Tasty (2 page)

The old philosopher's claim that flavor was impervious to scientific inquiry is now moot. Flavor science made great strides in the twentieth century; in the twenty-first it has moved ahead at astonishing speed. Receptors for all five basic tastes have been found, and it looks like fat may be identified as the sixth. Scientists are beginning to understand the connections between mind, brain, and body: why you think you
have
to have that cheeseburger or glass of wine.

This book is a brief biography of flavor. The narrative begins at the dawn of life on earth and ends in the present, and explores the structure of this unique sensation, from its molecular building blocks through more sophisticated levels of body, brain, and mind. Flavor has grown deeper and more complicated at each stage in its development over millions of years. It has driven evolution, and lately human culture and society, in new directions. It is a kind of slate on which human struggles, aspirations, and failures have been written, erased, and written again. We owe our existence and our humanity to it—and, in many ways, our future depends on it, too. As science unlocked flavor's secrets, its influence over what we eat and drink exploded. From the food labs of large corporations to the kitchens of the world's finest restaurants to the bar down the street, science shapes surprising and sometimes alarming new sensations tied to both our DNA and our deepest drives and feelings.

In March 1998, scientists at the National Institutes of Health (NIH) in Bethesda, Maryland, found themselves on the cusp of one of these paradigm-changing advances. They were searching for a sweet taste receptor, a kind of protein on the tongue specially tailored to snare sugar molecules out of the slurry of mashed-up food and drink in the mouth. More than two thousand years after Democritus and Alcmaeon, science was finally closing in on the mechanism of taste that enables us to transform the molecular arrangements in food into sensory perceptions and, ultimately, culinary art.

Over the previous decade, the science of genetics had made startling advances. For the first time, scientists were decoding the entire length of human DNA, the helical, ladderlike molecules found in chromosomes in the nucleus of every cell. The total human genome is made up of three billion pairings of four amino acids; each pair forms a rung in the ladder. The variations in the pairs constitute a code that maps out the blueprint for the body and all its functions. Every person gets two of these blueprints, one from each parent. Uncoiled, the DNA in a single cell would be about six feet long; if all the DNA in the human body were laid end to end, its length would be the equivalent of seventy round-trips from Earth to the sun.

Isolating genes—discrete segments of DNA that carry out specific biological instructions such as making proteins, the body's basic building blocks—had enabled scientists to find and treat diseases and to better understand human evolution. Now, genetics offered a way to quantify the intangibles of flavor, to explain its bewildering diversity. The nose's receptors for smell had been isolated and their genes decoded seven years earlier, an effort that later won a Nobel Prize. The smell receptors had been comparatively easy to find. They
were plentiful and concentrated in the small patch of tissue on the roof of the nasal cavity; live ones can be harvested with a Q-tip.

But the search for taste receptors had dragged on. They had proven almost impossible to isolate: Not only are there relatively few taste-­detecting cells to begin with, but it is difficult to coax a reaction from them. The body has a vast apparatus to detect all kinds of cues, from hormones on the inside to heat, cold, pressure, light, and chemicals on the outside. Most of these reactions are very sensitive. It takes only a tiny dose of adrenaline to get a rise out of the receptors that detect it. But taste receptors are about a hundred thousand times less sensitive. This is because they interface with the chaos of the world around us. Given the sheer volume and variety of sensations the tongue encounters in a single meal, the brain would overload if every molecule lit up the taste receptors. Taking a sip of Coke would be like staring into the sun.

The NIH scientists, led by Nick Ryba, had finally leaped many of those hurdles. They were examining taste bud cells while also searching stretches of the genome, hoping to match up a taste receptor protein with the gene that created it. They harvested DNA from the taste buds of rats and mice, whose sense of taste is similar to our own. The trick was finding the right individual gene: a short, specific stretch of code tucked somewhere among vast, unmapped tracts of DNA. Having that blueprint would enable them to clone copies of the receptor so they could easily study its structure and workings.

In a short span of time, science had become quite adept at slicing, dicing, and sorting these once-indistinguishable molecular strands. At NIH, scientists found a way to turn the scarcity of taste receptors to their advantage: They used a technique that plucked only the most unusual DNA snippets
from across the tongue, separating them from more generic strings. Some of these had to contain the material for taste. Next, they took each fragment and injected it into taste cells harvested from rodents. If it latched onto the DNA in the cell, it was a taste receptor gene. This was, roughly speaking, like putting a toddler in a room with a woman you think may be the mother: if they hug, then you know they're related.

