How does the tongue taste?

Your tongue is almost all muscle, about 10 cm long, wrapped in a moist pink lining. Four muscles on each side, the extrinsic muscles, tie it to the chin, the hyoid bone, the skull and the soft palate and move it around. Four more inside it, the intrinsic muscles, run lengthwise, across and up and down, and change its shape: long and thin, short and fat, curled or flat. It has no bone at all.

The top is covered in tiny bumps, the papillae. There are four kinds. Filiform papillae are the most common: little spikes that feel texture but have no taste buds. Fungiform papillae are the red dots, thickest on the tip and edges. Foliate papillae are folds at the back of the sides. And a row of 8 to 12 big circumvallate papillae forms a V near the back, each sunk in its own moat.

The taste buds hide in the fungiform, foliate and circumvallate papillae, with a few more on the soft palate and throat. An adult has about 2,000–4,000 of them. Each is an onion-shaped cluster of about 50–150 cells with a tiny opening, the taste pore, where chemicals dissolved in spit reach the cells' tips.

Three nerves carry taste to the brain: the facial nerve (VII) from the front two-thirds, the glossopharyngeal (IX) from the back third, and the vagus (X) from the throat. A fourth, the hypoglossal (XII), moves the tongue. Three pairs of salivary glands, the parotid, submandibular and sublingual, make about 0.5–1.5 L a day of saliva. See where taste ends up in the brain in BrainClear.

Taste cells can tell apart five basic tastes. Each receptor cell carries only one kind of receptor, a protein in its skin that catches one kind of chemical.

Sweet is caught by a pair of receptors, T1R2 + T1R3, that clamp shut on sugars like the sucrose in jaggery. It tells you "energy". Umami, the savoury taste of tomato, dal, broth and cheese, uses T1R1 + T1R3, which grips glutamate, a sign of protein.

Bitter is a warning sense. Many poisons made by plants are bitter, so we have about 25 kinds of bitter receptor, the T2Rs, between them catching thousands of chemicals, like those in karela. You can taste bitter at far weaker strengths than sugar.

Sour and salty work differently: they are channels, tiny doors that let charged particles into the cell. Acids, like the tartaric acid in tamarind, release protons (H⁺) that flow in through OTOP1. Salt releases sodium (Na⁺), which flows in through ENaC.

Either way, the cell becomes active and signals a nerve fibre. The brain reads the pattern across all five, so tamarind comes out "sour and sweet" at once.

You may have seen a tongue map in a school book: sweet on the tip, salty and sour on the sides, bitter at the back. It is wrong. Try it: dab a little sugar water on the back of your tongue and you will taste sweet.

The myth began with a careful German study in 1901. David Hänig dripped tastes on the edges of people's tongues and found that some spots needed a slightly weaker or stronger solution before people noticed it. In 1942 the psychologist Edwin Boring redrew his results as a graph without units. Later writers read it as "each taste lives only in one zone", and the map spread into textbooks.

In 1974 Virginia Collings tested it again and found every taste everywhere. Today we know why: every taste bud holds cells for all five tastes. What really changes across the tongue is how many taste buds there are. The tip, the edges and the V at the back have lots. The middle of the top has few, because it is mostly covered by filiform papillae, which have none.

The burn of a chilli is not one of the five tastes. It is a feeling of heat and pain, sensed not by taste buds but by the bare tips of nerve fibres all over your mouth (and your skin: that is why chilli on your fingers stings your eyes).

On those nerve endings sits a tiny channel called TRPV1. Its real job is to warn you about burning heat: it opens above about 43 °C. Capsaicin, the molecule that makes chillies hot, fits into TRPV1 and opens it too. So your brain gets the same message as from a hot pan: fire! Your face flushes and you sweat, even though nothing is actually hot. David Julius found TRPV1 in 1997 using capsaicin, and shared the 2021 Nobel Prize with Ardem Patapoutian, who found touch sensors.

Mint works the other way round. Menthol opens TRPM8, a sensor for cold, so mint chutney feels cool at room temperature.

Heat is measured in Scoville heat units (SHU). A bell pepper has none, a Kashmiri chilli about 1,000 to 2,000 and a Guntur chilli tens of thousands. In 2007 the bhut jolokia from north-east India became the first chilli certified above a million. Capsaicin is oily, so water barely washes it off. Milk and curd work much better: their protein, casein, and fat carry it away. The spices themselves are ground in MixerClear.

Your tongue senses just five tastes, yet you can tell mango from papaya, and masala chai from plain tea. The difference is smell. As you chew and breathe out, air carries aroma molecules from the back of your mouth up into your nose, to a small patch of smell sensors in its roof. This is called retronasal smell, and the brain files it under "taste", so it feels as if it comes from your tongue. How smell sensors work is in NoseClear.

Try it with a sweet: hold your nose and it tastes only sweet and a little sour. Let go, air flows again, and suddenly it tastes of strawberry or mango. That is why food seems dull when you have a cold.

Flavour is the brain's mix of taste, smell, touch (crunchy, creamy, the dry pucker of amla or strong tea), temperature, and chilli or mint on the pain and cold sensors. Scientists are still arguing about extra tastes: kokumi, a "rich, mouth-filling" feeling, and fat. Both have receptors on taste cells, but neither is yet agreed to be a sixth basic taste.

Taste needs saliva. Molecules must dissolve to reach the taste pores, so a grain of dry salt on a dried tongue tastes of almost nothing. And people differ: about a quarter of people in studies are supertasters, who find some bitter chemicals (like PROP) very strong, and tend to have more fungiform papillae; about a quarter barely taste them.

This chapter shares general facts only. If your sense of taste changes, or your tongue is sore, talk to a doctor or dentist.

Always renewing. Taste cells wear out fast. On average they last about 10 days, though some live for weeks, and stem cells at the base of each bud keep making new ones. That is why a tongue scalded by very hot tea usually tastes normally again once it heals, often within days to a couple of weeks.

What can change taste. Taste and smell often fade slowly with age. Smoking dulls both. A cold or COVID-19 can take away smell, and with it most flavour; for most people it comes back within weeks, but for some it takes much longer. Some medicines, a dry mouth and mouth or tooth problems can change taste too. Brushing your teeth and gently cleaning your tongue keep the mouth healthy.

A clever muscle. The tongue is a muscular hydrostat, like an octopus arm or an elephant's trunk: muscle with no bone that keeps the same volume, so squeezing it one way makes it longer another way. Its shapes make the sounds of speech, and when you swallow it squeezes food back into the throat while the soft palate and epiglottis close off the nose and windpipe. Food then heads down to the stomach: see StomachClear.