by John McQuaid
Overseen by University of Pennsylvania scientist Sarah Tishkoff, this arduous study aimed high. By returning to Africa, the scientists hoped to finally identify the evolutionary forces behind our diverse bitter tastes. The answer would explain a lot about food, flavor, and human biology.
Tishkoff’s team assumed the taste differences had to be tied to what people ate, and surveyed areas relatively untouched by modern society. “They’re not eating McDonald’s,” said Michael Campbell, who worked with Ranciaro and Tishkoff. For the most part, the people had been eating the same foods for thousands of years, perhaps as far back as the original exodus from Africa. People eating a meat-heavy diet might benefit less from an aversion to bitterness than a hunter-gatherer group harvesting pungent berries and roots. Over thousands of years, a meat-eating community might sift bitter-tasting genes out of the population.
When the results came in, however, the diet hypothesis—the theory of taste virtually all scientists subscribed to—turned out to be wrong. There was no evidence that food choices influenced genes—at least, not for the past five thousand years. Some older, deeper force seemed to be at work. This raised a provocative question: what if this ancient evolutionary signal was about something more than just taste?
It had been clear for a while that bitter sensitivity was part of a more complicated system in the body that reached beyond flavor. In the 1970s, Linda Bartoshuk, a taste scientist at Yale, noticed that many PTC tasters’ sensitivity extended to sour, sweet, and salty tastes. They tended to avoid the powerful kick of chili peppers, wasabi, and ginger, too.
Bartoshuk set aside molecular biology and looked into the mouths of her volunteers. Many bitter tasters had a radically different anatomy from non-tasters, in that they had more fungiform papillae on their tongues. This meant they had more hardwired connections between the mouth and the brain: they perceived more intense taste sensations and more flavor information overall than other people. Some were ten thousand times more sensitive. She dubbed this group “supertasters.” Their flavor experience differed from that of non-tasters. Their food was full of garish neon colors rather than gentle pastels.
Bartoshuk’s finding could only mean that, in concert with many other genes, taste genes influenced the anatomy of the tongue and nervous system. Hundreds of studies show that the split between bitter tasters and non-tasters extends beyond food preferences: There are more women tasters than men. Alcoholics tend to be non-tasters. Tasting has been linked to diabetes, bad teeth, eye disease, schizophrenia, depression, gastrointestinal ulcers, and cancer. Some of these correlations may be random, but the preponderance of them indicates that bitter taste biology influences the whole body. Since the DNA of taste receptors was decoded over the last decade, it has been found all over the body: along the digestive tract, in the pancreas and liver, in the brain, and in the testicles. (Smell receptors have also been isolated in the liver, heart, kidneys, sperm, and skin.) “You can imagine a simple organism, a protozoan with a sort of mouth where the taste gene is expressed,” said Roberto Barale, a biologist at the University of Pisa, in Italy. “When he starts to evolve and increase his size, the gene which controls expression of taste follows along. So the expression expands throughout the body, from mouth to stomach to intestine. And so on.”
Receptors of all kinds make up a sprawling sensory web that wends through the cells in every living thing. Without them, life would be blind and inert. Certain yeasts use receptors to recognize the sugars they feast on, as well as pheromones, so they can mate. Fruit flies have them embedded on the outside of their bodies to sense changes in light and to trace the scent of ripe fruit wafting by. Plants have them too. Vertebrates have between one thousand and two thousand different types. There is no cell in the human body that doesn’t contain them. They alert the body to changes in temperature and chemicals in the water or air. They are the switches for the body’s internal communications, and thus the preferred target for most drugs, medicinal and illegal. They detect the presence of nerves firing, and rising or falling levels of hormones or neurotransmitters that move us to act on appetite, fear, or love. And they power sight, smell, and taste.
Still, the notion that “taste” would be at work all over the body was truly odd. Scientists at the Monell Center and the University of Pennsylvania tried to understand what use bitter receptors might be in the nose. They used cultures of human sinus cells with TAS2R38 receptors. To verify they had the right cells, they dosed them with PTC to see how they would respond. The receptors generated a faint electric current—the same signal they produce when they bond with a bitter molecule on the tongue. Bingo.
