Few kitchen jobs are as unpleasant as chopping an onion. After a few seconds, your eyes start to sting, and soon tears are running down your face. For a long time, people believed that onions simply had a strong smell. Today, scientists understand that the onion is actually fighting back.
An onion grows under the ground, where it can easily be eaten by animals and insects. To protect itself, it has developed a clever chemical defense. Inside its cells, the onion stores sulfur compounds and, separately, special proteins called enzymes. As long as the cells are whole, the two never meet.
When you cut an onion, however, you break thousands of cells. The enzymes mix with the sulfur compounds, and a chain of chemical reactions begins. Within seconds, the onion produces a gas that floats up into the air. When this gas reaches your eyes, it irritates the nerves on their surface. Your brain responds by telling your eyes to produce tears in order to wash the irritating gas away.
There are several ways to reduce the tears. A sharp knife is a good start, because it cuts cleanly and breaks fewer cells than a blunt one. Putting the onion in the refrigerator for half an hour before cutting it also helps, since cold slows down the chemical reactions. Some cooks cut onions near an open window or under a kitchen fan, which carries the gas away from their faces.
Scientists in Japan and New Zealand have even developed onions that do not cause tears. Still, many cooks are not worried. For them, a few tears are a small price to pay for the flavor that onions add to almost every dish.