Why does food taste bland when you have a cold?
― Most of what we call "flavour" is made by the nose, not the tongue
On a day when your nose is blocked, your usual meal tastes muted. It's not that your tongue has stopped working. Most of what we call "taste" is actually smell, carried by air moving from the back of your mouth up into your nose.
You have a blocked nose one morning and drink your usual miso soup. You can tell it's salty. But the savoury aroma of the stock is simply gone.
Strawberry ice cream is the same. You can tell it's cold, and you can tell it's sweet. But the "strawberry-ness" has vanished completely.
Your tongue is working exactly as usual. What's missing is the part your tongue was never in charge of.
There are two main reasons
Sweet, salty, sour, bitter, umami. Those are the only five categories the tongue can distinguish. The individual character of "strawberry" or "coffee" isn't among them.
As you chew, aroma compounds rise up from the food inside your mouth. They curve round the back of your throat and reach the cavity behind your nose. When your nose swells up, this route gets blocked.
In other words, a cold doesn't take away "taste" — it takes away "aroma." The tongue's five categories are still delivered as normal, which is why saltiness or sweetness feels like all that's left.
The tongue's job is surprisingly rough-and-ready
The surface of the tongue is covered in small receptors called taste buds — humans have a few thousand of them. When food compounds dissolve and touch these, a signal is sent to the brain.
But the number of categories these receptors can tell apart is very small. Sweet, salty, sour, bitter and umami are thought to cover almost everything. Spiciness isn't a taste at all — it comes from the mechanism that senses pain and heat. Astringency, too, is thought to be the sensation of proteins on the tongue's surface shrinking.
Just five is fewer than you might expect. Try imagining telling "melon" and "banana" apart using only those five. Both are sweet — that alone won't separate them. The reason we can identify a food in a single bite is that other information is arriving alongside it.
Aroma has two separate routes
The cells that sense smell sit in a ceiling-like patch deep inside the nasal cavity, roughly between the eyes. Once air reaches them, the smell registers. The question is where that air comes from.
There are two routes. The arrow on the left in Figure 1 is the route air takes when you breathe in through your nostrils — the familiar way of sniffing a flower or a dish held close to your nose. The other is the arrow on the right in Figure 1: a route that rises from inside the mouth, curving round the back of the throat. Every time you chew or swallow while eating, air inside your mouth gets pushed out and travels this way.
This second route is the star of the show during a meal. The aroma that rises the instant you chew arrives at the brain at almost the same moment as the tongue's five signals. The brain then bundles the two together into a single sensation — "strawberry flavour." That's why something that's actually smell feels as if it's happening inside your mouth.
With a cold, the lining deep inside the nose swells, and this passage narrows. Aroma compounds can no longer reach the sensors. The tongue's five signals still get through as normal, which produces that strange sensation: you can tell it's sweet, but not sweet as what.
Aroma compounds only reach the sensors once they turn into gas from the food. Lower temperatures mean less gas forms, so aroma comes through more faintly. This is also why ice cream tastes richer once it's slightly melted — warming in the mouth releases more aroma. The same effect is thought to be a major reason cold soup tastes underwhelming.
You may have seen a diagram showing the tip of the tongue handling sweetness and the back handling bitterness. This came from a misreading of an old experiment that spread widely, and it's now been disproved. All five tastes can be sensed across almost the entire surface of the tongue.
Summary
The tongue handles only five categories, and almost all of a food's individual character is carried by aroma. That aroma, in turn, doesn't arrive through the nostrils — it arrives from inside the mouth, curving round the throat. A cold blocks this hidden route, so the taste itself stays intact while the character disappears.
The "taste" of food isn't built on the tongue —
it's built along the air passage linking mouth, throat and nose.
There are plenty of other cases where smell acts as a hidden clue. Where the savoury aroma of grilled food comes from is covered in "Why does grilled food smell so good?", and how smell travels through the air is covered in "Why does it smell a certain way when rain starts falling?" The way your fingertips sense "roughness" isn't simply reading shape directly either — see "How do fingers tell "rough" from "smooth"?" for more.
- Get a fruit juice, or two different flavours of the same-coloured gummy sweets.
- Pinch your nostrils firmly with your fingers, then put the food in your mouth and chew well. You should be able to tell it's sweet or sour, but struggle to tell which fruit it is.
- While still chewing, let go of your nose partway through. At that instant the fruit's individual character springs into being. That's the aroma that just travelled up via the throat.
Trying this as a guessing game with family makes the difference obvious. If it doesn't work well, the pinch is usually too loose, or you're not chewing enough.