Then they gave the cells a sinus infection, flooding them with mucusy gunk. The bitter receptor alarms went off. The neurons sent an electrical message and released nitric oxide, a signaling molecule. Tiny cilia in the sinus cells waved faster and mucus production increased in response. That’s how noses expel bad bacteria, or try to. Tasters, it seemed, were less likely to develop sinus infections than non-tasters. This would have been an advantage—unrelated to diet—for early humans colonizing colder climes.
There’s also the gut. Like many small communities along the Mediterranean coast, the people of Calabria, the province that comprises the toe of Italy’s boot, tend to be especially long-lived. Perhaps it’s a consequence of the Mediterranean diet’s emphasis on ingredients such as olive oil, fish, and red wine. But it might also have something to do with their sense of taste. Calabrian cuisine makes significant use of bitter vegetables, including eggplant, cauliflower, and spinach. The Bergamot orange, more bitter than grapefruit, is a local staple.
Roberto Barale was curious about the Calabrians’ bitter tasting genes. As it happened, a group of Calabria residents was already taking part in a study tracking their health over the course of decades. Barale studied them, too, and found something provocative: a mutation in their TAS2R16 bitter gene was more prevalent in the elderly. And the older they got, the more likely they were to have it. Since people without it were dying earlier in life, something about this mutation seemed to contribute to longevity. Barale speculates this may be the result of bitter receptors in the intestines—although their function is currently unknown—somehow facilitating metabolism.
Until recently, most taste studies focused on the biology of the tongue and on perception. Now a new frontier has opened up. The bitter receptors dotting the body are part of a kind of shadow taste system. Unlike those on the tongue, they don’t register in consciousness, and what they do is still mostly unclear. Flavor, in other words, is only the capstone of a vast, hidden system. It starts in the mouth with a burst of deliciousness, then disappears in darkness down into the gut, and from there its hand reaches everywhere in the body. It is an infinite mesh of sensors furiously sending and receiving messages as the whole body marinates in the chemical flux of the world.
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The body’s “taste” for bitterness may be responsible for the ubiquity of bitter foods today. Humans need bitter compounds; many are beneficial in low doses. Chewing willow bark is an ancient folk remedy for pain or fever—salicin is an anti-inflammatory compound related to aspirin. Bitter melon, a fruit grown in Asia, Africa, and the Caribbean, contains a suite of unpleasant-tasting chemicals that mildly lower blood sugar. As humans populated the globe, bitter-sensitive people may have helped their groups survive by detecting poisons; non-tasters might have tried more new things, bringing the others along when they found something with potential.
Humans can get used to anything. The Aymara people of Bolivia’s Altiplano farm a very bitter type of potato, and their taste perceptions have adapted accordingly. Tests done in the 1980s showed the tribe was less sensitive to bitterness than Americans—yet every person tested was a PTC taster. Their sensitivity had dulled because their diets demanded it. Even at this new threshold, however, their tastes remained discerning: the point at which potatoes started to taste too bitter to them wa
s exactly the point where they became toxic.
Like much else in the body, the flavor sense is a dialectic between genes and life experience. As people age, and sample ever-expanding varieties of foods, the networks of neurons in the brain responsible for aversive reactions shift. Bitterness mellows; for some it does a volte-face, becoming pleasurable. This capacity for contradiction, the strange yen to embrace the aversive, is what makes cuisine come to life.
It might have gone like this: Many generations after Bab el-Mandeb, a group of humans had migrated north of the Mediterranean, setting up camp in a valley. Culling the underbrush for food, they found sprigs atop twisted roots: plants from the genus Brassica, the wild ancestors of broccoli and mustard. What these lacked in tastiness, they made up for in nutrition: chemicals called isothiocyanates that stimulate the immune system and provide protection against cancer. To the eternal regret of President Bush and all modern broccoli-haters, this assured Brassica’s future.
