Tasty (10 page)

He who eats a peach, for instance, is first of all agreeably struck by the perfume which it exhales; he puts a piece of it into his mouth, and enjoys a sensation of tart freshness which invites him to continue; but it is not until the instant of swallowing, when the mouthful passes under his nasal channel, that the full aroma is revealed to him; and this completes the sensation which a peach can cause. Finally, it is not until it has been swallowed that the man, considering what he has just experienced, will say to himself, “Now there is something really delicious!”

The mouth and nose aren't located far apart, but they are radically different in structure and function—it's surprising taste and smell are able to come together at all. Scientists have found just five basic tastes, programmed by a few dozen genes. Each is distinct, unvarying, and immediately identifiable in the complex mix of flavors in food and drink. Smells, on the other hand, are virtually infinite: there are as many as a million distinct ones, detectable by our four hundred kinds of olfactory receptors. The receptors bond with aromatic molecules in combinations far more complex than those involved in tastes. Smells are also subtler sensations. They blend seamlessly into flavors, submerging their identities into the whole. This combination of superior range and nuance makes smell the single most powerful component of flavor.

The human brain elegantly summons clarity out of the flux of smells and aromas. One day in 1974, Gordon Shepherd, a neurobiologist, went to a Maryland supermarket and bought a hunk of strong cheddar cheese. Shepherd intended to map how the brain interpreted aromas, which was then largely a mystery. The obstacle, as it had been since the time of the ancient Greeks, was analyzing a subjective experience. The activity of the brain's living neural networks was inaccessible. Ordinary X-ray machines could not capture neurons firing or blood flow in the brain. Implanted electrodes were sometimes used in animals and humans, but this was crude and imprecise.

Using a new method, a forerunner of today's fMRI scans, Shepherd and his colleagues at the National Institutes of Health injected rats and rabbits with a radioactive isotope that attached itself to areas of the brain where neurons were firing. As the animals sniffed the cheddar, elaborate patterns of activity were mapped in their olfactory bulbs. Unfortunately for the animals, the only way to see this was to examine their brains directly. After forty-five minutes of sniffing, the animals were euthanized, cross-sections of their olfactory bulbs exposed to X-rays, and the films studied with a microscope.

Each aroma produced a unique pattern that resembled an abstract pointillist painting. Smell, Shepherd concluded, was something like sight; each scent created its own distinct “image.” In the eye, the retina converts light that strikes it into patterns of firing neurons in the brain that we experience as images. The olfactory bulb encodes scents and aromas in another type of pattern, which we experience as a smell, or as part of a flavor. The brain then refines these smell images further, adding contrast, creating crisp patterns as recogniz
able in their own way as the Washington Monument or the
Mona Lisa
.

Aromas, especially those in the complicated flavors of fermented foods, are instantly recognizable, yet elude description. Instead, they are typically described by analogy to something else: “a coffee smell,” “smokiness.” In this way they are a lot like faces. “We're very good at recognizing faces, but are very poor at describing them in words,” Shepherd said. “Smells, too, are irregular patterns, ones we're not conscious of, yet the brain has to connect to the cognitive process underlying language to describe them. It's also difficult, after hearing them, to describe musical passages.”

The waves of microbial activity in cheese leave behind a chemist's smorgasbord of alcohol, acids, aldehydes, esters, and sulfurous substances. Many launch into the air attached to evaporating water or alcohol molecules, forming evocative aromas. One substance found in Camembert, acetaldehyde, produces a pungent, nutty, yogurty flavor. A compound called 2-methylpropanal, found in gouda, has a malty banana flavor with a hint of chocolate, while butyric acid (gouda and cheddar) has a typical cheesy, sweaty, putrid aroma similar to sweat. Methional (cheddar) has scents evoking cooked potatoes, meat, and sulfur.

These aromatic images become delicate portraits of experience, etched into the nervous system as they're routed to parts of the brain involved in memory (the hippocampus) and decision making (the orbito­frontal cortex). In other words, smell literally links past and present. Because of its ancient roles mapping our surroundings and driving the evolution of the brain, smell is the only sense whose receptors connect so directly to these structures, only two synapses removed from the outside world. This gives it both immediacy and instant
context, as the slightest whiff of a familiar scent can trigger a cascade of memories and feelings.

