Taste is not what you think. Every schoolchild learns that it is one of the five senses, a partner of smell and sight and touch, a consequence of food flitting over taste buds that send important signals—sweet or bitter, nutrient or poison?—to the brain. Were it so simple.
In the past decade our understanding of taste and flavor has exploded with revelations of the myriad and complex ways that food messes with our consciousness—and of all the ways that our biases filter the taste experience. Deliciousness is both ingrained and learned, both personal and universal. It is a product of all five senses (hearing included) interacting in unexpected ways, those sensory signals subject to gross revision by that clump of nerve tissue we call the brain.
Let's start at the beginning: Food enters your mouth, meets your teeth and begins to be broken down by enzymes in your saliva. The morsel soon moves over your papillae, the few thousand bumps that line your tongue. Each papilla houses onionlike structures of 50 to 100 taste cells folded together like the petals of a young flower about to bloom—taste buds, we call them. These cells have chemical receptors attuned to the five basic tastes—bitter, sweet, sour, salt and umami, the last a word borrowed from Japanese that describes the savory flavors of roast meat or soy sauce.
These five tastes are enough to help determine if the thing we just put into our mouth should go any farther—if it's sweet or savory and thus a probable source of nutrients or if it's bitter and potentially poisonous. Yet they can't get close to communicating the complexity of the flavors that we sense.
For that, we turn to the nose. As you take in a piece of food, a little air is forced up passageways at the back of the mouth, where scent receptors in the nasal cavity detect thousands of volatile chemicals that add up to complex flavors [see interactive]. This retronasal olfaction, as it's called, has almost nothing to do, physiologically, with the act of sniffing your food. Your brain knows where your smell signals are coming from—through your nostrils or from your mouth. And in the case of the latter, it ropes them together with the signals from the taste buds. Retronasal olfaction produces a completely unique sense—neither smell nor taste alone but a hybrid that we call flavor. It's a process as transformative and irreversible as turning fuel and oxygen into flame.
Our sense of taste doesn't end at the mouth. In recent years scientists have found taste receptors all over the body, discoveries that have solved some long-standing mysteries. For 50 years scientists had been trying to figure out why eating glucose produces a much sharper insulin release than injecting the same amount of glucose directly into the bloodstream. In 2007 they discovered that cells lining the small intestine also contain taste receptors. When these intestinal sweet sensors detect sugar, they trigger a cascade of hormones that ultimately ends with a squirt of extra insulin into the bloodstream.
Our sense of taste isn't just limited to the gut. For example, your nose is lined with cells that sense bitter chemicals. If there's poison in the air, they reflexively stop you from pulling it into your lungs. If the poison does get to the throat, bitter detectors in the trachea trigger cilia to help clear the airway.[break]
This physiology may explain what we mean by flavor—but anatomy doesn't much help us understand what we like. Our flavor preferences take shape over a lifetime, beginning while we are still in the womb. Babies whose mothers consume garlic while pregnant are more likely to enjoy the flavor of garlic in breast milk. Pregnant women who drink carrot juice are more likely to have kids who like carrots. The evolutionary justification is simple enough: If Mom ate it, it's safe.
Indeed, we use our friends and loved ones in much the same way that medieval monarchs used food tasters—let them try it first, then let's see how they are doing in 20 minutes. The principle holds all the way down the food chain. Rats hate the taste of cocoa, yet some enterprising scientists recently separated a rat from its brood and coaxed it to eat some anyway. The rat then returned to its group. When the other rats smelled the cocoa on its breath, they changed their minds and suddenly couldn't get enough cocoa.
Children are harder to convince—they have to try an unfamiliar food about nine times, on average, before they begin to like the taste. As any parent will attest, so much of the eventual enjoyment rests on how well Mom and Dad sell it. Moreover, the same holds true for adults, as decades of increasingly sophisticated food-marketing campaigns have demonstrated.
The environment sends many cues about how food should taste. In one experiment, researchers connected volunteers' tongues to a low-voltage electrical device, showed them pictures of food items and then sent a mild shock across their taste buds—a sensation not unlike licking a battery. The shock was supposed to impart a neutral taste. Asked afterward to rate how pleasurable the shock was, those volunteers who saw photographs of sweet or fatty foods rated the stimulus far more pleasurable than those who saw a low-calorie food.
The visual and auditory triggers can be so obvious as to appear comical. Potato chips taste crisper if you hear a crunch over headphones. White wine with a drop of red food coloring tastes like red wine—even to experienced wine tasters. People will eat less food off of a red plate. A block of cheese with sharp edges tastes sharper than one with round corners.
It's not all from our mouth, or our mouth and the back of our nose, or our mouth, and nose, and taste cells in the intestine. Deliciousness comes from our mother, our childhood, the room we are eating in, the plates we are eating on and the friends we are eating with. It's mental as much as chemical.
This hunger, this quest for deliciousness, has effects that reach far beyond our taste buds (and our waistlines). In this special issue of Scientific American, we have set out to explore some of the amazing ways that food continues to transform the world. We have organized the articles into three sections. The first, Feast, celebrates our love for eating and our long-standing ingenuity in making food taste delicious. The second, Fuel, examines the ongoing revolution in our understanding of how food changes us, from the surprising causes of the obesity crisis to the ancient rise of cooking that perhaps fueled our evolution into big-brained Homo sapiens. The third, Farm, profiles novel ideas for intelligently expanding the food supply.
As you'll see in these pages, we have learned much about food. Yet there is always so much more to know.