CHAPTER 4
Flavor Cultures
Absinthe, a green-hued alcoholic beverage made with anise and assorted herbs and extracts, has a reputation for mystery and danger. Invented in Switzerland in the 1700s as a medicinal elixir, it became the preferred drink of artists, writers, and bohemians, who were drawn to its sharp herbal flavors and the vivid highs it was said to produce. In fin de siècle Paris, it cast a spell. “What difference is there between a glass of absinthe and a sunset?” Oscar Wilde wrote. In For Whom the Bell Tolls, Ernest Hemingway described it as “opaque, bitter, tongue-numbing, brain-warming, stomach-warming, idea-changing liquid alchemy.”
Among absinthe’s ingredients is thujone, an intensely bitter chemical with a menthol fragrance obtained from the flowers of wormwood, a small shrub. (Wormwood extracts are still used as a traditional folk treatment for intestinal parasites, and to kill insects.) A century ago, high doses of thujone were thought to cause hallucinations and madness. Vincent van Gogh was a heavy absinthe drinker; in 1887, he painted Still Life with Absinthe, which depicts a tall, shimmering, pale green drink sitting next to a water decanter on a table in a Paris café. After he killed himself in 1890, the art world speculated absinthe had been responsible for everything from impairing his color perceptions—leading to his use of bright and off shades in paintings—to his mental deterioration, to his death itself.
Once known as the “green fairy,” absinthe came to be called the “green witch” and the “queen of poisons.” After a laborer went on an absinthe bender and shot his pregnant wife and two children in Switzerland in 1905, the Swiss government banned it. France followed suit in 1915; “absinthism” was blamed for eroding French culture. These fears lingered for decades. Even after the repeal of Prohibition in 1933, absinthe remained illegal in the United States until 2007.
Modern science has shown that absinthe got a bad rap. While thujone blocks the action of GABA receptors, one of the nervous system’s principal signaling tools, it would take a massive dose to have any effect. A 2008 study of thirteen century-old absinthes found that each contained only trace amounts of thujone; overindulging in absinthe would cause alcohol poisoning long before thujone could do any damage. Scholars now believe van Gogh’s decline was caused by some form of mental illness, combined with alcoholism.
With the legal strictures lifted, enterprising beverage makers set out to re-create and rehabilitate absinthe. One of them, Jedd Haas, built a distillery in a modest cinderblock room under an overpass in an industrial district of New Orleans in 2011. He and his partners named it Atelier Vie, French for “Life Workshop.”
Distillation—the process of heating, evaporating, and condensing a liquid to purify it—dates to the ancient world (the term “distilled spirit” was coined by Arab alchemists, who thought that vapor contained the spirit of a substance). Distilled alcohol is a comparatively recent invention. Doctors in Salerno, Italy, began consistently making it in the twelfth century for medical use; in China, a form of distilled wine became a popular drink among the upper classes a century later. Making these drinks takes many steps. First, an alcoholic beverage is required; this may be wine (the basis for brandy) or a barley mash (whiskey) or fermented molasses (rum). It’s heated in a still to a temperature above the boiling point of alcohol, 173.3 degrees Fahrenheit, but below water’s 212 degrees. The alcohol evaporates faster, giving the vapor a higher concentration than the original drink. In the simplest kind of still, this vapor flows from the heated kettle into a separate container, where it is cooled and condensed into droplets, which collect in a third vessel. The resulting drink then may be aged and/or flavored.
Absinthe adds an extra twist to this process. First, herbs are infused into a previously distilled clear spirit. Atelier Vie uses rum; its hint of sweetness balances the bitterness of the herbs. This alcoholic “tea” is then distilled. The redistillation makes absinthe one of the most potent drinks on the market; Atelier Vie’s is 136 proof, or 68 percent alcohol. (Scotch whisky is typically 40 to 50 percent alcohol.)
Serving absinthe involves a bit of chemical showmanship. First, Haas poured some of his absinthe into a glass. Instead of herbal green, this drink was a deep red, created by a secondary infusion of natural colors including hibiscus flower. He then placed an ornate, slotted silver spoon across the rim, set a sugar cube in the bowl of the spoon, and poured chilled water over it. As the sugar water and absinthe mixed, dissolved herbal compounds coalesced, turning the drink cloudy; this milkiness is called the louche, French for “shady.”
