Slice into an onion and, within a few seconds your eyes begin to feel the burn. Why do onions make you cry.
Unless you’ve avoided cooking, you’ve probably cut up an onion and experienced the burning and tearing you get from the vapors.
When you cut an onion, you break cells, releasing their contents. Pearly white onions are chock-full of vitamins C, B1 and B6, along with a healthy dose of potassium, phosphorus and fiber.
Though onions aren’t universally appealing to the human palate, they have the potential to lower bad cholesterol, relieve hypertension and minimize blood clotting.
On the flip side, onions exact a brief uncomfortable physiological toll on anyone who dare to slice through their white, yellow or red-tinted skins.
Note that the tears you shed when chopping onions aren’t emotional ones. There three categories of tears but we will discuss two only: Basal and Reflexive.
Since basal tears are the ones that hang around our eyes and eyelids to act as a lubricant, that leaves us with reflex tears.
The lachrymal glands above the eyelids regulate the release of tears.
In the case of reflex crying, an external irritant, such as dust or smoke, triggers nerve endings in the cornea to communicate with the brain stem.
The brain registers the irritation in the eye then alerts the lachrymal gland to stimulate tear production to flush away the invader.
So what happens when chopping onions a few feet away from our eyes, what’s causing this weepy reaction? The answer begins in the soil.
Onions are part of the plant genus Allium, along with garlic, chives, leeks and about 400 other cousins. These vegetables absorb sulfur in the earth, which helps form a class of volatile organic molecules called amino acid sulfoxides.
These sulfoxides are the real tear-jerkers when onions go under the knife.
As we chop up an onion, it releases lachrymatory-factor synthase enzymes. These catalysts instigate the chemical chain reaction that makes you to cry over the kitchen counter.
These enzymes react with the sulfoxides and convert them into sulfenic acids [source: Scott]. Sulfenic acids are highly unstable and rearrange into a compound called syn-propanethial-S-oxide [source: Library of Congress].
When syn-propanethial-S-oxide (a combination of sulfuric acid, sulfur dioxide and hydrogen sulfide) enters the air around our faces and approaches our eyes, it elicits the reflexive tear response described above [source: Norton].
Multiple nerve endings in the cornea register the sensation of the syn-propanethial-S-oxide as a substance that could harm our eyes.
Consequently, the brain stem phones the lachrymal glands, and we commence to sniveling.
But there is goodnews; instead of shedding tears because of onion why not try this few tricks before cutting your onion and avoid crying:
- Chop an onion beneath running water.
- Turn on a fan while cutting an onion to scatter the sulfur compounds.
- Chill or cook an onion before chopping it.
- Use an onion chopping container.
- Wear goggles or glasses to protect your eyes.
Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn’t burn your eyes.
While refrigerating or cutting the onion under water slows the reactions and changes the chemistry inside the onion.
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