When you cut into an onion, you slice through the onion cells that contain sulfuric compounds. The cutting of the onion allows certain enzymes to mix with sulfenic acid which forms a new compound. This highly volatile compound enters the air quickly and is immediately detected by our tear ducts, which begin to produce tears to wash them away before they harm our eyes.
Cooking the onion de-activates the enzyme which triggers this combination - so this is why cooked onions don't make us cry.
One trick I've heard (via Food Network) is to refrigerate onions before cutting them - as somehow this slows down the chemical process that creates the sulfuric gas. Others say you can cut an onion under running water and the gasses will be washed down with the spray - rather than coming up toward your eyes.
The answer is: its cells, which shelter a sulfur-based chemical that irritates both eyes and nose. When you cut onions, the chemical is released and, once it is in contact with air and water produced by the lacrimal glands, it causes a chemical reaction.
What follows from this equation? A sulfuric acid and a lot of pain. As the more we shed tears, the more we hurt, and the more we get hurt, the more we'll shed tears, over and over again. So ... quickly what to do?
There is no a miraculous product. There are however some simple steps that we can take:
- Mouth breathing when cutting onion. In this way, we will relieve the nose and eyes of a large quantity of gas.
- If you like chemistry, you have at your hand a most laborious method: mix onion with parsley to oxidize some sulphates. This combination improves the bad breath after consumption of onions.
- To cut onions in water or under running water, because in this way gaseous substance released will react with water and no with tears
- If onions will be cool down, about an hour before cutting, can be helpful, because sulfates will be more difficult to evaporate.
- Mechanical Method: Put some glasses on, that will act as a shield for your eyes.