Want to know more? ― Terms, formulas, and textbook linksEach item is labelled by level, from middle-school science to university-level specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Biology Basics / Biology"
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university-level specialist content (sensory physiology, neuroscience)
- ResearchNot yet settled even at university level — an area researchers are actively investigating
MSTerminology: this phenomenon has names
- Basic tastes: the five categories the tongue is thought to sense independently — sweet, salty, sour, bitter, and umami.
- Retronasal olfaction: the way aroma reaches the sensors deep in the nose by travelling from the mouth round the back of the throat. Also called "mouth aroma" or "return aroma" in Japanese.
- Flavour: the single sensation the brain builds by combining taste and aroma with temperature, texture, and other stimuli. What we normally call "taste" in everyday speech is almost always this.
MSHSChecking the numbers: how big is the gap in distinguishing power?
Let's check, order of magnitude, just how different the "drawers of distinction" available to the tongue and the nose really are. All the figures below are estimated counts, and the unit throughout is "types."
| Basic tastes the tongue distinguishes | 5 types |
| Types of smell sensor active in humans | estimated at ~400 types |
| Compounds found in coffee aroma | estimated at 800+ types |
| Ratio of drawer counts | 400 ÷ 5 = 80 |
| Ordered pairs when combining two sensor types | 400 × 399 = 159,600 |
| Removing order duplicates | 159,600 ÷ 2 = 79,800 |
Just counting the drawers already gives an 80-fold gap. Smell is further represented by combinations of many sensor types firing together, so even counting pairs alone gives about 80,000 possibilities. In reality, sets of three, four or more overlap, so the number of smells that can be distinguished is on a completely different scale. This shows just how rough-and-ready the tongue's five categories really are.
HSHS+Taste and smell are picked up differently
HSBoth taste and smell start when a receptor protein on a cell membrane binds to a specific molecule. That binding generates a signal inside the cell, which travels via nerves to the brain. Think of it like a key fitting a lock.
HS+Sweetness, umami, and bitterness are handled by receptor proteins of this kind. Saltiness and sourness, on the other hand, are thought to mainly work through channels that let ions flow directly into the cell. So even within "taste," the underlying mechanism splits into two separate systems. Smell, by contrast, is entirely picked up via receptor proteins.
UnivEach cell uses only one type
Each smell-sensing cell is thought to select, in principle, just one of the roughly 400 available receptor types. Cells that use the same type are then wired together to a single fixed point in a structure called a glomerulus, at the entrance to the brain. The result is a map at the brain's entrance showing "which type fired, and by how much." On this view, the identity of a smell is represented as a pattern across that map. The genes for these receptors were discovered in 1991, a finding that led to the 2004 Nobel Prize in Physiology or Medicine.
ResearchWhat's still not fully understood
- Smell can't be predicted from molecular shape. Unlike colour, which can be predicted from a light wavelength, we still can't reliably predict how a molecule will smell just by looking at its structure. There are known cases of very similar shapes producing completely different smells.
- Where taste and smell become "one." Exactly where in the brain, and to what extent, signals arriving via separate nerves get merged into the single unified sensation of flavour is still being worked out.
- Smell that doesn't return after a cold. Sometimes sense of smell stays impaired for a long time after an infection, and it's still debated whether this is a problem with the sensor cells themselves or with the wiring further along. If it persists, seeing an ENT specialist rather than self-diagnosing is recommended.
In other words, this article too reflects "the current state of understanding." It may be surprising that smell, something so familiar, still leaves some of the biggest unsolved questions among the five senses.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: stimuli and response (sense organs) | What the tongue and nose each pick up |
| HS | Biology Basics / Biology — receptors and nerves | Receptors and how signals travel |
| HS+ | Biology — membrane proteins and ion channels | The two systems behind taste |
| Univ | Sensory physiology / neuroscience | One cell, one receptor type, and the map at the brain's entrance |
| Research | Olfactory science / chemical senses | Molecular shape vs. smell, and the integration of flavour |
| ― | Everyday connections | Why cold food feels underwhelming, the nose-pinching experiment |
- The Nobel Foundation: 2004 Nobel Prize in Physiology or Medicine (smell receptors and the organisation of the olfactory system)
- Buck, L. and Axel, R., A novel multigene family may encode odorant receptors, Cell, 1991 (the paper reporting discovery of the olfactory receptor gene family)
- Japanese Association for the Study of Taste and Smell (日本味と匂学会), ed., The Science of Taste and Smell (味と匂いの科学) (an explanation of basic taste and olfactory receptor mechanisms)
- Gordon M. Shepherd, Neurogastronomy (美味しさの脳科学, Japanese edition) (a general-audience book on how retronasal aroma builds flavour)
※This article is a general-audience science explainer. The figures given are approximations intended to aid understanding of the underlying mechanisms. Sense of smell and taste vary between individuals and can be affected by physical condition. If a symptom such as smell not returning persists, please consult a medical professional rather than self-diagnosing.