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.