Here in the Atomic Kitchen, we bring the gamut of emotions. We like to make you laugh. Today, we might make you cry. Fair warning: you may need a box of tissues as we explain why cutting onions makes you cry.
Your tears are primarily water. Saline. Salt water.
Onions can be divided into two categories: Sweet and Storage. Sweet onions have a much higher water content, and have a limited shelf life. They have a milder reaction with your eyeballs and tear ducts because of that high water content. We're going to address yellow, white and red onions used in cooking – the “storage onions” sometimes referred to as “dry onions,” with a relatively low water content.
You're embarking on a fantastic recipe. You've got a sharp knife at the ready. You peel the skin off the onion and place it on the cutting board. Shortly after making that first slice, your eyes tear uncontrollably. These do not feel like “Oh, the suffering of humanity!” tears. They burn. What is going on?
Lurking within your onion are a mishmash of chemicals separated by cell membranes. In the whole onion state it is stable. But inside some of those cells are many amino acids, among which are sulfoxides. When you cut into the onion, you break the cell membranes, releasing a chemical medley through the onion, creating instability in Onion World. The sulfoxides become sulfenic acids. These are free to combine with other molecules, and one of the by-products is propanethiol S-oxide, a gas which takes a vapor form. Propanethiol S-oxide wafts upward, interacting with the moisture in your nose and those saline tears in your lachrymal ducts to create small amounts of sulfuric acid.
Sulfuric acid burns. Your tear ducts create more tears to wash the irritant away, and before you know it, you look like you just watched “Ol' Yeller.”
To avoid the waterworks, chill the onion for 30 minutes before cutting it.
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