Why do onions make us cry?
As we know while cooking an onion a very strong aroma is produced that most people enjoy. But cutting up an onion stings your eyes and makes them water uncontrollably. If you love to include onions in your dishes, this probably frustrates you all the time. So just what is it in onions that make them such an ordeal to prepare?
When you slice through an onion, you break open a number of onion cells. Some of these cells have enzymes inside them and when they are sliced open, the enzymes escape. The enzymes then decompose some of the other substances that have escaped from sliced cells. Some of these substances quickly rearrange themselves into a volatile gas. The gas reaches your eyes and reacts with the water that keeps them moist. This change produces a mild sulfuric acid, which irritates the eyes. The nerve endings in your eyes are very sensitive and so they pick up on this irritation which is why our eyes sting when we slice onions. The brain reacts by telling your tear ducts to produce more water, to dilute the irritating acid so the eyes are protected. Your other reaction is probably to rub your eyes, but this will actually make the irritation a lot worse, of course, if you have onion juices all over your hands.
Oddly enough, this volatile compound is also responsible for a lot of the great taste in onions, as well as the pleasant aroma when you cook the vegetable. You will also get these same acids by cutting up garlic among other vegetables, but they don’t form the same irritating gas, just a strong smell.
There are all kinds of remedies for dealing with this irritating phenomenon, some more effective than others. As a general rule, move your head as far away from the onion as you can, so the gas will mostly disperse before it reaches your eyes. Peeling the onion and then chilling it in the refrigerator before you slice it will minimize the release of gas somewhat, because the change in temperature alters the compounds in the onion. Another easy solution is to cut the onion under water or run the tap over it as you slice.