Why Do Onions Make You Cry?

Onions make you cry because cutting them sets off a chain of enzyme reactions that releases a volatile sulfur gas, syn-propanethial-S-oxide, which drifts up and triggers pain-sensing nerves in your eyes. Your eyes respond the only way they know how: by flooding themselves with tears to wash the irritant away. A whole onion doesn't do this. It's a defence system that only switches on when cells are broken.

What's the chemical that makes you cry?

An onion's tear gas doesn't exist until you cut it. The ingredients are stored separately inside each cell, like a two-part glue waiting to be mixed.

• In the main body of the cell sit sulfur-containing compounds, mainly one called isoalliin.
• Locked away in a separate compartment, the vacuole, is an enzyme called alliinase.

When your knife slices through the cell, the walls rupture and the two meet. Alliinase snips isoalliin apart, producing an unstable molecule called 1-propenylsulfenic acid. A second enzyme, lachrymatory factor synthase, then rearranges that molecule into syn-propanethial-S-oxide, the actual tear-maker. That second enzyme was only discovered in 2002, which surprised researchers who'd assumed for decades that the tear gas formed on its own.

The whole sequence happens fast. Gas production peaks within about 30 seconds of cutting and then tapers off, which matches the experience of tears building quickly once you start dicing.

The same reactions also produce onion's raw flavour. The sulfenic acids that don't become tear gas join up to form thiosulfinates and related compounds, which give raw onion its sharp, pungent bite. So the tears and the bite share a common origin.

Why does onion gas sting your eyes?

Syn-propanethial-S-oxide is volatile, so it floats up from the cutting board straight to your face. When it lands on the surface of your eyes, it activates sensory nerve endings in the cornea, mainly through a receptor called TRPA1, which responds to irritating chemicals. (The same receptor fires for wasabi, mustard and tear gas sprays.) The nerves send an "irritant!" signal to your brain, which triggers the tear glands to flush the eye.

You may have read that the gas turns into sulfuric acid in your eyes. That's an oversimplification. The key event is the gas setting off pain-sensing nerves directly, and your reflex tears are a protective response to that signal.

The whole system is almost certainly a defence. An animal that bites into an onion bulb gets a mouthful of harsh sulfur compounds and an eyeful of irritant gas. Most creatures learn the lesson. Humans just learned to cook.

Chef Fizz says: An onion is a tiny chemistry kit that only opens when you cut it. Respect the kit and it'll go easy on you.

Why don't garlic or cooked onions make you cry?

Both comparisons reveal how specific the chemistry is.

Garlic uses a very similar system. Its cells store a cousin of isoalliin called alliin, and crushing garlic lets alliinase loose on it. But garlic makes very little lachrymatory factor synthase, so instead of tear gas it produces allicin, the compound behind fresh garlic's punch. Garlic can sting your nose and tongue, but it won't make you weep. For more on garlic's version of the reaction, see why crushed garlic tastes stronger.

Cooked onions have lost their enzymes. Alliinase and lachrymatory factor synthase are proteins, and heat unfolds and inactivates them, so once an onion is sizzling in a pan it stops making new tear gas. Any gas already released at the board mostly evaporates or breaks down. Heat also transforms the remaining sulfur compounds into gentler ones, while the onion's natural sugars come forward and start to brown. That's the story in why onions get sweet when you cook them.

Which tricks actually stop onion tears?

Every cook has a favourite remedy. Some rest on solid chemistry; others are folklore. Here's how they stack up.

Why does a sharp knife help so much?

A dull blade doesn't slice through onion cells; it crushes and tears them, bursting many more cells on either side of the cut. More burst cells means more alliinase meeting isoalliin, and more gas. A keen edge passes cleanly between layers and leaves far more cells intact. It's also much safer, because a sharp blade goes where you aim it instead of skidding off the onion's slick, curved surface; there's more on that in why a sharp knife is safer.

How cold should the onion be?

About 30 minutes in the fridge, or 10 to 15 minutes in the freezer, makes a clear difference. Cold slows enzyme reactions and lowers how readily the gas evaporates. Don't leave onions in the freezer longer than that before cutting, though: ice crystals rupture cell walls and the onion turns soft and watery.

Why are some onions worse than others?

Onions vary a lot in pungency, and the main factor is how much sulfur the plant has stored. That depends on the variety and on the soil and weather it grew in.

• Yellow storage onions are bred to keep for months and are usually the most pungent, with the strongest bite and the most tears.
• Red onions are often a bit milder raw, though they vary.
• Sweet onions such as Vidalia, Walla Walla and Maui are grown in low-sulfur soils and tend to have higher water content and fewer sulfur compounds, so they bite less and make you cry less.
• Tearless onions bred to produce very little tear gas have appeared in some markets in North America and Japan. They're made by conventional breeding and selection, and they taste much milder raw.

Labs measure onion pungency by testing for pyruvic acid, a by-product of the same alliinase reaction that leads to tear gas. Sweet onions score low; sharp storage onions score high.

If you love raw onion but find it too harsh, soak sliced onion in ice water for 10 to 15 minutes, then drain. Many of the sharp sulfur compounds wash away, leaving crunchy, milder slices for salads and burgers. A quick soak in vinegar with a pinch of salt and sugar mellows them even more and turns them into fast pickles.

Want to practise the chemistry and the knife work? The Cooking Scientist app has short lessons on why cooked onions turn sweet and how to keep your knife sharp, each with a mini game.

Try it at home

The cold-onion challenge needs two similar yellow onions, a sharp knife and a timer.

1. Put one onion in the fridge for 30 minutes. Leave the other on the counter.
2. Peel and halve the room-temperature onion. Start a timer and dice it at your normal pace, noting how many seconds pass before your eyes begin to sting.
3. Step away, rinse your eyes with cool water if needed, and let them settle for 10 minutes. Air out the kitchen or turn on the fan, then turn it off again.
4. Repeat with the chilled onion, using the same knife and the same cutting style.

Most people notice the chilled onion takes noticeably longer to sting and causes milder tears. You've slowed two enzymes and reduced how fast the gas evaporates, just by using your fridge.

For a second round, dice half an onion with a dull butter knife and half with your sharpest knife. The crushed side releases a much stronger smell and stings sooner, because every crushed cell is another little gas factory. Take care with the dull knife; go slowly and keep your fingers well clear.

Common mistakes

• Using a dull knife. It crushes cells instead of slicing them, releasing more gas and making slips more likely. Hone your knife before you start and sharpen it regularly.
• Leaning in over the board. The gas rises straight up into your face. Stand tall, keep your head back and let a fan move the air.
• Hacking at the onion with lots of extra cuts. Every unnecessary cut breaks more cells. Learn a clean dicing method with the root end left on to hold the layers together.
• Rubbing your eyes mid-chop. Onion juice on your fingers makes the sting worse. Rinse your hands, then rinse your eyes with cool water if needed.
• Freezing onions for hours before cutting. A long freeze bursts the cells, so the onion turns soft and watery. Stick to 30 minutes in the fridge or 10 to 15 in the freezer.
• Cutting onions far ahead and leaving them uncovered. Cut onion keeps releasing sulfur compounds into the air and the fridge. Store it in a sealed container and use it within a few days.