Why do Onions Make You Cry?

(Lansing State Journal, Feb. 12, 1997)

Like other plants, onions are made of cells. These cells are divided into two sections separated by a membrane. One side of the membrane contains a chemical called an enzyme which helps chemical processes occur in your body. The other side of the membrane contains molecules that contain the element sulfur. When you cut an onion, the contents on each side of the membrane can mix together freely and the enzyme causes the sulfur compound to undergo a series of chemical reactions. These reactions produce molecules such as ethylsufine which make your eyes water. The products from these reactions are called "transient species", meaning that they only "live" for a very short time. Their "lives" are actually so short that scientists are just now learning what they actually are. If you want to prevent crying when you cut onions, there are a few things you can do. One is to cut the onion under a stream of cold water. The sulfur compounds dissolve in water so they will be rinsed down the sink and not be able to get into you eyes. Another way to prevent crying is to put the onion in the freezer for 10-15 minutes before you cut it. Cold temperatures slow down the reaction between the enzyme and the sulfur compounds so fewer of the burning molecules can reach your eyes.

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