience made permanent: a world of sweet, sour, salt, bitter, umami, and a general blankness where a whole universe of identity used to live. It is a loss easy to underestimate until one imagines honey that tastes only of sweetness, coffee that tastes only of bitterness, and a Sunday roast reduced to salt and savoury greasiness, with all the roast itself — the browned, nutty, oniony, herbed particularity of it, the very subject of our next chapter — simply gone.[^2]

The vault above

To understand why pinching the nose works such sudden violence upon flavour, we must climb — in imagination, at least — to the roof of the nasal cavity, a region so awkwardly placed that it receives only a modest fraction of each ordinary breath. Most of the air we inhale streams along the floor and middle of the nasal passage, bound for the lungs by the most efficient route available; only a small, turbulent eddy curls upward into the vault, where a patch of tissue perhaps the size of a large postage stamp — the olfactory epithelium — waits to intercept it. This inefficiency is worth pausing over, for it explains why we sniff: a hard, deliberate sniff redirects a greater turbulent share of incoming air up into that vault, which is precisely why one sniffs a glass of wine rather than merely breathing in its general vicinity. It also explains, in passing, why the human nose is such a comparatively modest instrument next to a dog's: the area of olfactory epithelium in a dog is roughly forty times greater than in a human being, folded into scrolls of bone to multiply its surface, which is as good a reason as any not to challenge a bloodhound to a smelling contest.

The olfactory epithelium itself is a small marvel of specialised architecture. Embedded within it are several million olfactory sensory neurons, each sending a single fine hair-like process, a cilium, out into a thin layer of mucus coating the tissue. It is in that mucus that airborne — or, in our chief interest here, mouth-borne — odorant molecules first dissolve, diffusing through to make contact with receptor proteins studding the cilia. Each sensory neuron, remarkably, manufactures only one type of receptor protein out of several hundred possible kinds, a piece of extraordinary genetic housekeeping: the neuron commits itself, for its entire working life, to detecting one narrow class of chemical shapes and ignoring all the rest. Behind this modest patch of tissue lies a most privileged piece of neural real estate. The olfactory sensory neurons send their fibres directly through perforations in the skull — the cribriform plate, a bony sieve riddled with holes like a saltcellar, and, incidentally, vulnerable enough that a sharp blow to the head can shear those fine fibres and cause a permanent, otherwise inexplicable loss of smell — straight into the olfactory bulb, which sits just above, tucked beneath the frontal lobes. No other sense enjoys so short a commute; sight, hearing, and touch must all first report to a relay station deep in the brain called the thalamus before their signal is permitted anywhere near conscious cortex. Smell alone skips the queue. We shall return to the considerable consequences of that shortcut before the chapter is through.

One further practical consequence of this arrangement deserves the cook's attention. A volatile compound, true to its name, must actually volatilise — must leave the surface of the food and enter the air, or the mouth's warm cavity — before any receptor can meet it, and the rate at which a molecule does so rises steeply with temperature. This is why a chilled soup or a cold cut of meat can taste curiously muted next to the same dish served warm, quite apart from any question of seasoning: the aroma molecules are all present in the cold dish, in exactly the same quantity, but too few of them are escaping into vapour to reach the olfactory epithelium in useful number. It is also why a wine is customarily warmed a little in the palm before serious sniffing, why a chef tastes a sauce from a spoon still faintly steaming rather than one left to go cold on the pass, and why the dedicated must always taste and re-taste a dish once more just before it reaches the table, since heat is not merely a means of cooking food but, quite separately, a condition of smelling it properly at all.

