We probably wouldn't go through the trouble of cutting onions up if they didn't make our dishes so tasty. Unfortunately, we have to sacrifice a bit of our eyesight for a couple of minutes while we prep onions for a meal. But why does slicing onions make us cry anyway?
The fact that slicing onions will make us cry is such a common piece of household knowledge that we hardly even question it anymore. It's like knowing that the fire is hot and water is wet. It's just something that happens. Of course, like most other things that occur in the universe, however big or small, there's an explanation for why slicing onions makes us cry.
You probably know that an onion releases a gas of some sort as a defense mechanism. What exactly does an onion release, where does it come from, and why does it affect us the way it does?
Onions grow underground, where a few hungry critters also live. So imagine that you're an onion. You're newly ripe, you're quite round and plump, and best of all, you're just there. You have no spikes or spines that can protect you. You're quite defenseless against critters with sharp teeth. Thus, you have your own nature-patented onion spray that keeps hungry critters away.
As the onion grows, it interacts with the sulfur in the dirt. This interaction results in the production of amino acid sulfoxides, which can turn into gas. When an animal bites into an onion and the onion's flesh breaks apart, it releases the amino acid sulfoxides as well as the onion's enzymes. When these compounds react, they produce sulfenic acid. When sulfenic acid reacts with onion enzymes, they produce the gas syn-propanethial-S-oxide.
It's this gas that keeps animals away from the onion and makes humans cry when cutting onions up. Though the process to produce the gas sounds simple, it's actually quite complicated.
The good news is that the gas isn't actually harmful to humans. It may sting your eyes for quite a while, but it won't cause damage. Different people also have different levels of tolerance towards the gas. Some lucky people won't feel any effects at all, while others will tear up and even get a runny nose as well.
Not all onions are equal, at least in terms of the syn-propanethial-S-oxide gas they excrete. White, yellow, and red onions have more of the enzymes necessary to produce syn-propanethial-S-oxide. Green onions, sweet onions, and scallions don't have as much of the same enzymes.
Fortunately, tearing up when you're slicing onions isn't inescapable. There are some things you can do to protect your eyes from an onion's gas. If you're particularly sensitive to the gas and it makes you very uncomfortable, you can wear protective goggles while slicing onions. You can also use eye drops to dilute the effects of the gas. However, doing these things may seem like too much of a hassle for such a small chore. Thus, you can try cutting onions up in a bowl of water and see if that works.
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