This Is Why Onions Make You Cry (It's All in the Chemistry)

At least once every meal prep session, I find myself standing in front of the refrigerator with the freezer door open and my head stuck as far in as I can get it. It's the only "home remedy" for onion tears that's ever really worked for me, even though it doesn't keep the reaction from happening, only makes it less painful.

You know the pain, but why do onions make you cry? And is there anything you can do to keep that reaction from happening? Let's look at the science behind the onion and a few things you can do to keep the tears at bay.

why do onions make you cry

An onion is a class of plant called an alum. Other alums are shallots, garlic, chives, scallions, and leeks. Alums grow underground and get their sharp bite from sulfur in the soil, specifically from chemicals known as amino acid sulfoxides. Onions that grow in soil with less sulfur don't have as strong a bite, which is why Vidalia onions (grown in Georgia) have that sweeter taste. Basically, like wine and coffee, onions have terroir, or the characteristic taste that comes from the environment in which a food is produced.

We're not the only ones who find alums tasty. Underground critters do as well, and so onions developed a defense mechanism to keep from being attacked. And now we're going to have to get into a little bit of chemistry.

Why do onions make our eyes water?

why do onions make you cry

Onions produce an enzyme that, when mixed with the the sulfur compounds in the onion, creates sulfenic acid. When the cell walls break down -- basically, when you cut into an onion -- you're creating a chemical reaction between the onion enzymes and sulfoxides. The sulfenic acid then turns into thianosulfinate, the chemical that creates the distinctive odor of an onion, and syn-propanethial-S-oxide, a gas that causes the stinging pain which makes our eyes water.

The gas produces this intense reaction in our eyes, and that is the goal of the onion who just wants to be an onion in the ground and not the basis of Wednesday dinner. This defense reaction is short-lived, though. It only takes about five minutes for the chemical process to happen and all the enzymes to be used up, though the gas can hang around for a bit longer than that.

Fortunately, the reaction doesn't cause any lasting harm to your eyes. If you can find a way to handle the temporary pain while cutting onions (thus my head in the freezer trick), you'll be fine.

How to beat the onion tears

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Everyone has a way to deal with this reaction. Some people swear by holding a piece of bread in their mouth while others deal with the problem by simply buying the pre-cut stuff in the grocery store. Professional chefs use a sharp knife and cut quickly, since the cleaner the cuts are, the less reaction there is, meaning a smaller gas cloud and fewer tears.

You can lessen the amount of gas, but the techniques for doing so, like soaking an onion in water, also take away some of the flavor and texture of the onion. You can freeze an onion; the cold makes the gas produced less volatile, but it also makes the onion harder to cut safely.

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What works is moving the gas cloud away from your face. If you cut onions in front of a fan or near an exhaust vent, you can move the gas cloud away from your eyes. Cutting the onion under running water is the same idea (breaking up the fumes), but you should only try this tip if you've got some solid knife skills and a level surface under that running water on which to chop the onion.

Farmers and researchers are working on tearless onions. Growers in Nevada and Washington are now offering Sunions, which claim to be mild and crunchy onions that produce no tears when you chop them. And other researchers in New Zealand are using gene suppression to create an onion that produces no tears.

Whether or not these new onions will keep you from crying in the kitchen remain to be seen, so until then, remember to keep your kitchen ventilated when chopping (or just indulge in a good cry from time to time).
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Watch: How to Chop an Onion Like a Pro

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