Onions get their distinctive smell from sulfur-containing compounds that are released the moment you cut into the bulb. These compounds don’t exist in their final, pungent form inside an intact onion. Instead, they’re created through a rapid chemical reaction triggered by cell damage, which is why a whole onion sitting on your counter barely smells at all, but slicing one fills the entire kitchen.
The Chain Reaction Inside a Cut Onion
An intact onion stores sulfur-based molecules called cysteine sulfoxides in its cells, kept separate from an enzyme called alliinase. Think of it like a two-part epoxy: the components are harmless on their own but reactive when mixed. The instant a knife breaks through the cell walls, alliinase contacts the sulfoxides and cleaves them apart, producing a group of highly reactive molecules called sulfenic acids.
From there, the sulfenic acids take two paths. Some spontaneously combine with each other to form the thiosulfinates and disulfides responsible for onion’s sharp, lingering odor. These are volatile, meaning they easily become airborne and reach your nose within seconds. Others get grabbed by a second enzyme, lachrymatory factor synthase (LFS), which converts them into a specific compound called propanethial S-oxide. That’s the one that makes you cry. It drifts upward from the cutting board, dissolves into the moisture on your eyes, and triggers a stinging, tear-producing reflex.
The whole sequence, from knife strike to tears, takes only seconds. And because both the smell compounds and the tear-inducing gas are transient and reactive, they continue breaking down and recombining into new sulfur molecules, which is why the smell of a freshly cut onion shifts and mellows over time even before you cook it.
Why Onions Evolved This Way
This chemical defense system isn’t an accident. Onions and their relatives (garlic, leeks, chives) evolved sulfur-based weaponry to deter animals and insects from eating them. When a pest bites into the bulb, the same enzyme reaction fires, releasing irritating and foul-tasting compounds. Sulfur is central to this strategy because sulfur-containing molecules are inherently reactive and pungent at very low concentrations, making them effective deterrents without requiring large amounts of energy to produce. The plant essentially booby-traps its own cells so that destruction of the tissue is what activates the defense.
What Changes When You Cook Onions
Heat transforms onion chemistry dramatically. Cooking deactivates alliinase and LFS, shutting down the enzyme-driven reactions that produce the sharp, raw-onion smell. At the same time, heat breaks down the sulfur compounds that are already present into a different set of molecules, many of which have sweeter, more mellow aromas. This is why sautéed or caramelized onions smell rich and savory rather than sharp and eye-watering.
Cooking also increases the availability of certain amino acids and flavor-active nucleosides in onion tissue, contributing to the deeper, more complex taste of cooked onions. The longer and slower you cook them, the more complete this transformation becomes. A quick sear keeps some bite; a long, low caramelize converts nearly all the pungent sulfur compounds into gentler ones.
Why Some Onions Smell Stronger Than Others
Not all onions pack the same punch, and the difference comes down to how much of those precursor sulfur compounds the bulb accumulated while growing. Sulfur nutrition is the key variable. Research has shown a direct correlation between the amount of sulfate available in the soil and the final pungency of the bulb. Onions grown in sulfur-rich soil produce more cysteine sulfoxides, which means more raw material for the smell-producing reaction when cut.
That said, the relationship has a ceiling. A study testing onions across different soil types and sulfur fertilizer levels found that once the soil already contained sufficient sulfur, adding more didn’t significantly increase pungency. This helps explain why “sweet” onion varieties like Vidalias are traditionally grown in low-sulfur soils in specific regions. The genetics of the cultivar matter too, but the soil sets the upper limit on how much sulfur the plant can stockpile.
How Storage Affects Pungency
The smell-producing potential of an onion isn’t fixed after harvest. It shifts during storage. Research tracking onions stored at around 3°C (standard refrigeration temperature) over four months found that the amount of lachrymatory factor produced from crushed tissue changed significantly over time, and the pattern depended on the cultivar. One mild variety showed a steady linear increase in pungency over the storage period, while a more pungent variety initially decreased before climbing back up. So the onion sitting in your pantry for a few weeks may be more or less pungent than it was the day you bought it, depending on the variety and how long it’s been stored.
Because the tear-inducing compound is volatile and reactive, even the timing of measurement matters. In crushed onion tissue, the concentration of the lachrymatory factor peaks and then declines as it either evaporates or reacts with other byproducts. This is why letting cut onion sit for a few minutes before bringing it near your face can slightly reduce the sting.
The Same Compounds That Smell Also Have Health Effects
The sulfur molecules responsible for onion’s pungency are biologically active in ways that go beyond flavor. The same propanethial S-oxide that triggers tears has demonstrated antibacterial and antiplatelet properties in lab studies. Other sulfur compounds in onions, particularly disulfides, have shown the ability to inhibit both harmful bacteria types that commonly cause foodborne illness and selectively affect bacteria in the digestive tract.
On the cardiovascular side, certain onion sulfur compounds have been shown to reduce the secretion of a protein involved in lipoprotein production in human liver cells, which could lower cholesterol levels. Animal studies have confirmed that onion extracts can reduce blood lipid levels in rats with high cholesterol. Related compounds have also shown blood-sugar-lowering effects in diabetic animal models. These findings don’t mean eating onions is a medical treatment, but they do explain why allium vegetables consistently show up in dietary research as protective foods. The very chemistry that makes your eyes water is what makes these compounds biologically potent.