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Why do you cry when you cut onions?

While chopping vegetables for dinner, the time comes to slice into an onion. As soon as you get a few cuts in, you become overwhelmed with gratitude and sadness at the onion’s sacrifice to flavor your meal. Your eyes begin to well with tears. Suddenly, you're crying.

No, that’s not what really happens, but it makes for a good story.

The real reason that chopping onions brings tears to your eyes is a chemical combination involving enzymes and the onion’s ability to absorb sulfur. Here’s the truth behind your teary-eyed experiences with onions.

Making enzymes

Person's hand holding sharp knife, slicing red onions on wooden cutting board
Credit: Africa Studio/ Shutterstock

During the growing process, most plants use sulfur as a key nutrient. Onions use a bit more sulfur than most, and they’re especially great at absorbing large amounts of it. Onions use sulfur to make a mixture of amino acids and enzymes. The acids and enzymes are separated and stored in different regions of the onion’s cells, which are called vacuoles.

Breaking the cells

Overhead view of chopped white onions on a rustic wooden cutting board
Credit: MaraZe/ Shutterstock

While the onion remains whole, the amino acids and enzymes in the onion’s cells remain separated. Once you cut into the onion, everything mixes together and that’s when the fun begins.

When you slice open an onion, you’re breaking open thousands of cells that contain sulfuric chemical components. When the two substances are combined, they form a chemical known as syn-Propanethial-S-oxide, or lachrymatory factor (LF). LF is an irritant that’s easily vaporized when it reacts with the air.

Crying shame

LF isn’t strong enough to affect tougher parts of your body such as your skin, but it can irritate more sensitive regions. As the vapors waft up towards your eyes, your eyes will begin to sting. Your body — sensing the irritant — will release a torrent of tears in an attempt to wash the chemicals from your eyes. This results in the embarrassing tear stains that you’ll have to wear to the dinner table. Luckily, LF can’t do any serious damage, even in high quantities. It just makes you tear up.

Defense mechanism

For plants, life is all about procreation. Many plants grow delicious berries and fruits that are tasty for animals. When animals eat the fruit, the seeds are transported elsewhere where they grow into a new plant.

Onions, however, are root vegetables. They reproduce by growing seeds at the top that scatter in the breeze upon maturity. If the onion is eaten before that can happen, then it doesn't reproduce.

Producing LF is the onion’s way of defending against anything that may want to eat it. As soon as an animal bites into the root, its eyes start to burn and it’s reminded to stay away from onions. Unfortunately for onions, humans are rather persistent.

Other plants that produce LF

Several large bulbs of garlic placed atop a wooden cutting board
Credit: al1962/ Shutterstock

Of course, onions are the most famous producers of LF, but they’re not alone. Onions are members of the allium family of plants, which also includes:

  • Shallots
  • Scallions
  • Garlic
  • Leeks
  • Chives

Most alliums produce LF in some quantity, but they won't have you inexplicably crying as much as onions do.

Stop the tears

Up close view of person's hand using sharp knife to cut red onion
Credit: Sergey Skleznev/ Shutterstock

While it’s impossible to stop the onion from producing the tear-inducing chemicals, there are steps that you can take to make food preparation more bearable:

  • Use a sharp knife. The sharper the knife, the less damage to the onion’s cells and less LF released into the air.
  • Place cut pieces face down. Once you cut an onion in half, put the pieces face down. That way, the LF will be somewhat contained by the cutting board.
  • Cool the onions. If the onion is cold, LF won’t evaporate as easily. You’ll still feel some irritation, but it won’t be as bad. You can refrigerate onions, but never freeze them. They’ll be mushy when they thaw.
  • Use a fan or vent. Clear away or scatter the vapors before they reach your eyes.
  • Hold a piece of bread in your mouth. You might look ridiculous, but the bread will absorb the vapors before they reach your eyes.
  • Wear goggles. Your family might still make fun of you, but at least you’re not crying or holding a piece of bread in your mouth.

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