What the tongue actually detects

Taste, in the strict sense physiologists use, is a narrow channel. Receptor cells packed into taste buds on the tongue, the soft palate and the throat respond to a short list of chemical qualities and send that information to the brain. Everything else about a strawberry, from its perfume to its seeds, comes from other senses.

Five qualities, five kinds of sensor

The working list has five entries: sweet, sour, salty, bitter and umami. Over the past quarter century, research groups have identified the molecules that detect most of them. A 2006 review in Nature, The receptors and cells for mammalian taste, summed up the picture then emerging: separate cell types carry separate receptors, and each type is tuned to one quality.

Sweet is detected by a receptor made of two proteins, T1R2 and T1R3, working as a pair. It responds to sugars and also to the very different molecules of artificial sweeteners, which is why those can taste sweet at all. Umami uses T1R1 paired with the same T1R3. It responds to glutamate, and its response is amplified by ribonucleotides such as inosinate, the compound that makes dried bonito and meat stock so effective when combined with glutamate-rich ingredients like kelp or tomato. Kikunae Ikeda, a chemist in Tokyo, isolated glutamate from kombu broth in 1908 and gave the taste its name.

Bitter is handled by a family of roughly 25 receptors in humans, called T2Rs. The number makes sense for a warning system. Bitter compounds come in thousands of chemical shapes, many of them plant toxins, and a broad net catches more of them. Genetic differences in these receptors partly explain why some people find raw broccoli and other cabbage-family greens harsh while others barely notice.

Sour turned out to be the hardest to pin down. In 2019, two research groups showed that an ion channel called OTOP1 lets hydrogen ions into the sour-sensing cells, and that mice lacking it respond far less to acids. Sourness is, in effect, a measurement of acidity.

Salt is still partly open. In mice, low concentrations of sodium are picked up through a channel known as ENaC, and that response is the attractive one. High concentrations taste unpleasant, and that signal seems to recruit the cells for bitter and sour. How closely the human system follows the mouse version is still being worked out.

The map that never was

Generations of schoolchildren learned that the tip of the tongue tastes sweet, the sides sour and salty, and the back bitter. The idea traces to a 1901 German dissertation by David Hänig, who measured how sensitive different edges of the tongue were to each quality. He found small differences in threshold, nothing more. In 1942 the Harvard psychologist Edwin Boring replotted those data in a history of sensory psychology, and the relative figures were read as all or nothing. Textbooks turned the curves into zones.

Every part of the tongue that has taste buds responds to every quality. The simple test is to put a few grains of sugar on the back of the tongue, supposedly the bitter zone. The sweetness registers at once.

How much comes from the nose

Chewing and swallowing push air from the mouth up behind the palate into the nasal cavity. Aroma molecules travel with it and reach the smell receptors from behind, a route called retronasal smell. The brain files the result under taste, because the food is in the mouth. Pinching the nose shut while eating a jelly bean shows the effect clearly: it tastes sweet and faintly sour, and the fruit flavor appears only when the nose is released.

Popular writing often puts smell’s share of flavor at 75 to 95 percent, with 80 the favorite. The experimental psychologist Charles Spence went looking for the source in a 2015 review in the journal Flavour. He found that the quotations rarely cite anything, and traced the figure back to a 1977 study by Claire Murphy, William Cain and Linda Bartoshuk. Six trained panelists rated solutions of a sweetener mixed with ethyl butyrate, a fruity odorant. Closing the nostrils removed up to 80 percent of the “taste” the panel had attributed to the odorant. That result belongs to one sweet-smelling compound, not to food in general.

Spence concluded that smell’s contribution changes from one food to the next and cannot be pinned to a single percentage. Most researchers still agree that smell usually dominates the experience of eating. A ripe cheese owes more to the nose than a plain slice of cucumber does.

Sensations that are neither

Some of what people call flavor comes through the nerves of touch and pain. The burn of chili is capsaicin acting on a heat receptor called TRPV1, the same one that fires at scalding temperatures. The sting of mustard and the cool of menthol travel by the same general route. Scientists call this chemesthesis, and it explains why a dish can be described as hot while sitting at room temperature.

Candidates for a sixth taste keep appearing. Fat is the strongest of them: in 2015 researchers at Purdue proposed the name oleogustus for the faint, slightly unpleasant taste of free fatty acids, which tasters could tell apart from the other five. The list of basic tastes has grown before, when umami joined it, and it may grow again.