It worked: the scientists found half of a rodent gene for a sweet receptor; the second half was found soon after, and then the analogous human gene for sweetness. Their double genes mean that sweet receptor molecules come in two parts that fit together like a train coupling. They are bizarre, Lovecraftian-­looking things, tangled skeins of seven coils stuck in the surface of a taste cell. One coil reaches out into the void to snag sugar molecules floating by. When it does, an electrochemical chain reaction begins that travels all the way to the brain, igniting a burst of pleasure.

Elsewhere, scientists were starting to crack another once-intractable problem, the subjectivity of taste. A few years after the sweet receptor discovery, volunteers in an experiment at the University of Groningen in the Netherlands lay on a table with pacifiers connected to long straws in their mouths. They were then slid into a magnetic resonance imaging (MRI) scanner that recorded their brain activity as they sipped bitter tonic water through the straws. Later, they were scanned while looking at photos of people grimacing in response to a taste of a drink, and again as they read brief scripts intended to evoke distaste or disgust. The purpose of this experiment, run by neuroscientist Christian Keysers, was to explore the relationship between tastes and emotions. During the 1990s, the emergence of this type of scanner, called a functional MRI (fMRI), allowed scientists
to see which parts of the brain were active when a person ate or drank, smelled an aroma, or read—anything that could be done while one's head was immobilized.

There were limits to this approach. It showed associations between real-world actions and arcing networks of neurons in the brain, but not exactly what those associations meant. But it was a revealing waypoint between the chemical reaction of taste on the tongue and the mind itself.

Their findings were strange. As volunteers imagined bitterness in a story, or saw photos of a wince of distaste, their brains experienced a “bitter” reaction. These patterns varied slightly in each part of the experiment, reaching out to encompass different parts of the brain. Taste seemed to be a cornerstone of higher functions such as imagination and emotion.

The next twist in this story is still being written; it hinges on certain lingering mysteries. Flavor remains frustratingly paradoxical. Like other senses, it's programmed by genes; unlike them, it is also protean, molded by experience and social cues, changing over the course of a lifetime. This plasticity is wild and unpredictable: people can learn to like or dislike almost anything, which is why the range of flavors in the world is seemingly infinite, and why the old tongue map was useless.

Everyone lives in his or her own flavor world, which takes form during early childhood and evolves over the course of our lives. This world is created by the clash of ancient evolutionary imperatives meeting a lifetime's worth of high-octane processed foods, cultural cues, and commercial messages.

The flavor preferences of my own children, born two years apart, were apparent as soon as they began to eat solid food. Matthew, the elder, relished extremes. He started eating jalapeños in preschool and liked coffee from the time he was nine.
Every so often, usually in the summer, he would sit down with a lemon or lime, quarter the fruit, add salt, and devour it with the peel. His sister, Hannah, craved bland, rich flavors, and the foods she ate tended to be white or beige: cheese, rice, potatoes, pasta, chicken. She preferred chamomile tea to coffee, and milk chocolate to dark. Yet both were picky eaters: they knew what they liked and rarely departed from it. Getting them out of their respective comfort zones to try something new was nearly impossible.

This combination of divergent tastes and limited likes turned grocery shopping or restaurant-going into a kind of Rubik's cube challenge; only pizza satisfied everyone. I made most family dinners, and struggled to get them out of a rut dominated by the same handful of dishes presented with only slight variations in a weekly cycle: pasta, roast chicken, or chicken nuggets for Hannah, hot dogs or shrimp in Szechuan sauce for Matthew. My wife, Trish, and I were more adventurous, but the convenience of this routine dragged us in, too.

The appetites of children are a crucible where the forces of chemistry and culture collide. The sweet tooth, the scourge of modern nutrition and dentistry, is crucial to childhood development. In newborn babies, sugar acts like aspirin, soothing pain. The Monell Chemical Senses Center in Philadelphia, a think tank that studies taste and smell, found that children with a strong taste for sweets also had higher levels of a hormone tied to bone growth. A yearning for sweets pointed early human children to then precious sugars in fruits and honey, and when combined with sourness, to citrus fruits packed with vitamins C and D.

Picky eating is likely a holdover from the same epoch, when humans lived together in small migratory groups and
children—thanks to their tendencies to wander and to shove random things in their mouths—faced a constant threat of poisoning. Today, a limited diet is a danger to long-term health, and in its most extreme form pickiness has been labeled an eating disorder, called food neophobia.