A patchwork quilt of code

For the better part of the twentieth century, the mechanism by which a nose distinguishes a rose from a rotten egg, or a ripe peach from an unripe one, remained genuinely mysterious — mysterious enough that its eventual solution earned the Nobel Prize in Physiology or Medicine. Through the 1980s, the American researchers Linda Buck and Richard Axel, working together at Columbia University, set out to hunt for the genes responsible, reasoning that odorant receptors, whatever they turned out to be, would very likely belong to a large family of related proteins rather than each being invented from scratch. It amounted, in essence, to searching for a needle in a haystack without knowing quite what the needle looked like; the pair used the newly available tools of molecular cloning to search for a family of genes sharing certain structural fingerprints characteristic of a class of proteins already known from other contexts, and in 1991 they published their answer in the journal Cell: an enormous family of genes — in the rat tissue they were studying — a family so vast it would in time be found to comprise close to a thousand distinct genes, on the order of three per cent of the entire genome — each encoding a slightly different receptor protein, each tuned to recognise a particular range of molecular shapes. Humans, it later emerged, retain a rather smaller working set. Of some eight hundred or so olfactory receptor genes surviving in the human genome, only around four hundred remain functional; the remainder are pseudogenes, evolutionary fossils switched off long ago, presumably as our primate ancestors came to lean ever more heavily on eyesight and ever less on the nose. Buck and Axel received their Nobel Prize jointly in 2004, "for their discoveries of odorant receptors and the organization of the olfactory system" — a citation that undersells, in the polite way such citations often do, quite how thoroughly the pair had rewritten the textbook.

The genius of the system they uncovered lies not in the raw number of receptor types — four hundred is, after all, a modest number set against the tens of thousands of distinct odorous molecules a good nose might plausibly encounter across a lifetime — but in how those four hundred are put to use. Each individual receptor is not a specialist tuned to one single molecule, the way a lock admits only one key; rather, each receptor responds, with varying enthusiasm, to a range of related molecular shapes, and — this is the crucial reciprocal point — each individual odorant molecule typically activates not one receptor but several, in a particular combination and at particular relative strengths. A given scent, arriving at the olfactory epithelium, therefore does not light up a single dedicated "peach receptor" or "rose receptor"; it lights up a whole pattern across the field of four hundred receptor types, rather as a handful of primary-coloured threads, woven in different proportions, can produce every shade required by a tapestry. The Nobel committee reached for a similar image in its own citation, likening the resulting neural signal to "the colours in a patchwork quilt or in a mosaic." It is this combinatorial coding — a modest alphabet of receptors combined into an immense vocabulary of patterns — that allows so few types of sensor to distinguish so vast a range of smells. The classic estimate, the figure the Nobel committee itself cited in 2004, held that humans can recognise on the order of ten thousand distinct odours. A more recent study, published in 2014, attempted to push that estimate very much further, to well over a trillion discriminable olfactory stimuli; the calculation proved controversial, and statisticians have since argued the underlying method could support numbers anywhere from ten thousand upward, so the true ceiling remains genuinely unsettled. What is not remotely in dispute is the general principle at stake: a small combinatorial alphabet, not a vast dedicated one, is what gives the nose its almost unreasonable powers of discrimination.

This same genetic patchwork, moreover, is not identical from one nose to the next, and its small variations turn out to matter rather more than one might expect. Because each of us inherits a slightly different assortment of working and non-working receptor genes, smell varies between individuals more than the other senses do — a fact confirmed by the existence of specific, well-documented anosmias, in which a person is blind to one particular smell while perceiving all others quite normally. A 2007 study identified the receptor gene OR7D4 as the chief source of disagreement over androstenone, a musky steroid compound found in pork and truffles: depending on which variant of that one gene a person carries, androstenone may register as an unpleasant urinous reek, a pleasant muskiness, or nothing at all. A comparable discovery, made in 2012 by researchers combing through the genetic data of more than fourteen thousand volunteers, traced the ancient dinner-table argument over coriander leaf — cilantro, to give it its other common name — to variation near a gene called OR6A2, which in its more sensitive form renders the leaf's characteristic aldehydes not herbaceous but unmistakably, unpleasantly soapy. One's private olfactory world, in short, is dealt from the same deck of four hundred-odd receptor types as everyone else's, but shuffled into a hand that is subtly, permanently one's own.

Once the pattern reaches the olfactory bulb, one further piece of tidiness awaits it. All the sensory neurons that happen to carry the same type of receptor — scattered, as it happens, more or less at random across the epithelium — send their fibres converging upon the very same tiny structure in the bulb, a globe of tissue called a glomerulus, of which there are some two thousand. The effect is of a great sorting office: millions of individual reports, arriving from every corner of that postage-stamp patch of tissue, are filed correctly by type before being passed onward to the cortex for interpretation. Only there, integrated across many glomeruli at once, does the pattern finally resolve into what the mind experiences as a single, seamless smell — "peach," "rose," or, more usually at this volume's table, simply "dinner."

