Smell and Taste – The Chemical Senses

Walk past a kitchen where someone is toasting cinnamon in butter, or into a secondhand bookshop thick with the smell of old paper, and a memory can surface before you have had a single conscious thought. Sometimes it is decades old, arriving fully formed and carrying a feeling with it. No other sense does that quite so quickly or reliably.

Smell and taste are the chemical senses. Rather than detecting light, sound or pressure, they detect actual molecules drifting into your nose or dissolving on your tongue. Between them they create most of what we experience as flavour, and they do a surprising amount of quiet work we rarely notice until it stops.

Locks, Keys and Chords

High inside your nose is a small patch of tissue called the olfactory epithelium, packed with millions of olfactory receptor neurons. Each carries one type of receptor protein, which you can picture as a tiny, precisely shaped lock. An odour molecule that has evaporated from something and drifted up your nostril is the key. When the right molecule fits, the neuron fires a signal a very short distance into the brain's olfactory bulb.

Humans have several hundred receptor types, each tuned to a different family of molecular shapes. Real smells such as coffee or cut grass are mixtures of dozens or hundreds of molecules, so the brain reads a pattern of activity across many receptors and learns to recognise it, much as you might recognise a chord on a piano from its combination of notes.

The genes behind these receptors were identified in 1991 by the American scientists Linda Buck and Richard Axel, work that earned them the 2004 Nobel Prize in Physiology or Medicine. They showed that each olfactory neuron expresses just one receptor type. A few hundred receptors combined in different ways can distinguish a vast number of odours, the way a small alphabet builds a huge vocabulary. Exactly how many is debated; some statistical estimates run as high as a trillion, though other researchers dispute that figure.

The Shortcut to Memory

Most sensory information passes through a central relay in the brain called the thalamus before being sent on, like mail through a sorting office. Smell largely skips that step. Signals from the olfactory bulb travel with very few stops to the amygdala and the hippocampus, structures central to emotion and long-term memory.

That direct wiring helps explain why a scent can bring back a vivid, emotional memory faster than almost anything else, sometimes called the Proust effect after the French novelist who wrote about memories released by a small cake dipped in tea. It is not mystical, just anatomy. For more on how such memories are laid down, see our article on how the brain stores a life.

Five Tastes and a Debunked Map

Taste is detected by taste buds, clusters of receptor cells mostly on the tongue but also elsewhere in the mouth and throat. Its toolkit is far smaller than smell's, with five well-established basic tastes, though scientists still discuss whether others deserve a place:

• Sweet, signalling sugars and therefore energy-rich food.
• Salty, detected largely through channels that respond to sodium.
• Sour, signalling acids, often a hint of unripe or spoiled food.
• Bitter, detected by an unusually large family of receptors, probably because many toxic plant compounds taste bitter.
• Umami, the savoury taste linked to glutamate in meat, mushrooms, aged cheese and soy sauce, identified by the Japanese chemist Kikunae Ikeda in 1908 but only widely accepted in Western science many decades later.

The tongue map you may have learned at school, with sweet at the tip and bitter at the back, grew out of a misreading of a modest German study from 1901 and has been thoroughly debunked. Taste buds that detect all five tastes are spread across the whole tongue, with only slight regional differences.

Flavour Is Mostly Smell

This is the part that surprises most people. What you call the flavour of food is overwhelmingly smell. A strawberry and a raspberry register on the tongue as broadly similar sweet-sour combinations; everything that makes one taste like strawberry comes from your nose.

As you chew, volatile molecules rise through the back of your throat into the nasal cavity from the inside, reaching the same receptors that detect smells through your nostrils. This back-door route is called retronasal smell, and the brain blends it with taste, texture and temperature into one seamless sensation. That is why food tastes flat with a head cold: with your nose blocked, you are left with the tongue's blunt five-colour palette.

Chilli and Mint Are Not Tastes at All

The burn of chilli comes from capsaicin, which switches on a receptor called TRPV1 that normally warns the brain about dangerous heat. Nothing in your mouth has got hotter, but your brain reads the signal as burning. Mint is the mirror image: menthol activates TRPM8, a receptor that normally signals cold. Scientists call these chemically triggered feelings of temperature, touch or pain chemesthesis, a hijacking of the warning system explored in pain and what it is for.

Sensitivity also varies for real biological reasons. Differences in bitter receptor genes mean some people find raw broccoli or Brussels sprouts almost unbearably bitter while others barely notice. Roughly a quarter of people, more often women, are so-called supertasters, with a higher density of the mushroom-shaped bumps called fungiform papillae that house taste buds, and they tend to experience bitterness more intensely. Ageing generally reduces both taste buds and smell neurons. Other animals differ again: dogs have hundreds of millions of olfactory receptor cells to our roughly six million, while cats, like several other strict carnivores, carry a broken version of a gene needed for a working sweet receptor and cannot taste sweetness at all.

When Smell Goes Missing

Losing the sense of smell, called anosmia, shows how much it normally does. It can follow a cold, a viral infection, a head injury or other conditions, and eating often loses much of its pleasure. It also removes an early warning for smoke, gas leaks and spoiled food. Sudden smell loss has been linked to certain viral infections and, in some studies, flagged as worth watching in some neurological conditions, but most causes are ordinary, and no firm conclusion can be drawn from smell loss alone.

There is good news in the biology. Olfactory receptor neurons are among the few nerve cells regularly replaced throughout life, renewed from stem cells every few weeks to couple of months. Many people who lose smell to a temporary cause recover it, sometimes helped by smell training, regularly and attentively sniffing a small set of strong scents, though recovery depends on the cause and is not guaranteed.

Breathing In the World

When a smell stops you mid-step and pulls up a long-forgotten memory, or a blocked nose leaves a favourite meal tasting hollow, it is no coincidence. It is the steady work of a few hundred molecular locks in your nose, wired unusually directly to memory and feeling, alongside a smaller set of chemical detectors on your tongue, together turning breathing and eating into some of the richest experiences the body offers.