The week you have a cold, the strangest thing about dinner is what isn't there. The soup still steams. The bread still crunches. The pepper still stings a little on the tongue. But the food itself — the thing that was supposed to taste like something — has quietly evacuated the room, leaving behind a set of textures and temperatures pretending to be a meal.
Most people describe this as losing their sense of taste. That's almost always wrong.
The tongue is fine. The taste buds are working. What has gone offline is smell, and smell is doing far more of the work of flavor than the tongue ever gets credit for.
The apple-onion trick
There's a classroom demonstration food scientists have been running for decades. Cut a slice of raw apple and a slice of raw onion into matched cubes. Pinch your nose firmly shut. Close your eyes. Have someone hand you one, then the other.
Most people can't reliably tell them apart.
Both are crisp. Both are wet. Both register as vaguely sweet with a whisper of sour on the tongue. The thing that makes an apple obviously an apple and an onion obviously an onion — the aromatic signature — never reaches the brain, because the airway from mouth to nose has been closed off.
The perceived flavor of food depends heavily on olfaction. The tongue handles five categories: sweet, salty, sour, bitter, umami. Everything else — the specific identity of a peach versus a mango, a cabernet versus a merlot, cilantro versus parsley — comes in through the nose.
Two ways smell gets in
Smell reaches the brain along two different paths, and the second one is the one nobody teaches you about.
The first path is the obvious one: you inhale, molecules travel up through the nostrils, and receptors at the top of the nasal cavity fire. That's orthonasal smell — the coffee across the café, the neighbor's basil, the rain on hot asphalt.
The second path runs in the opposite direction. When you chew and swallow, warmed volatile molecules from the food in your mouth get pushed up through the back of the throat, into the nasal cavity from behind, and hit those same receptors from the wrong side. That's retronasal olfaction, and it's the mechanism doing almost all of the heavy lifting when you eat.
The trick is that your brain doesn't file retronasal smells as smells. It files them as taste. It attributes the sensation to the mouth, not the nose. This phenomenon — sometimes called "oral referral" — explains why almost no one intuitively understands that most of what they call taste is actually a smell arriving through a back door.
Where the brain assembles it
For a long time, researchers couldn't say exactly where the tongue signal and the nose signal became one experience called flavor. The mechanism was clear. The address was not.
In September 2025, a team at Sweden's Karolinska Institutet published an answer in Nature Communications. Putu Agus Khorisantono, Maria Veldhuizen and Janina Seubert trained 25 participants to associate particular aromas with particular tastes, then scanned their brains while delivering the aroma alone and the taste alone, and used machine learning to work out which regions could tell one flavor from another.
One of the best-performing algorithms, as Science News reported, was trained on images from the insula — a region tucked deep in the brain that was previously thought to handle only the taste side of flavor. When a participant experienced the smell associated with a flavor and, separately, the taste associated with the same flavor, the insula lit up in strikingly similar patterns.
The insula appears to be an early stage in flavor assembly — the place where the two signals stop being separate and start being one thing.
The insula is a hard region to study. It sits deep. It also monitors internal organs and quietly shapes eating behavior and how people feel in their bodies, which is why the finding matters beyond flavor science. The room where you decide a mango is delicious is next door to the room where you decide you're full.
Why a cold flattens dinner
A head cold doesn't damage taste receptors. Sweet still registers. Salt still registers. What a cold does is swell the nasal passages and coat them in mucus, which physically blocks volatile molecules from reaching the olfactory receptors at the top of the nose.
Retronasal smell is the first casualty. The molecules can leave the mouth on their way up, but they can't get through to the receptors, so nothing arrives at the insula on the smell channel. The tongue signal shows up alone.
Sweet-salt-sour-bitter-umami, with no specificity attached. Soup that tastes like warm salt water. Coffee that tastes like hot bitter. An orange that tastes like cold sugar with a stinging edge.
The stinging edge, incidentally, isn't taste either. It's the trigeminal nerve responding to irritants — the heat of chili, the fizz of soda, the sharp cool of mint. Those still work when you have a cold, which is why a spicy soup feels like it has some structure even when the flavor is gone. You're feeling the food. You're not tasting it.
The scale of the loss
Anosmia — the total loss of smell — got a great deal of public attention during the COVID-19 pandemic, when it turned out to be one of the more distinctive symptoms of infection. What surprised people wasn't the loss of smell itself. It was how much of eating went with it.
Patients described food not as bland but as absent. Some stopped eating properly. Others developed a specific kind of grief for meals they used to love and could no longer recognize. The National Institute on Deafness and Other Communication Disorders treats smell loss as a real health problem rather than an inconvenience: it changes how much people eat, in severe cases it can lead to depression, and a smell disorder can be an early sign of conditions including Parkinson's disease and Alzheimer's disease.
Which is another way of saying that when the flavor of your morning coffee stops arriving, something more than breakfast is at stake.
The multisensory layer
Flavor isn't only tongue plus nose, either. Flavor emerges from the dynamic integration of gustatory, olfactory, tactile, visual, and even auditory cues into a single percept.
The crunch of a chip changes how fresh it seems. The color of a drink changes how sweet it tastes. The weight of a bowl in the hand shifts how satisfying the soup inside it feels. When food-industry researchers try to reduce salt or sugar without losing pleasure, they lean heavily on aroma and texture to compensate — adding volatile compounds that make a low-sodium broth taste saltier than the sodium in it would predict.
Expectation matters too. Wine described as expensive tastes more expensive. A meal eaten in silence tastes different from one eaten with music. The insula and the orbitofrontal cortex are not just receiving flavor signals — they're weighting them against memory, context, and belief.
Why the ingredient economy relies on this
Most of what a food company is selling, when it sells "flavor," is aroma. The little bottle labeled strawberry flavoring in a bakery aisle is a blend of volatile molecules — some derived from actual strawberries, most synthesized to mimic the compounds strawberries produce. On the tongue, strawberry flavoring registers as almost nothing. In the nose, delivered retronasally through a warm cupcake, it registers as summer.
This is also why spices matter so much in cooking with less of something else. A pinch of the right aromatic can carry an entire dish. The 75,000 crocus flowers that go into a single pound of saffron are worth it because saffron's aromatic footprint is so distinctive that a few threads recolor a whole pot of rice.
The volatile compounds are the argument. The tongue is the audience.
Try the trick, then try the reverse
The apple-onion demonstration is worth doing at least once, because reading about it and experiencing it are different things. The moment when you unpinch your nose halfway through chewing and the flavor floods in, arriving from what feels like the wrong direction, is one of the more disorienting sensations available in an ordinary kitchen.
The reverse works too. Take a food you know well — a strawberry, a piece of dark chocolate, a slice of ripe tomato. Chew it with your nose open. Then chew an identical piece with your nose pinched. Then release. What returns in that release isn't a memory of the food. It's the food itself, arriving late.
That returning wave is what a cold takes away for a week. It's what a bad sinus season steals for a month. It's what some people never get back after a virus, an injury, or the slow narrowing that comes with age.
Which is a quiet argument for paying attention to it while it's here. The next ripe peach of the summer is only a peach because a stream of molecules is making the trip from the back of the mouth up into the nose, unnoticed, on the exhale.
