An onion's sharp, pungent bite isn't an accident. It's a sophisticated chemical defense system, evolved to ward off pests and predators. The components for this defense are stored harmlessly inside the onion's cells, kept carefully separated like a two-part epoxy. Only when the cell is damaged do the ingredients mix, triggering a rapid chemical cascade.

The main players are a family of sulfur-containing molecules called S-alk(en)yl-L-cysteine sulfoxides (ACSOs). In an onion, the most important ACSOs are methiin, isoalliin, and especially propiin. These are flavorless, odorless precursors, patiently waiting in the cell's cytoplasm. Their specific ratio determines the onion's potential for pungency and flavor before any heat is ever applied.

The trigger for this chemical reaction is a powerful enzyme called alliinase (pronounced al-ee-in-ase). Its sole job is to break down the ACSOs. To prevent a premature reaction, the onion sequesters alliinase inside a membrane-bound sac within the cell called the vacuole. This elegant compartmentalization keeps the system stable until the moment of injury.

When you slice, dice, or crush an onion, your knife blade ruptures thousands of these cells. The cell walls break, and the vacuole membranes tear. Suddenly, the alliinase floods into the cytoplasm and mixes with the patiently waiting ACSOs. The reaction is immediate.

Alliinase rapidly converts the ACSOs into a new class of highly unstable molecules called sulfenic acids. In onions, the most abundant precursor, propiin, is converted into 1-propenyl sulfenic acid. These sulfenic acids are the true source of the onion's initial sharp, "hot" flavor. They are also the branching point for all the other volatile compounds that follow, including the one that makes you cry.

While most of the newly formed 1-propenyl sulfenic acid contributes to flavor, some of it is immediately hijacked by another enzyme, Lachrymatory Factor Synthase (LFS). This enzyme was only identified in 2002 and performs a very specific task: it rearranges the sulfenic acid into a volatile sulfur compound called syn-propanethial S-oxide.

This compound, known as the lachrymatory factor, is light and volatile enough to float up from the cutting board and into your eyes. When it dissolves in the moisture of your eyes (your tears), it forms a trace amount of sulfuric acid. This acid irritates the nerve endings in your cornea, causing the stinging sensation and triggering your tear ducts to flush the irritant out. It’s a brilliant, if annoying, defense mechanism.

This initial chemical state—a flood of reactive sulfenic acids and the tear-inducing lachrymatory factor—is the baseline for a raw onion. It's sharp, aggressive, and pungent. From this point on, any cooking process is a race between the natural breakdown of these compounds and the new reactions introduced by heat.

Now that we understand the onion's initial chemical potential, we can explore how applying heat transforms these sharp, volatile compounds into deep, sweet, and savory flavors.