Why Do Onions Make You Cry?

There are not many foods that fight back. A carrot does not object to being chopped, and a potato goes quietly. The onion is different. You can hold one, sniff it and roll it across the table without a single tear, but slice into it and within seconds your eyes are stinging and streaming as if you had just watched the saddest film ever made. The onion has done nothing to deserve such a reputation until the knife arrives, and then it does rather a lot.

People have been weeping over onions for a very long time. They have been eaten, and cried over, for at least five thousand years. For most of that history nobody knew why. The tears were simply part of the price of dinner, like washing up. What makes the answer so pleasing is that it turned out to be a small, precise piece of chemistry, and one step of it stayed hidden until surprisingly recently.

Here is what happens. An onion keeps its ingredients in separate compartments. Some cells hold sulfur-containing compounds, and others hold an enzyme called alliinase, and in an uncut onion the two never meet. A knife tears the cells open and mixes them. Alliinase breaks the sulfur compounds down into substances called sulfenic acids, and a second enzyme then rearranges one of those into a compound with a magnificent name: syn-propanethial S-oxide. It is volatile, which means it drifts into the air, and the air above a chopping board is exactly where your face happens to be.

When that vapour reaches your eyes, it irritates the nerves in the surface of the cornea. Your body treats it as an attack and responds the only way it can, by flooding the eye with tears to wash the irritant away. So the crying is not sadness at all. It is a rinse cycle. Which raises a lovely question: if you could stop that second enzyme working, would you still get the flavour of onion without any of the weeping?

That is the question the scientists asked, and the story of how we know is a good one. The tear-making compound itself was identified in the 1970s, and for years it was assumed to form on its own, as a by-product of the reaction that also gives onions their flavour. In 2002, researchers in Japan showed in the journal Nature that this was wrong. The compound is made on purpose by its own enzyme, which they named lachrymatory-factor synthase, from the Latin word for tears.

In 2017 a team at Case Western Reserve University in the United States worked out that enzyme's shape by growing crystals of it, which revealed step by step how it builds the irritant. The work matters beyond the kitchen, because it shows the flavour and the tears come from different reactions. A group in New Zealand used this to grow a tearless onion in 2008 by switching off the gene for the enzyme. Until one reaches your kitchen, the simplest advice is to chill the onion before you slice it, which slows the release of the fumes and gives your eyes a gentler afternoon.