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Sandy asks, "Why do onions make you cry and how do you stop it?"
Hi Sandy, are you familiar with the acronym VOC? VOC stands for "volatile organic compound." People are getting more and more familiar with the acroynym VOC, because people who are concerned about health and allergic reactions try to avoid VOCs. For example, if I'm in a house that has been recently painted, my throat will start to close up a bit, my whole mouth will start to itch like crazy, and I'll start to sound like Darth Vader when I'm breathing. The culprit? VOCs in the paint. So when we painted our new house, I had to look for low-VOC or VOC-free paints. And when the school where I teach was being painted, I had to let them know that they had to choose between me and paint with VOCs.
My point? You might be surprised to know that onions contain VOCs. We tend to think of VOCs as artificially produced chemicals that are used as additives, but VOCs also occur naturally. After all, the "O" in in VOC stands for "organic" so it does make sense that we'd find these chemicals in nature. So what exactly is a VOC? It's any organic compound that emits as a gas. VOCs are found everywhere - perfumes, gasoline, fuel oil, deodorants, shampoos, paints, candles, laundry detergents and fabric softeners...the list goes on and on. What do all of these have in common? You can smell them. That's a pretty good rule of thumb; if it has a fragrance, there's a good chance it has VOCs, because fragrances are carried on the gases emitted.
So, yes, onions have VOCs. The particular compound that causes crying is syn-Propanethial S-oxide (C3H6OS), which is called an organosulfur compound, because it is an organic compound containing sulfur. What does it do? It irritates the glands that produce tears.
Now, to answer the second part of your question, I want you to think about a puddle of water. Eventually that puddle will all evaporate, right? But what conditions will make that puddle evaporate more quickly? The higher the temperature, the more quickly the water will turn to gas (that's what evaporation is). The same reasoning works for VOCs - higher concentrations of VOCs will be emitted at higher temperatures. Therefore, if you want to cry less, stick the onion in the fridge (maybe even the freezer) for a while, until the entire onion is at much cooler temperatures. Then when you're ready to cut it, take it out and cut it immediately, before it has a chance to warm back up again. The release of the tear-inducing compound will be lessened.
And if that doesn't take care of the problem? Some people wear goggles while cutting onions!