Science Explains Why Some Onions Make You Cry More Than Others

I recently learned that “purple” garlic really does taste different to its white counterpart.

It’s a “hardneck” variety, meaning it’s got a wooden stem in its centre: this makes the cloves larger, sweeter, milder, and more evenly-sized.

Which left me wondering ― is there an equivalent rationale behind why some onions are more intense than others?

Anyone who regularly cooks will know that picking your alliums often feels like Russian roulette. The eye-wateringly sharp kinds seem to come out of nowhere, and often appear in the same pack as their less offensive peers.

But why does it happen, and can you tell which onions will make your eyes water before you slice into them?

To understand why some onions are more likely to get you to “cry,” we need to look at what causes the reaction.

Per Britannica, onions are excellent at absorbing sulphur as they grow. This is used for amino acids in the bulb.

But when you slice into an onion, its cell walls break down. That allows enzymes to meet sulphur-rich amino acids, creating a chemical known as syn-propanethial-S-oxide (some research suggests two enzymes are needed for its release).

This is unstable and volatile, meaning it easily evaporates from the plant and gets into your eyes. Tears are a means of washing the offending chemical away.

This is an effective way of repelling hungry mammals, Bon Appetit writes ― whether hungry rabbits or me, making a spag bol.

Spring onions and sweeter onions absorb less sulphur, so they’re less liable to elicit tears. But yellow, red, and white onions are all potential tear-jerkers.

The strength of the response is linked to the concentration of sulphur-rich amino acids in the onion, which may change according to the soil the veg is grown in.

Writing for The Conversation, dietitian Dr Duane Mellor of Coventry University wrote: “It’s also possible that no two onions from the same bag will have the same effect, so cutting into the vegetable may be the only way to know if it will make you cry.”

Avoid cutting the root of the onion, as this has the highest concentration of sulphur.

“The sharper the knife, the less damage is done to the onion, and fewer irritations are released into the air as a result,” Healthline adds.

Chilling your onions in a bowl of cold water 30 minutes before slicing them may inhibit the enzymes, they say, producing less irritating gas.