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Word of the Week: Volatile

By Paul Adams Published

What’s really going on when your kitchen smells delicious.

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.

The broth’s been simmering ever-so-gently for hours, and the whole house is filled with a delicious smell. That’s delightful, sure, but all that aroma you’re smelling is flavorful molecules that are no longer in the soup because they have evaporated and floated into your nose.

It’s a tradeoff. Keep the pot tightly lidded—or use a pressure cooker—and the house won’t smell like much of anything, but the broth will hang on to all those flavors, and they’ll rise from the bowl when it’s served. If you compare side-by-side a broth that was cooked in a covered pot versus one that was open to the air, you’ll notice significantly more delicate aroma in the covered one.

In order for us to smell anything, molecules have to evaporate from that thing, waft through the air, and physically enter our nose or mouth. And for that to happen, the molecules have to be small and light enough to take flight and make that journey. They have to be volatile.

Sniff a clean, empty metal saucepan, or a plate, or a bowl of pure white sugar, and you won’t smell much. Sucrose, a.k.a. table sugar, is too large a molecule to evaporate. It’s nonvolatile. (Though, if you sniff the sugar that’s in your sugar canister right now, it probably will have a faint smell: even at room temperature, sugar very slowly caramelizes and produces some small volatile molecules.)

The substances that give foods their smell are small, light molecules: the diacetyl that characterizes butter, the 2-acetyl-1-pyrroline of jasmine flowers and jasmine rice, the cis-3-hexenol that’s a component of fresh green vegetables, and hundreds of thousands more. Vinegar has a smell because its main ingredient, acetic acid, is volatile; other common food acids like citric, malic, and tartaric acid are nonvolatile and odorless.

The more volatile a substance is, the more readily it escapes from its surroundings by evaporating. Though it’s not a hard-and-fast rule, the smaller and lighter a molecule is, the more volatile it tends to be. And the higher the concentration of volatile molecules in the air you’re sniffing, the stronger a smell is.

Though volatiles evaporate at room temperature, they do it more and faster at higher temperatures. That’s why warm food is more flavorful than cold food: Quite a bit more of its flavor is floating out of it (and into your retronasal passage) rather than staying put.

You can separate volatile from nonvolatile molecules by encouraging the volatile ones to evaporate: most commonly, by heating them. Bring your pot of broth to a rolling boil, and more and more of the delicate fragrance molecules evaporate and fill the air. The neighbors are envious. But no matter how long and hard you boil it, when all the water is gone, everything nonvolatile is still going to be in the pot. The sticky brown sludge left behind contains the salt, the sugar, the fats, the proteins, the starches, and only those volatiles that remain dissolved in fat or enmeshed in protein gels. Everything else has flown away.

Graphics by Jay Layman.