Chemists have their own rough-and-ready way of finding out which molecules, among the hundreds or thousands present in any given food, are the ones actually doing the work of announcing its smell to a nose — a technique with the unglamorous name of gas chromatography-olfactometry, and the rather more evocative nickname of the "sniffing GC." A complex aroma extract, laden with every volatile compound a food contains, is separated by the instrument into its individual components, which emerge one by one over the course of perhaps half an hour; and rather than trusting a detector alone to judge which of them matter, a trained human assessor sits with her nose pressed to a small sniffing port at the machine's exit, sniffing patiently as compound after odourless-to-the-detector compound streams past, and calling out, on a scale of the sort no instrument yet rivals, exactly which fleeting molecules smell of anything to a human being at all, and what. It is, in its modest way, as fine an image of this whole volume's method as any: the instruments of modern chemistry, married to the oldest and most patient of naturalist tools, a trained nose taking notes.

A gallery of aroma molecules

It is one thing to describe the machinery of smell in the abstract; it is quite another to meet, by name, a few of the molecules that keep it so continuously employed. The volatile compounds that furnish flavour number in the many thousands across the whole of nature's larder, but a handful are so potent, so widely encountered, or so instructive in what they reveal about chemistry and cookery that they deserve individual introduction here — a small gallery of portraits, if the reader will indulge the conceit of a naturalist's specimen case.

Vanillin is where any such gallery must begin, if only because it is the aroma compound most people could name without prompting. It is the chief flavour and aroma constituent of the cured vanilla bean — of which this chapter's Field Note has a good deal more to say — a modest molecule, 4-hydroxy-3-methoxybenzaldehyde to give it its full and rather unlovely chemical name, that the human nose detects at vanishingly small concentration. Curiously, the natural bean owes its particular richness not to vanillin alone but to the company it keeps: more than two hundred other volatile compounds accompany it in a properly cured pod, which is why a good vanilla bean tastes rounder, warmer, and more complicated on the tongue than a bottle of the pure synthesised compound, however chemically identical that single note may be to its counterpart in the pod.

Diacetyl announces itself as butter before the mind has consciously registered the word. Chemically a simple diketone, it arises naturally as a by-product of fermentation — in butter and cheese, in wine, and, notoriously, in beer, where brewers labour to coax their yeast into scrubbing it away before bottling, since a beer left with too much diacetyl tastes unmistakably, and quite unintentionally, of cinema popcorn. That association is no accident: diacetyl is precisely the compound the snack-food industry has, for decades, added to purified popcorn oil to simulate butter without the expense or perishability of the genuine article, and it is potent almost beyond reason — the human palate can detect it in a glass of chardonnay at concentrations of a few tenths of one part per million.

Geosmin is, molecule for molecule, perhaps the most sensitively detected odorant known to the human nose, recognisable at concentrations of only five to fifteen parts per trillion — a figure so small that a single drop, diluted through several hundred Olympic swimming pools, would still announce itself to a patient sniffer. It is produced by soil-dwelling bacteria of the genus Streptomyces, among others, and it furnishes two of the more improbable bedfellows in the whole gallery of aroma: it is both the smell of rain falling on parched earth — the scent known as petrichor, released when raindrops strike dry soil and fling tiny aerosols of bacterial residue into the air — and the earthy note at the heart of a freshly sliced beetroot, a vegetable whose partiality for absorbing geosmin from the surrounding soil is so pronounced that plant breeders have occasionally tried, with mixed success and decidedly mixed public approval, to breed the trait out of it.

Isoamyl acetate smells, without argument, of banana — indeed it is the compound most artificial banana flavouring is built around, to the perpetual disappointment of those who find it tastes rather more like a banana sweet than like an actual banana, whose true flavour is a subtler chord of several dozen compounds of which isoamyl acetate is merely the loudest voice. The same molecule performs an entirely different office in the insect world: it is the principal component of the alarm pheromone honeybees release from the sting apparatus when a hive comes under attack, a chemical shout that recruits nearby workers to converge upon, and sting, the very same unfortunate target. Beekeepers, accordingly, know better than to eat a banana while working a hive.

