Why onions make you cry

When it comes to cooking, onions often get a pretty raw deal, and most cooks complain about the tears and stinging that comes with chopping onions. But why do they make you cry when you cut them?

Onions contain a set of molecules called syn-propanethial-S-oxide molecules which are released when you cut into them. Because the molecules are so tiny and light, the chemical, a known irritant, then reacts with the air as it wafts up to your eyes. This then reacts with the water on your eyes and your lachrymal glands to create a burning sulphuric acid, making them release tears. Because of their unique sulphur compounds, onions are the only vegetable to make your eyes water.

Onions are actually pretty clever; they’ve evolved their own chemical defence system to warn off hungry critters on the hunt for food when they’re growing in the ground. Each onion cell is filled with enzymes that, when cut, break open, mix with other chemicals in the cell and release their contents.

But not all onions are created equal – the yellow onions that we use in lots of different dishes are the most likely to trigger a teary response, whilst sweeter onions are kinder on the eyes. And like every onion is slightly different so are we – what makes you well up might not plague your partner in the same way and vice versa. This is down to the individual chemistry of your eyes and your sensitivity to sulfuric acid.

We’ve all heard an old wives’ tale about how to stop onions making you cry, like sticking a wooden spoon in your mouth as you chop, but there are actually a few more scientific ways to tackle the tears.

Freezing or refrigerating your onions before you chop them changes the chemistry inside the onion and causes a slower release of the enzymes. The same goes if you cut your onion under water. Goggles also work but aren’t necessarily a good look for the kitchen... are they Nic?!!