When Marcel Proust's narrator bites into a tea-soaked madeleine at the start of
In Search of Lost Time
, he is transported to his childhood village of Combray:

But when from a long-distant past nothing subsists, after the people are dead, after the things are broken and scattered, taste and smell alone, more fragile but more enduring, more unsubstantial, more persistent, more faithful, remain poised a long time, like souls, remembering, waiting, hoping, amid the ruins of all the rest; and bear unflinchingly, in the tiny and almost impalpable drop of their essence, the vast structure of recollection.

Shepherd and his daughter Kirsten Shepherd-Barr, an English professor at Oxford University, combined their talents to explore what was happening in Proust's narrator's brain. The madeleine, they wrote, is an ideal vehicle for flavor; the vapors of tea carry its volatile aromatic compounds through the retronasal pathway to the olfactory epithelium, where the receptors lie. A single one of those vanilla or lemon flavorings, with its unique molecular shape, might recall a fragment of the narrator's early memories; the brain then can use the fragment to summon the whole. This neural architecture helps to make flavor flexible and adaptable. Food is written into memories and emotions. The reverse is also true. As memories accumulate, they come back to shade flavor perceptions in the present. This is one way that flavor continually evolves.

• • •

Unlike smell, the sense of taste is less emotional than existential. The primitive wants and aversions that tastes generate are basic survival responses. The signals from taste receptors ping through the oldest parts of the cerebral anatomy, where instincts and urges play out. When they reach the neocortex, they are processed by the insula, which contains distinct regions where neurons fire to salty, sweet, sour, bitter, and umami. The insula appears obscure. In each hemisphere, it is hidden away beneath a layer of tissue called the operculum, tucked into the brain's cortical shell at the temple. But in fMRI studies, it pops up again and again as a critical node in networks of brain activity for many different things. It seems to shape the overall tone of experience itself.

The insula seems to be where the body's internal state and external circumstances are sorted, assessed, and relayed to consciousness. Along with tastes, it processes other messages about the state of the body such as thirst, sexual arousal, temperature, the metabolic and cardiovascular stresses of exercise, and the need to use the bathroom. It aids in tasks involving perception, including the ability to distinguish one's own face in the mirror from that of someone else or a scrambled image; keeping rhythm to music; and processing emotions such as sadness, happiness, trust, empathy, beauty, and “state of union with God.” It activates when we're engaged in sophisticated tasks, such as keeping time, recognizing an image being revealed piece by piece, or making choices. The insula, in other words, helps create the special, ever-shifting quality of now.

• • •

The unity of taste and smell in flavor is like a good marriage. The differences are profound, but each partner has comple
mentary strengths and weaknesses. Their paths through the brain unite—along with those of all the other senses—in the orbitofrontal cortex, located above the orbits of the eyes. Relative to body size, humans have larger orbitofrontal cortices than any other animal—this was one of the most important evolutionary upgrades in the emergence of
Homo sapiens
. Flavor is only one in its array of sophisticated cognitive responsibilities, which includes decision making. It's the brain's food critic, connecting to areas governing emotions and judgment, and anatomically structured to process pleasure and aversion. Moving from the center out, its pleasure-sensitive neurons give way to displeasure-sensitive ones. This may explain our tendency to rank favorite or most hated foods: our brains are literally organized that way.

But the core of flavor perception is the way that the orbitofrontal cortex weaves the senses together, and with them all the elements of flavor. Thus, we perceive dark chocolate, grilled fish, and absinthe, rather than the long lists of distinct tastes and smells that comprise them. Individual tastes and aromas work in concert, reinforcing each other, fusing into something new.

The umami taste plays a special role in this process. Cooking, curing, and fermenting release prodigious quantities of umami, which dominates in seared meats, cheeses, tomatoes, pickles, and especially Asian foods such as soy and fish sauces and miso. Umami receptors detect glutamates—the salt of a particular amino acid, amino acids being one building block of proteins. (Umami is often dubbed the “protein taste,” as sweetness is the taste for sugars, though its exact purpose is unclear. In nature, proteins are found mostly in milk and in raw animal flesh, which aren't very savory.) The surge in
umami from the new foods of civilization was a nutritional as well as a taste bonanza. Glutamates fuel digestion and make it possible for the neurons in our brains to fire. In pregnant women, the placenta uses glutamates as an energy source. Umami receptors are found not just on the tongue but also lining the small intestine: extra glutamates stimulate better digestion and absorption of nutrients.

