After a serious look at onions last week, now it’s time to learn some funny details about them, some tips on using onions, and the answer as to why onions make you cry.
Onions – why the tears?
It’s all about chemistry!
Onions absorb sulfur from the soil, which helps form a class of volatile organic molecules called amino acid sulfoxides; they form sulfenic acids. When you cut an onion, you break cells, releasing their contents. Enzymes that were kept separate now are free to mix with the sulfenic acids to produce propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.
Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn’t burn your eyes. Aside from wearing safety goggles, a snorkel mask or running a fan, you can keep from crying by refrigerating your onion before cutting it (slows reactions and changes the chemistry inside the onion) or by cutting the onion under water. This can be very tricky and may lead to knife mishaps. Instead, use a sharp knife which reduces the crushing effect of slicing onions.
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