Ask Dr Bec: Why do onions make you cry when you cut them?

Narrabri-based scientist Dr Bec Thistlethwaite contributes a column to The Courier on the science in our daily lives.
Readers are invited to send their science questions in to Dr Bec via [email protected]

Onions are sneaky little vegetables.

The moment you cut into one, you break open thousands of tiny cells packed inside its layers.

Each cell is like a miniature chemistry lab, storing different ingredients separately.

When those barriers are sliced open, the ingredients mix for the first time.

Enzymes from one part of the cell react with sulfur-containing compounds from another, and together they form a new chemical: a gas with the tongue-twisting name syn-Propanethial-S-oxide.
This gas floats up from the cutting board and heads straight for your eyes.

Your eyes, of course, don’t appreciate being gassed.

Their built-in defense system kicks in immediately, producing tears to flush the irritant away.

That is why, after a few chops, you start blinking, tearing up, and maybe even sniffling.

The more onion you chop, the more of that gas is released, and the more your eyes stream.

Not all onions are created equal.

Pungent brown onions tend to release more of the tear-inducing chemical, while red, white, or spring onions often make you cry less.

There are even ‘tearless’ onion varieties bred by scientists to produce fewer of these compounds.

And if you chill an onion before chopping, the cold slows the chemical reaction, meaning less gas escapes into the air.

Cooking works too.

Once heat destroys those enzymes, the crying stops, which is why caramelised onions taste sweet and never sting your eyes.

So the next time your eyes water in the kitchen, don’t blame your emotions … blame chemistry.

Every onion you chop is running its own tiny experiment, proving that science doesn’t just live in labs or classrooms.

Sometimes it happens right on your chopping board, with a side of tears for extra flavour.