Would you ever consider throwing sulfuric acid into your face? Hopefully, the answer is no. Yet whenever you chop an onion, what happens isn’t that different from getting acid in your eyes.
Onions, tasty and delicious though they might be, are the bane of many a cook’s existence, since it’s nearly impossible to cut into one without inducing a veil of tears. The bad news is that cutting onions causes an unavoidable chemical reaction that burns your eyes; the good news is that there are ways to make those tears a thing of the past.
Why We Cry
Onions grow underground, where they absorb sulfur from the surrounding soil. The sulfur turns into a substance called sulfoxide, and when you slice into an onion, cutting through membranes and cell walls, those sulfoxides mix with another amino acid, called lachrymatory-factor synthase enzyme. When these two substances combine, the result is sulfenic acid, a highly unstable compound.
When the sulfenic acid hits the air and mixes with oxygen, it changes again, this time into a compound called syn-propanethial-S-oxide, made up of sulfuric acid, sulfur dioxide, and hydrogen sulfide—not exactly chemicals you’d want around your eyes. The gas wafts upward toward your face, and when it hits your eyes, your corneas instinctively recognize it as a caustic, harmful substance. They instruct the lachrymal glands (located behind your eyes) to produce tears to wash the gas away. The tears caused by onions aren’t emotional tears; they’re reflex tears, which the body produces in response to an irritant. They protect the eyes by washing away dust, dirt, smoke, and other debris. Most people instinctively reach up to wash away the waterworks, but think twice when you’re handling onions—your hands will probably be covered with the same chemical that’s irritating your eyes.
Luckily, syn-propanethial-S-oxide is both water-soluble and sensitive to heat, so washing the compound with water or heating it up will render it inert. That’s why washed and cooked onions don’t have the same uncomfortable effects as raw onions do.
If You Can’t Stand It, Beat It
If you just can’t imagine life without French onion soup, fret not—there are several ways to mitigate onions’ tear-inducing properties.
1. Wear goggles. They protect your eyes from the harmful gas, and they’re easy to find at most kitchen-supply stores. However, they’re bulky and expensive, not to mention that they make it hard to see what you’re doing. Some people feel that wearing contact lenses is enough to protect corneas from irritation.
2. Freeze it. Put the onion in the freezer for ten to fifteen minutes before you chop it. The offending compounds and the water in the vegetable will freeze a bit, and won’t mix as readily when cut. Just make sure not to freeze the onion entirely, or it will end up mushy.
3. Light a candle. Some people believe that if you light a candle near your work surface or turn on a burner on a gas stove, the flames will burn up some of the sulfurous gas as it’s produced. Flames also create convection, which pulls the fumes closer and incinerates them.
