Science Matters: No use crying over cut onions

A bit extreme: A snorkel and mask might help with avoiding the tears when cutting onions, but is not very practical.
A bit extreme: A snorkel and mask might help with avoiding the tears when cutting onions, but is not very practical.

The other day my husband walked in to the kitchen and found me in tears.

What had happened? I’d been fired? A terrible accident? Actually it was much worse that any of that. I was just chopping an onion.

I love onions – they’re delicious, and they also have pretty high levels of antioxidants, so they’re good for us too. But I really hate chopping them.

When we chop an onion we damage all the tiny cells that the onion is made of. And when we damage cells, we release their contents.

In onions, this results in the release of certain enzymes, which then, through a chain of reactions, produce a tear-inducing chemical called syn-propanethial S-oxide. This compound is volatile, so it easily turns into a gas and travels through the air – and into our eyes.

While dressing up in a full hazmat suit might be one way to prevent onion tears, scientists have taken a different approach.

When we get syn-propanethial S-oxide it is recognised as a threat by our nervous system, so we produce tears to try and wash it out. It also reacts with the moisture and produces an acid, causing stinging.

It’s thought that this might be a defence mechanism for the onion – to deter animals from eating them. And this is why I hate chopping onions.

There are a whole bunch of theories floating around about how you can prevent onions from making you cry. Some of these are downright ridiculous, like holding a piece of bread in your mouth. But some have a slightly more scientific basis. Chopping your onions under the rangehood can help, as the rangehood sucks up gas.

Getting your onion nice and cold can also be a winner. The idea is that freezing your onion will reduce the chemical reaction in the onion as you cut it.

Or if you don’t mind looking a bit silly, you can buy special goggles for chopping onions, which have a foam seal around the edges to prevent the gas reaching your eyes.

While dressing up in a full hazmat suit might be one way to prevent onion tears, scientists have taken a different approach.

We’ve now been able to create “tearless” onions by using genetic engineering. The technique switches off one of the onion genes that produces an enzyme involved in making syn-propanethial S-oxide. No gene equals no gas.

Not everyone is on board with the idea of genetically modified food yet, but another group of scientists have produced tearless onions, called “sunions” using more traditional selective breeding techniques.

Although tearless onions are not available in Australia yet, through the power of science, I might be able to enjoy a tear-free onion experience in the future.

Read more Science Matters: