Science

Why Do Onions Make You Cry? Ask Paul

The real chemistry behind the tear-inducing nature of chopping onions, and what you can do about it.
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Published Dec. 6, 2023.

It’s hard to cook without onions, which add their distinctive allium savor to many a dish. But it’s hard to cook with onions, too, since chopping the bulbs is a recipe for runny eyes. Let’s take a look at the chemistry.

Why Do Onions Make You Cry?

Like garlic, radishes, and kale, onions form a lot of their flavor only when they’re cut open. The cells of an intact onion abound with certain flavorless compounds called ACSOs (S-alkyl cysteine sulfoxides), and also, sequestered within special compartments, particular enzymes.

Slicing the vegetable breaks open some of its cells, allowing the ACSOs to encounter the enzymes and react. Immediately, the ACSOs are transformed into strongly flavored thiosulfinate compounds; some of these continue to react, forming additional flavor compounds, all of which gives an onion its richly savory complexity.

In evolutionary terms, these reactions form a defense mechanism against pests trying to eat the onion: The pungent sulfur-based compounds—which humans find delicious—drive away many other would-be onion-eating species. Similarly, many of the other vegetables that produce defensive compounds just wind up making themselves more delicious.

But onion has another trick up its papery sleeve. After the first enzymatic reaction happens, one of the flavor compounds produced gets transformed in turn by another enzyme, forming a new compound called lachrymatory factor, or LF. The word derives from Latin lacrimāre, to weep; LF is a volatile compound that blasts off into the air. Even very small amounts are enough to irritate our eyes and mucous membranes, making us cry.

How Can You Cut Onions Without Crying?

There are some techniques that result in less production of LF, which leads to less tearfulness. Using a sharp knife rather than a dull one is always advised, because it breaks open fewer cells, releasing fewer ACSOs to undergo the transformative reactions. 

Rinsing the cut surfaces of a sliced onion removes the precursor flavor compounds, and chilling an onion before cutting it slows down the enzymatic reactions that form LF.

But there’s a problem with those methods, because the creation of LF goes hand in hand with the creation of onion flavor. Techniques that produce less LF ultimately lead to a less tasty final product. It’s preferable to allow the onion to generate as much LF as it wants, but keep that stuff out of your sensitive eyes.

Swimming or safety goggles are a tried-and-true way to look silly in the kitchen while protecting your eyes—it really works. Or create a cross-breeze with an open window, a range hood, and/or a table fan to move the volatile molecules away from your work area as swiftly as possible. As soon as you’re done chopping, put the onion in a covered container to contain the fumes.

Ask Paul Adams, senior science research editor, about culinary ambiguities, terms of art, and useful distinctions: paul@americastestkitchen.com

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