Children have strange tastes because they are bizarre creatures. Taste and smell develop earlier than other senses, so a fetus's sensory universe consists almost entirely of the smells and tastes in amniotic fluid. This makes a lasting impression. In another Monell study, the babies of women who drank a steady diet of carrot juice during their pregnancies or during breastfeeding later took a shine to carrot-flavored cereal.

Then, between birth and the ages of two and three, a baby's synapses—the connections between neurons that form networks in the brain—multiply from about 2,500 per neuron to 15,000 (an adult has 8,000 to 10,000). This temporarily ties the senses together. Young children live in a fugue of overlapping sensations, one reason why early flavor experiences evoke not just meals but entire moments. As children age, experience gradually trims the thicket of neurons, and better sensory connections emerge. During this process, kids' tastes vacillate between conservative stretches and probing, adventurous periods.

During the teen years, intense tastes fade along with the physiological demands and evolutionary imperatives of childhood. A subtler palate takes their place, though the original likes and dislikes never quite disappear. This muting allows the range of tastes we can experience to increase, and our reservoir of food memories and associations deepens. Sensations bubble up, synapse by synapse, from chemical reactions in the nose and mouth. Meanwhile, food engages the other senses,
tapping the mind's capacity for learning, understanding, and appreciation. Back and forth it goes: the mind shapes taste, and experience shapes the mind. A version of this dialogue has gone on through billions of meals since life first developed an appetite.

CHAPTER 2

The Birth of Flavor in Five Meals

T
he first inklings of flavor appeared as early life-forms began to sense the world around them and the taste of nutrients floating by in seawater excited primitive nervous systems. Countless meals were consumed as life evolved over the hundreds of millions of years that followed. Like Russian nesting dolls, our modern tastes contain those experiences. No matter how cultured one's palate or subtle the ingredients in a dish, a taste summons raw urges out of the deep past, echoing evolutionary twists and long-ago life-and-death struggles over food. Five ancient meals, each taking place at a turning point in evolutionary history, help explain where the sense of flavor, and
Homo sapiens
' talent for culinary invention, came from.

The First Bite

The creature resembled a scarab. About an inch long, with a soft, ribbed carapace, it scuttled across the sand in a primordial coastal shallow. Then it sensed a threadbare tapestry of
smells, vibration, and shifting light. Its wormlike prey burrowed into the sand, trying to undulate its way to safety. But it was too late. The predator ripped it open with its pincer-­like mandibles, sucked it into its mouth and down its gullet, then continued on its way, searching for a sheltered spot to digest.

Evidence of this 480-million-year-old meal was discovered in 1982, when a scientist named Mark McMenamin on a survey expedition spotted a tiny fossil imprint in a gray-green slab of shale. Without giving it much thought, he chiseled the impression out of the rock and bagged it along with dozens of other samples. Then a graduate student, McMenamin was surveying the geology of the Sonoran Desert for the Mexican government, picking over the flanks of Cerro Rajón, a summit about seventy miles southwest of Tucson in the Mexican state of Sonora. The ancient seabed had ended up on a mountaintop.

To the untrained eye, the fossil looked like a series of faint scratches barely a quarter-inch long. When he studied it back at the lab, McMenamin recognized them as traces of the movements of a trilobite, etched into petrified mud. Trilobites were the ancestors of nearly everything in the animal kingdom: fish, flies, birds, humans. They left countless fossils in seabeds, making them a fixture in natural history museums. Many had shells with multiple segments and looked like a cross between a horseshoe crab and a centipede. This fossil's pattern of markings was well-known, and even had a scientific name,
Rusophycus multilineatus
. McMenamin kept it and wrote about it in his PhD thesis. He thought little about it until more than twenty years later, when he was a professor of geology at Mount Holyoke College, studying the early evolution of life.

McMenamin was examining the fossil again when he saw
something he had previously overlooked. “It had this additional feature, not just the trilobite, but another sinuous trace fossil right next to it,” he said. “These things are rare.” He concluded the fossil contained evidence of an encounter between two creatures. The extra trace was an indication of a smaller, wormlike organism's attempt to burrow into the mud. From the arrangement of the markings, it appeared the trilobite had been right on top of it. McMenamin employed Occam's razor: the simplest explanation was that the trilobite had been digging for lunch. This was, he wrote, evidence of the “first bite,” the oldest known fossil of a predator eating its prey.

What did this meal taste like? Is it even possible to imagine?

Before this era, known as the Cambrian Period, flavor did not exist in any meaningful sense. Life on earth consisted mostly of floating, filtering, and photosynthesis. Bacteria, yeasts, and other single-celled creatures nestled in the furrows of granite and between grains of sand. Some joined together into slimy mats of cells. Organisms shaped like tubes or disks rode the ocean's currents. “Eating” meant absorbing nutrients from the sea. Sometimes one organism enveloped another.