(Z)-3-hexenal, sometimes called leaf aldehyde, is the smell of a lawn newly mown, of a tomato vine brushed in passing, of grass torn rather than cut cleanly. It scarcely exists, in any meaningful quantity, in an undisturbed leaf; it is manufactured in an instant, enzymatically, the moment plant tissue is wounded, as fatty acids in the ruptured cell membranes are broken down by a swift cascade of enzymes into a handful of highly volatile, intensely green-smelling fragments. The human nose is exquisitely sensitive to it, detecting it at a mere quarter of one part per billion — which may say something about how useful it once was to a forest-dwelling ancestor to know, by smell alone and without looking up, that vegetation nearby had just been damaged, whether by browsing animal, falling branch, or blade.

2-Acetyl-1-pyrroline, the least pronounceable name in this gallery, rewards the effort of learning it, for it is responsible for one of the loveliest smells in the whole of cookery: the toasty, faintly popcorn-like fragrance of jasmine and basmati rice, of the pandan leaf used throughout South-East Asian kitchens, and — most relevant to the chapter that follows this one — of a fresh loaf's crust. It forms, in every case, by essentially the same chemical route: a reaction between amino compounds and sugars under gentle heat, a foretaste, in miniature, of the browning chemistry to which we shall very shortly turn our whole attention. Aromatic rice varieties manufacture it quietly as they grow; bread manufactures it violently as it bakes; and the nose can detect it at a mere tenth of one part per billion, unmistakable in both contexts once it has been learned — proof, if any were needed, that the same handful of chemical tricks recur again and again across nature's kitchen.

<figure> <svg xmlns="http://www.w3.org/2000/svg" width="100%" viewBox="0 0 640 660" role="img"> <title>A Wheel of Aromas — six aroma families, fruity, floral, green, roasted, earthy and buttery, each with a representative compound</title> <rect x="12" y="12" width="616" height="636" fill="none" stroke="#43331f" stroke-width="1"/> <rect x="15.5" y="15.5" width="609" height="629" fill="none" stroke="#43331f" stroke-width="0.5"/> <g fill="#43331f"> <path d="M12,7 L17,12 L12,17 L7,12 Z"/><path d="M628,7 L633,12 L628,17 L623,12 Z"/> <path d="M12,643 L17,648 L12,653 L7,648 Z"/><path d="M628,643 L633,648 L628,653 L623,648 Z"/> </g> <text x="320" y="41" text-anchor="middle" font-family="Georgia, 'Times New Roman', serif" font-size="14" letter-spacing="2" fill="#43331f">A WHEEL OF AROMAS</text> <line x1="170" y1="56" x2="470" y2="56" stroke="#6f5a40" stroke-width="0.5"/> <circle cx="320" cy="395" r="215" fill="none" stroke="#43331f" stroke-width="1"/> <circle cx="320" cy="395" r="135" fill="none" stroke="#6f5a40" stroke-width="0.5"/> <circle cx="320" cy="395" r="45" fill="none" stroke="#43331f" stroke-width="1"/> <g fill="none" stroke="#6f5a40" stroke-width="0.6"> <path d="M297.5,356 L212.5,208.8"/><path d="M342.5,356 L427.5,208.8"/> <path d="M365,395 L535,395"/><path d="M342.5,434 L427.5,581.2"/> <path d="M297.5,434 L212.5,581.2"/><path d="M275,395 L105,395"/> </g> <path d="M522.0,468.5 L529.6,471.3 M484.7,533.2 L490.8,538.3 M357.3,606.7 L358.7,614.6 M282.7,606.7 L281.3,614.6 M155.3,533.2 L149.2,538.3 M118.0,468.5 L110.4,471.3 M118.0,321.5 L110.4,318.7 M155.3,256.8 L149.2,251.7 M282.7,183.3 L281.3,175.4 M357.3,183.3 L358.7,175.4 M484.7,256.8 L490.8,251.7 M522.0,321.5 L529.6,318.7" fill="none" stroke="#a9906c" stroke-width="0.6"/> <path d="M320,383 L324,391 L332,395 L324,399 L320,407 L316,399 L308,395 L316,391 Z" fill="#9c5a2c"/> <g font-family="Georgia, 'Times New Roman', serif" font-size="11" letter-spacing="1" fill="#43331f" text-anchor="middle"> <text x="320" y="305">FRUITY</text><text x="397.9" y="350">FLORAL</text> <text x="397.9" y="440">GREEN</text><text x="320" y="485">ROASTED</text> <text x="242.1" y="440">EARTHY</text><text x="242.1" y="350">BUTTERY</text> </g> <g font-family="Georgia, 'Times New Roman', serif" font-style="italic" font-size="9.5" fill="#43331f"> <text x="320" y="220" text-anchor="middle">isoamyl acetate</text><text x="320" y="233" text-anchor="middle">banana</text> <text x="471.6" y="307.5" text-anchor="start">linalool</text><text x="471.6" y="320.5" text-anchor="start">floral</text> <text x="471.6" y="482.5" text-anchor="start">cis-3-hexenal</text><text x="471.6" y="495.5" text-anchor="start">cut grass</text> <text x="320" y="570" text-anchor="middle">2-acetylpyrroline</text><text x="320" y="583" text-anchor="middle">toast</text> <text x="168.4" y="482.5" text-anchor="end">geosmin</text><text x="168.4" y="495.5" text-anchor="end">beetroot</text> <text x="168.4" y="307.5" text-anchor="end">diacetyl</text><text x="168.4" y="320.5" text-anchor="end">butter</text> </g> </svg> <figcaption><em>Plate 4. A small wheel of aroma families — fruity, floral, green, roasted, earthy, and buttery — the principal continents of smell, upon whose coasts every individual aroma molecule makes its home.</em></figcaption> </figure>

The nose that remembers

We promised, at the head of this chapter, to explain why a smell can return a grown adult, without warning or consent, to a particular kitchen at a particular hour of a particular childhood — and the explanation lies in that shortcut through the skull mentioned earlier, the one that spares olfactory signals their customary detour through the thalamus. Sight, sound, and touch all pass through that structure, a kind of neural switchboard, before being routed onward to conscious awareness; smell alone bypasses it, sending its signal from the olfactory bulb by short, direct connections into two structures buried deep in the temporal lobe: the amygdala, which assigns emotional weight — fear, pleasure, disgust — to experience, and the hippocampus, the seat of memory formation. A smell, in other words, is only one or two synapses removed from the very machinery that decides how we ought to feel about something and whether we ought to remember it at all. No other sense enjoys so direct a line to the emotional and mnemonic core of the brain, which is as good an explanation as neuroscience presently offers for why a whiff of a particular soap, or bread, or hospital disinfectant, can deliver an entire vanished afternoon back to us whole — complete with its light, its temperature, and the people who were in the room — before we have consciously identified the smell at all.

The blind and deaf-blind have often written about this more precisely than the sighted, perhaps because they are not distracted by the eyes' habitual insistence on going first. Helen Keller, denied both sight and hearing from infancy, called smell "a potent wizard that transports us across a thousand miles and all the years we have lived," and described being carried instantly, by the scent of a particular fruit, back into a peach orchard of her Southern childhood. Marcel Proust would make an entire literary monument of much the same phenomenon a few years later, though it was a madeleine dipped in lime-blossom tea, and not the nose precisely, that did the work in his famous telling — a matter to which this volume returns properly in a later chapter devoted to the psychology of deliciousness. For now it is enough to note the underlying mechanism: emotion and memory are not incidental to the sense of smell, decorative flourishes bolted on after the fact; they are wired directly into its architecture, a fact any perfumer, or any grandmother's kitchen, could have told the neuroscientists long before the neuroscientists were able to draw it on a diagram.

