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Why tears fall when cutting onions

It’s a well-known fact that  onions make you cry. We’ve all experienced this while cooking; we start  chopping an onion, our eyes start stinging and the next thing we know we have  tears rolling down our cheeks.

Blinking or rubbing our eyes only makes it worse. It’s happened to us all, but have you ever wondered why?

When you slice into an onion you’re breaking apart the cells inside. These release their contents which form sulfenic acids. These  acids mix with enzymes to produce a gas called propanethiol S-oxide. As this gas reaches our eyes and mixes with the  water in them, it forms sulfuric acid.

This acid makes our eyes  sting, which results in more tears and this added water produces more acid, which in turn produces more irritation.  Cooking  the onion then deactivates the enzyme and our eyes return back to normal. There are a number of different suggestions about how we can avoid this. As you will see, some of them are quite logical and useful; others are just downright impractical or even dangerous.

Cut the onion under, or near, running water

In  theory this method ensures that the gas gets absorbed by the running water instead of your eyes. It’s not very practical from a saving water point of view though, and cutting anything under water is quite likely to end in an  injury. Other tips based on this theory include chopping onions with your tongue sticking out!

Chill your onions first

This is supposed to slow down the amount of acid that is released. The onion doesn’t need to be frozen, just chilled in the freezer for 10-15 minutes and is one of the more sensible options.

Wearing goggles

It is possible to actually buy “onion goggles,” although tight-fitting swimming goggles would likely do the same job. They just have to stop any gases from reaching your eyes. It may seem excessive, and you may be ridiculed by your family for standing in your kitchen in swimming goggles, but at least you’ll know that you can chop as many onions as you like without crying!

Whichever method you use though, it is worth it because onions have lots of health benefits, from lowering cholesterol to being high in vitamin C. So maybe it’s worth all the tears after all.

Nothing in this blog post is to be construed as medical advice, nor is it intended to replace the recommendations of a medical professional. For specific questions, please see your eye care practitioner.

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