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Word of the Week: Retronasal

By Paul Adams Published

The back door to flavor.

Working at the intersection of food and science, we come across a LOT of scientific jargon and terminology, from chemical processes (see: nixtamalization), to physical phenomena, to species names (see: Aspergillus oryzae). We’ll be sharing many of these fun words with you in this ongoing series, Word of the Week. Get ready to impress at your next cocktail party.

When you hold a fresh-baked cookie up to your nose and inhale its rich, toasted aroma, that’s smelling—also known as olfaction. When you eat a bite of said cookie and relish its chocolatey, buttery flavor, that’s smelling too, of a slightly different type. As you chew, you will continue to breathe in and out, and when you breathe out, the odor of the cookie flows from your mouth directly into your nasal cavity, via a passage at the back of your mouth called the nasopharynx. Because of that backwards approach, this type of smelling is known as retronasal olfaction. Smelling what’s already in your mouth, as opposed to what’s around you, is a powerful, and less appreciated, way to smell.

Since retronasal smelling happens in sync with the taste buds’ tasting of food, the two sets of sensations, one from the nose and one from the mouth, are perceived by the brain as a single experience. The tastes sensed by the tongue—sweetness, saltiness, bitterness, sourness, and umami—merge with the more complex signals sent by the thousands of aroma compounds distinguishable by the brain, to create the composite phenomenon we know simply as flavor.

In his comprehensive book Neurogastronomy, Gordon Shepherd writes that, because retronasal smell works in tandem with taste, and because it’s actively stimulated by chewing, “it can truly be considered a separate type of smell.” Experiments have shown that the two types of smelling behave differently—a scent that’s familiar through the nostrils may not be recognizable at all when it comes in through the mouth. Even though it’s the same aroma molecule, the way the brain forms its impression of the scent depends completely on whether it comes in through the nose or, in the process of chewing, through the nasopharynx. In fact, there are particular regions of the brain that are only activated when you taste and retro-smell a food at the same time.

Retronasal smelling is essential to the perception of food—without it, we can’t really detect flavor. That’s why, if you close your eyes, hold your nose, and eat a jellybean, you can’t tell what flavor bean it is—just that it’s sweet. Once you release your nose and take a breath, the flavor is immediately apparent. (Try this test with jellybeans, if you haven’t. It’s cool.) That’s also why avid tasters of wine, coffee, soup, and so forth, slurp and swirl each sip, to maximize the amount of airflow—and thus aroma— that gets into the nasal passage. We do the same here at Cook’s Illustrated during tastings. While smacking your mouth when eating at a restaurant wins you no points, in the test kitchen it might just make you better at your job.  

And, to distinguish it from the retronasal kind, scientists call regular smelling through your nostrils orthonasal olfaction. Breathe in, breathe out.

Graphics by Sophie Greenspan.