Why do onions make you cry?

Minda Daughtry, Extension Agency, Agriculture – Horticulture, North Carolina State University
Need a handkerchief? Num LP Photo/Shutterstock

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

Why do onions make you cry? – Dana L., age 12, Belmont, Massachusetts

Onions are grown and used all over the world, and anyone who has cut into one knows that it can make you cry. This happens because onions release an irritating chemical that makes your eyes sting.

Onions are mostly water, plus some vitamins and sugar compounds. They also contain compounds that include sulfur, a natural chemical found in many smelly substances, such as skunk spray and garlic. This is one way that plants defend themselves – producing substances that repel creatures who might eat them. Other plants have thorns or stinging leaves, or are made of special cells that make them hard to chew.

One sulfur compound in onions, called propyl sulfoxide, escapes into the air when you slice an onion. When it comes into contact with moisture, such as water vapor in the air or the natural moisture around your eyes, it changes into sulfuric acid. Sulfuric acid has a strong smell and irritates your eyes, so they make tears to wash it away.

There are some tricks to avoid this “emotional” onion experience. Next time you’re getting ready to dice an onion, start by cutting off and throwing away a little bit of the root end, which has lots of stringy little roots hanging from it. This lets most of the noxious sulfuric compounds, which are found in the root, escape. Then you can remove the pointy tip of the onion, peel its skin and slice it with fewer tears.

Some cooks chill onions for 30 minutes before they cut them, which helps because the sulfur compounds don’t escape into the air as easily when they’re cold.

Onions add flavor to lots of our favorite foods, from spaghetti sauce to tuna salad, so don’t let the smell drive you away. And gardeners love to grow ornamental alliums – members of the onion family that are bred for their looks. Many are very attractive, with blooms that make balls of color on long straight stalks. And their onion-y smell helps fend off rabbits, deer and other animals looking for a tasty garden meal.




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Minda Daughtry is affiliated with Harvesting Hope NC., a not-for-profit community garden in Smithfield, NC.