The Japanese word
umami
is a combination of the characters for “delicious” and “taste.” Its meaning conveys a sense of wonder and satisfaction from food. Yet the umami taste is elusive. A sip of pure, dissolved glutamates is practically tasteless. But in concert with other flavors, umami's savoriness comes alive, and brain scans show it sparks patterns of brain activity roughly similar to those of sugar. The other four tastes boldly announce themselves, but umami operates by misdirection, helping other flavors bloom. It's like the Wizard of Oz, putting on a tremendous show from behind a curtain.

Two neuroscientists at Oxford, Edmund Rolls and Ciara McCabe, explored this phenomenon in a 2007 experiment. They treated twelve volunteers to an umami cocktail (monosodium glutamate—the chemical version of umami that food companies and Asian restaurants use—and a second substance, inosine 5'-monophosphate, which enhances MSG's effects) and a vegetable aroma. Separately, the drink and aroma were unpleasant. Together, their flavor was delicious. To map this curious effect, Rolls and McCabe tested the volunteers using fMRI scans. The umami-aroma combination ignited neurons in the orbito­frontal cortex in far greater numbers and for a longer time than one would expect from simply adding up the separate effects. Umami taste worked
in concert with smell to create a new and more powerful sensation.

This suggests an underlying reason why the taste of chicken soup or pizza is delicious: umami unites and heightens taste and smell, adding a burst of pleasure. Think of what Parmesan cheese does to enrich the flavors of pasta it's sprinkled on: it produces a Technicolor burst of boldness and robustness. For ancient peoples sampling cheeses or fermented soy, this effect must have been a revelation.

• • •

Beverages also offer potent fusions of taste and smell. Ethanol, the alcohol in all alcoholic beverages, is a promiscuous molecule. It affects the brain's taste, smell, and touch systems all at once. These merge into its powerful effects on mood. Take a sip of wine, beer, or bourbon, and the alcohol binds to sweet and bitter receptors, and to the heat-sensing receptors that trigger the burn from chili peppers. Depending on the strength of the drink, any one of those sensations can move to the foreground. Below a 10 percent concentration, alcohol creates a faint sweet sensation, and the brain's response echoes that for sugar. This isn't surprising, as yeasts feasting on sugars churn out ethanol molecules. It's also influenced by genes: people with a family predilection for sweets tend to drink more.

But in stronger drinks, bitterness and burning overwhelm the sweetness, which is why distilled spirits like vodka or tequila, concentrated at 40 percent alcohol or higher, have such a kick, the edgy mixture of pleasure and aversion that makes tossing back a shot so bracing. As ethanol molecules evaporate, they waft into the nose and attach themselves to the receptors for smell, accounting for the jolt in a mere sniff of absinthe.

Alone, ethanol is relatively flavorless. It serves as a scaffold for other by-products of fermentation. Some of these substances pique the taste buds, some operate via smell. Jiahu grog contained acids and bitter compounds that balanced out its sweetness. Tannins, chemicals found in grape skins, create a distinct puckering sensation and bind to proteins on the tongue to alter the composition of saliva. Science still has only a sketchy idea of how most of these flavor compounds work; tracing the connection between fleeting molecules and flavor is a daunting task. One class of aroma molecule found in cabernet grapes, methoxypyrazines, imparts a fresh vegetable-­like flavor similar to bell peppers.

• • •

Taste and smell blend so seamlessly in flavors that the different senses merge, becoming indistinguishable. The brain even mixes them up: in the mind, smells become tastes. Vanilla, an aromatic flavoring, is usually perceived as sweet. In a study, a majority of volunteers described the scent of strawberries and amyl acetate, a banana-flavored food additive, as sweet. Food formulators routinely add such aromatic essences to drinks to enhance their sweetness without using sugar. But this is a trick of perception: smells cannot be sweet. Sweetness is a taste, detectable only by receptors on the tongue. Somehow, the brain produces the sensation that the nose is tasting, or that the tongue is smelling—or both.