A drink such as Atelier Vie’s Toulouse Red owes its existence to centuries of refinements, technological advances, and cultural foment. It is many steps removed from the natural world, its original ingredients transformed beyond recognition. Its sensations and effects on the brain are like nothing found in any ancient hunter-gatherer’s meal. These differences can be traced back to the birth of civilization itself about twelve thousand years ago, when both culture and the tools people used changed profoundly, and flavor with them.
At that time, the great post-Africa migrations were ending. The Ice Age was over, glaciers were receding, and a climate of warm, dry summers and cool, wet winters settled in across Europe and Asia. Wild grasses such as wheat, barley, and rye thrived, spreading over the Fertile Crescent, the area spanning the Tigris and Euphrates river valleys. People began eating these grasses, then cultivating them. In mountains not far away, others learned the tricks of herding goats and sheep. Cultivated crops and domesticated animals replaced the more diverse foods found in nature.
With this simplification of diet came a flood of food and flavor innovations. One of these rivaled the taming of fire: humans harnessed fermentation. Today, fermentation is the source of much of the flavor in the world, its signature found in a galaxy of consumables, which, in addition to spirits, includes wines, beers, cheeses, yogurts, tofu, soy sauce, and pickles.
A basic biological force, fermentation is the metabolic action of certain types of bacteria and fungi. These single-celled organisms belong to the microbiome, the sprawling populations of microbes that cover human skin, line our insides, and infest every square inch of the planet. One of the microbiome’s most important jobs is decomposition: microbes feast on dead tissue, injecting its molecules back into the circle of life. Fermentation is a particular kind of decomposition, the breakdown of carbohydrates in the absence of oxygen. It has the felicitous result of making decomposing things taste better, rather than worse.
The by-products of fermentation include carbon dioxide, acids, alcohol, and a host of cast-off molecules. These were useless waste to the microbes, but they captured prehistoric imaginations. Their flavors were complex and provocative. Alcohol also altered brain chemistry, lowering inhibitions and smoothing social interactions. These new sensations shocked stunted palates, and altered the nature of flavor itself. Taste and smell are usually thought of as a series of chemical reactions in the mouth and nose. But flavor only comes to life at the ot
her end of this system, in the brain, where chemistry is transmuted into sensation and consciousness. Just as the advent of cooking unleashed new flavors and nutrients that influenced the course of evolution, fermentation impressed itself on human biology and the mind.
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There was no single “first” alcoholic beverage, cheese, or any particular fermented food. Like cooking, these items were probably invented a number of times, in more than one place. But they were profoundly different from cooked food. The tools of civilization gave prehistoric peoples a level of control over nature, specifically microbiology, that had never been achieved before.
All the elements for success were already in place in nature, waiting to be assembled. One of the most prolific, and useful, microorganisms on earth is a species of yeast called Saccharomyces cerevisiae. It is the hidden agent behind virtually all alcoholic beverages, as well as breads and other baked goods, which is why it’s also known as baker’s yeast. Saccharomyces cerevisiae is a supermicrobe. It can store lots of energy to survive lean periods, and it manufactures enough alcohol to kill off other yeasts, eliminating its competition. DNA from baker’s yeasts still clumped to the legs of ant-like insects entombed in amber found in Poland and the Dominican Republic shows that it is tens of millions of years old.
There’s an intriguing explanation for the ubiquity of baker’s yeast: wasps. Wasps carry yeasts in their guts, and are attracted to fruits. In wine country, wasp nests often grow nearby as grapes ripen each season. Scientists at the University of Florence, Italy, looked for evidence of a connection. They caught wasps from colonies in Italy and analyzed their insides. Among 393 distinct kinds of yeast, the baker’s variety stood out. The other yeasts waxed and waned during the course of the year, but baker’s yeast was always present. It survived the cold by riding out winters in the guts of the fertilized queens. When young wasps departed their hives in the spring to form new colonies, baker’s yeast went with them. In fact, the wasps are part of a global yeast transportation network; DNA evidence linked baker’s yeast at Italian vineyards to many places in Italy and beyond: breweries, palm winemakers, and bread ovens as far away as Africa.