Then, over tens of millions of years—suddenly, in geological terms—the seas filled to teeming with new creatures, including the trilobites, which became the most successful class of organism in the history of life; their dominion lasted more than 250 million years. Their emergence, about 500 million years ago, was when nature as we know it really began: for the first time, life began systematically devouring other life. Unlike their predecessors, these new creatures had mouths and digestive tracts. They had rudimentary brains and senses that allowed them to detect light, dark, motion, and telltale chemical signatures. They used this fancy new
equipment to hunt, to kill, and to feed. As Woody Allen's character Boris remarks in the film
Love and Death
: “To me, nature is . . . I dunno, spiders and bugs and big fish eating little fish. And plants eating plants and animals eating . . . It's like an enormous restaurant.”

No trilobites survive today, and fossils do not reveal much about their nervous systems, so assessing their sensory capabilities depends on educated guesswork. Certainly, they could perceive nothing like the complicated flavors of dark chocolate or wine. Human tastes, even the aversive ones, are full of subtleties and associations with other flavors, and to past events and feelings, the whole of our learned experience. Trilobites probably did not feel anything like pleasure, and retained only a few trace memories. Each meal would have tasted more or less the same. Its saliency would have come mainly from the slaking of hunger and the urge to attack.

Still, these primordial elements of flavor were an extraordinary evolutionary achievement, and human tastes share this same basic physiological structure. Of course, that's something like comparing a mud hut to Chartres Cathedral. But the foundation had been set.

Some big change occurred in living conditions on earth to trigger this predator-prey revolution, which is called the Cambrian explosion. Scientists disagree about what it was. Some believe it was caused by a prehistoric bout of global warming that had melted the polar ice caps after a long deep freeze. The seas rose hundreds of feet, and water rolled far inland, over low hills and rocks with lichens and fungi (trees, grasses, and flowering plants did not yet exist), carving out lagoons and shaping sandbars and shoals, creating warm, shallow cauldrons ideal for life to flourish. Others trace it to a shift in the orientation of the earth's magnetic field, still
others to mutations that brought about the emergence of the action potential, the ability of nerve cells to communicate over distances, or other fortuitous changes in the DNA code.

Whatever the precise sequence of events, an iron link was established between acute senses and evolutionary success. A biological arms race ensued as bodies and nervous systems adapted to rising threats and opportunities. The senses, once mere detection-and-response mechanisms, had to grow more powerful in order to guide complicated behavior. Flavor became the linchpin of this process. From the time of the trilobites to the present, foraging, hunting, and eating food have driven life's endless bootstrapping, culminating in our big human brains and the achievements of culture. More than vision, or hearing, or even sex, flavor is the most impor­tant ingredient at the core of what we are. It created us. The ultimate irony, McMenamin says, is that the introduction of killing into the world, and with it untold suffering, also expanded intelligence and awareness, and ultimately led to human consciousness.

Sweetbreads

Drawn by the scent of decomposition, the jawless hagfish burrows into the bodies of dead sea creatures and then devours their carcasses from the inside out. This has proven to be a highly successful evolutionary strategy. Jawless fish, the first vertebrates, appeared about 450 million years ago, roughly 30 million years after the “first bite,” and the fossil record shows they have changed little since then. They are older by about 200 million years than their rival for the title of champion survivor, the cockroach. The hagfish, an outlandish-looking
animal with an eel-like body and a sucker for a mouth, is sometimes called a living fossil. Humans are descended from some ancient relative of the hagfish; its anatomy and behavior offer a glimpse into the deep past, when the basic couplings between the brain and the senses were first established.

To early predators, the trilobites, taste and smell would have been virtually indistinguishable. But in jawless fish, they assumed different jobs, and would not reunite until humans appeared on the scene. Taste became a gatekeeper to the body's inner precincts. But smell reached out into the world. Hagfish swam through a shifting haze of scents. Smell created a picture of their surroundings in their brains: predators, potential mates, their next meal. To humans, the scent of rot usually triggers disgust. But this reaction is subjective. To the jawless fish, it meant survival and satisfaction.

