Why does cutting an onion make you cry? It's not because of the lyrics "peeling off my heart layer by layer"...

Onions are a vegetable that we often see in our daily lives. Not only can they increase our appetite, but they are also good for human health. But eating them comes at a price: People who chop them may soon feel tears flowing from their burning eyes. But why does chopping onions make you cry?

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The culprit is known as a lachrymogen, a chemical that irritates the eyes. When an onion is intact, a group of compounds called cysteine ​​sulfoxides remain separate from an enzyme called alliinase. But when you slice, dice, or crush an onion, the barrier separating the compounds and the enzyme is broken. The two combine, triggering a reaction: Alliinase turns cysteine ​​sulfoxides into sulfinic acid.

"Sulfuric acid isn't very stable," so they have to transform into something else, said Josie Silvaroli, a doctoral candidate in pharmacy at Ohio State University and lead author of a 2017 study on the tear factor that appeared in the American Chemical Society's journal ACS Chemical Biology. In onions, the sulfurous acid has two options. One option is that it can spontaneously condense, a reaction on its own, into an organosulfur compound. Organosulfur compounds are the source of onions' strong odor and flavor. A similar reaction occurs in garlic, which is why it also has such a pungent flavor, Silvaroli said.

But sulfenic acid option two is unique to onions and several other species of the Allium family, or the flowering plants that produce vegetables like onions, garlic, scallions and shallots. Another enzyme called lachrymatory factor synthase, which has been hiding in the cell, kicks into action, rearranging the sulfenic acid into lachrymatory factor.

The tear factor is a volatile liquid, which means it turns into vapor very quickly. That's how it reaches your eyes and stimulates the sensory nerves. Both the organosulfur compounds that give onions their rich flavor and the tear factor likely evolved as defense mechanisms. They were designed to deter insects, animals, or parasites that might damage the onion plant.

Image source: Pixabay

Tips to avoid tears

1. Microwave heating. Before cutting the onion, you can heat it slightly in the microwave.

2. Cut onions in running water. Cut onions in running water so that some of the water-soluble sulfur compounds can be washed away, so you won't cry.

3. Put the onions in the refrigerator to cool. Put the onions in the refrigerator before cutting them to reduce the pungent smell.

4. Plug your nostrils with something soft. Before cutting onions, you can gently plug your nostrils with cotton balls, and you won’t cry when you cut onions.

If burning tears are more than you can handle, you do have options. I recommend wearing goggles or a face shield to protect your eyes. (Contact lenses also provide a barrier against tear gas.) Using a sharp knife may also help, which damages fewer cells and produces less "tear gas" than a dull knife.

There are also efforts to create tear-free (or at least tear-less) onions. Cultivators describe them as "sweet and mild," developed using hybridization techniques to reduce the irritation. But Silvaroli is cautious. Because the compounds that cause the tear factor are the same organosulfur compounds that give onions their distinctive flavor, reducing the tear factor would also make the onions taste less spicy. For now, if you want that flavor, you may have to endure some burns.

This article is compiled from Live Science and Science Popularization China

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