The onion is indispensable in cooking, but causes crying when it is being cut.
The onion has compounds that, when mixed, produce sulfuric acid, which causes the eyes to burn and tear.
The onions contain sulfur compounds, responsible for their strong smell, and also a high concentration of amino acids, among which is cysteine, which contains sulfur. When the onion is cut, rupture occurs in its cell walls, which causes the contact between the amino acids and enzymes present in other cells, initiating a variety of chemical reactions that lead to the formation of volatile sulfur compounds, such as the disulfide of allylpropyl.
These substances are very volatile and come quickly to the eyes of those who are cutting the onion. When they come into contact with water, present in tears, another chemical reaction occurs, which turns the substance into sulfuric acid, an extremely strong acid.
Another phenomenon also occurs in this case. The enzymatic action that occurs on the amino acids of the cut onion causes the formation of the thiopropionaldehyde oxide, which is a tearing agent. To avoid blazing there are several tricks like washing the onion, using a match stick in the mouth or chopping the onion underwater. The principle of all is the same, to prevent the gas from reaching the eyes. However, it is important to note that phosphorus does not help at all because it also contains sulfur. But there are some tips that can help, such as cutting the onion with a fan on because the wind removes the volatile substances produced by the eye onions; Wet your hands before cutting the onion as this causes the substances to find water before reaching the eyes; Dip the onion in hot water for at least five minutes, this will denature your enzymes; or cut the onion in running water.
These simple tips can prevent many tears.