There is, besides, a plain evolutionary logic running beneath all this, the same logic that runs through every chapter of this volume. An animal that could not learn, quickly and permanently, to associate a particular smell with the illness that followed eating a particular fungus, or with the safety and comfort of its mother's milk, would not long survive its own education. The dependency begins astonishingly early: a newborn mammal, eyes barely open and legs barely able to bear its own weight, finds its mother's teat chiefly by smell, and a pup experimentally deprived of its sense of olfaction will, tragically, fail to feed at all, however near the milk may be. Smell had to be wired directly into memory and feeling because the stakes of getting it wrong — poison mistaken for food, food mistaken for poison, a mother mistaken for a stranger — were, across the long, slow classroom of evolution, frequently fatal. We inherit, at the dinner table, a nose built for consequence, which is perhaps why so humble an object as a bowl of soup can, on the right afternoon, undo us completely.

Field Note — The Vanilla Orchid & Vanillin

Of the twenty-five thousand or so species of orchid known to botany — surely the most extravagantly ornamental family in the whole plant kingdom — precisely one is grown for the table, and it very nearly did not survive its own economics. Vanilla planifolia is a climbing vine native to the forests of eastern Mexico, where its greenish-yellow flower opens for a single day and, if not pollinated before it closes, withers unfertilised and drops. In its homeland, pollination is attributed by long tradition to visits from small stingless bees native to the region — though even there, botanists now suspect, successful fruit-set has always been rare and unreliable, which is presumably why the vine, when carried elsewhere, so quickly proved sterile. French colonists carried vanilla cuttings to the Indian Ocean islands of Réunion and Mauritius in the 1820s, hoping to found a plantation industry there; the vines grew vigorously and flowered generously, and produced almost nothing, for whatever pollinators the plant relied upon at home had, naturally enough, stayed at home.

The impasse was broken in 1841 by a twelve-year-old enslaved boy named Edmond Albius, working on a Réunion plantation under a botanically minded owner. Adapting a technique he had seen used on melons, Albius discovered that a sliver of grass or a thin stick, together with a simple pinch of the thumb, could lift the membrane separating the flower's male and female parts and press the two together by hand — a procedure that takes a skilled worker perhaps thirty seconds per bloom, must be performed on the single day each flower opens, and must be repeated, flower by flower, across an entire plantation, year upon year, to this very day. There is still no economical mechanical substitute; nearly every vanilla pod sold in the world remains, in the most literal sense, hand-made. It is this labour, multiplied across the many months a pod then takes to cure — killed by heat, "sweated" in blankets, dried in the sun, and conditioned in boxes until its flavour fully develops — that makes vanilla, after saffron, the second most costly spice in the world.

The vanillin molecule at the heart of that flavour was first synthesised in the laboratory in 1874, by the German chemists Ferdinand Tiemann and Wilhelm Haarmann, initially from a compound found in conifer bark and shortly thereafter, by a competing route, from guaiacol. Today the overwhelming majority of the world's "vanilla" flavouring — well over ninety per cent of it — is this laboratory-made vanillin, chemically indistinguishable from the natural molecule down to the last atom, but missing the two hundred-odd companion compounds that give a true cured bean its rounder, more complicated voice. Natural vanilla extract, by contrast, accounts for only a sliver of world supply, and Madagascar's hand-pollinated vines still produce the greater part of it.

We have been speaking, for the length of this chapter, chiefly of aromas already present in nature, waiting in a vanilla pod or a blade of grass for a nose sufficiently attentive to find them. But the kitchen's most extraordinary trick is not the detection of aroma; it is the manufacture of it, molecule by molecule, out of raw materials that possessed no such smell to begin with. Put a plain, odourless piece of dough or a cut of pale, unpromising meat into a hot, dry pan, and something remarkable happens that no amount of sniffing the raw ingredient could ever have predicted. Reader, we go now to the fire.

[^1]: Jean Anthelme Brillat-Savarin, The Physiology of Taste, Meditation II, "Of Taste" (1825). [^2]: Anosmia has a stranger cousin, parosmia, in which the sense of smell returns after illness or injury but subtly mis-wired, so that coffee may suddenly smell of petrol, or roses of rotting meat. It is a vivid reminder that smell is not simply present or absent, like a switch, but a learned pattern — and evidently, on occasion, a mis-learned one.