Where did this additional sensory power come from? Sometimes, mutations in the genetic code do not merely change the body—they add to it. Entire strings of DNA can randomly duplicate themselves; when their biological instructions are carried out, the organism acquires an extra set of something. Redundant tissues can be deadly, mucking up the body's normal functions. But under the right circumstances, they can bring about significant evolutionary leaps. The original genes continue doing their established jobs, and natural selection works on the copies, which take on new tasks or build new body parts. The German writer and naturalist Johann Wolfgang von Goethe anticipated this powerful evolutionary force in the late eighteenth century, guessing that duplicate parts of the anatomy might transform themselves into something different. The structure of a leaf might be the basis for the flower petal. A skull might be a modified vertebra.

In the jawless fish, receptors for smell were duplicated and the extras altered to detect new scents. Their immediate ancestors likely had only a handful of smell receptors; hagfish have more than two dozen. As life evolved, this process repeated itself many times over: some animals have as many as 1,300 kinds of smell receptors; humans have more than 300.

The new sensations bombarding the first jawless fish would have been a cacophony to the brain of the average trilobite. So as the sense of smell grew sharper, the hagfish brain adapted. The olfactory bulb is a way station between the nose and the brain of all animals, converting smells to patterns of nerve impulses. In the hagfish, a new structure grew upward from the bulb, like a flower springing out of the earth. This structure was the forerunner of the cerebrum, the topmost part of the human brain that gives conscious form to virtually everything we do: it processes senses, perceptions, movement, and speech. In humans, the same sets of genes still jointly govern development of the olfactory anatomy and the brain's basic structure. Smell has been the biological currency of feeling and action for almost as long as animals have had nostrils. It is the human sense of smell that gives flavors their vast range and subtleties. Proust, whose novel
In Search of Lost Time
is a reverie inspired by the scent and taste of a madeleine cookie dipped in tea, might have been taken aback to hear that carrion feeding was the starting point for humanity's deep connection between smell and memory.

Ant Soufflé

About 250 million years ago, the global dinner table was abruptly cleared and reset. A wave of volcanic eruptions
across the Siberian steppes, possibly triggered by a meteor impact, sent lava pouring over nearly a million square miles. Ash blotted out the sun for millennia. Acid rains raked the face of the planet. Plant life in the oceans and on land died off, and the atmosphere grew thick with carbon dioxide, making it all but unbreathable. This cataclysm, called the Permian extinction, eliminated 90 percent of marine species and 70 percent of land species (even most insects, which often escape such catastrophes). It was the biggest mass extinction in the history of life, a bookend to the Cambrian explosion 250 million years earlier.

Into this blighted landscape sauntered two quite different kinds of animals: dinosaurs, and creatures that looked something like small furry lizards. The outlines of this story are familiar: dinosaurs dominated the planet until their own end came, while early mammals stayed out of their way, waiting their turn. But in the shadows and hollows where mammals skulked, a different storyline was unfolding.

One of these protomammals,
Morganucodon oehleri
, lived about 50 million years after the Permian extinction. It wasn't cuddly;
Morganucodon
laid eggs, and its long snout and ambling gait were reptilian. It had some mammalian traits: fur, warm-bloodedness, and a secondary joint in its jaw. But what really placed
Morganucodon
closer to the mammal camp were its heightened perceptions, forged around its endless hunt for food, which became the object of complex strategies and vivid gratification—the earliest stirrings of humans' grand culinary passions.

Morganucodon
was a wisp of a beast, shorter than a man's finger, but its whole body responded to the world. In a single moment, it could register the scent of a tiny lizard a hundred feet away, a termite mound over the next rise, and a dinosaur
across a bog. Its eyes could spy predators in the dark. It could sense animals moving nearby via slight shifts in airflow over its fur. Whiskers helped it root through the underbrush for food. It usually found what it was looking for: trails that led to anthills, worms and grubs under rotting tree trunks, tinier mammals skittering across its path. Mealtimes, which in earlier epochs were all about filling the stomach and closing the abyss of hunger, were now focused more on the delicate senses of the mouth, offering earthy flavors and hints of pleasure.

This was the world of the scavenger. If food wasn't quickly and efficiently obtained, eaten, and digested, a
Morganucodon
would die, either of starvation or as some dinosaur's snack. Mammals' signature advance—warm-bloodedness—reflects this desperation, and the crisp urgency of each meal. Cold-blooded dinosaurs could eat and rest at varying tempos depending on how hot or cold it was, husbanding their energy. Mammals had to be constantly on the hunt, and good at it, because the metabolic furnace that maintained their body temperature demanded far more calories (a modern mammal at rest consumes seven to ten times the energy of a reptile the same size). As time went by and dinosaurs grew larger, mammals had to pour still more energy into evading them.