Posted on Aug 9, 2021 126
“A grandfather is sitting, dressed in a hundred fur coats; he who undresses him, sheds tears”. The reason onions are so tearful is that of a special substance - lacrimator (lat. lacrima - tear). When an onion is cut open, lacrimator is released and dissolved in water and human tears. This produces sulfuric acid, which irritates the mucous membrane of the eye. If the onion is frozen before peeling, the lacrimator activity is decimated. And now there explains why onions are peeled by wetting them or a knife with water - the lacrimator dissolves in water and is practically not released into the air.
Onions belong to the Alliums group of plants, which includes onions, shallots, garlic and leeks. Since time immemorial, these species, like many other plants, have been at war with those who try to eat them. Our eyes also become victims of “chemical weapons” when we slice onions in the kitchen.
The defense strategy of many onions is quite sophisticated. Their cells contain enzymes that can break down certain molecules, such as a certain type of amino acid. Normally, these two substances are stored in different parts of the cell and do not come into contact. But when one takes the risk of biting into a piece of onion or garlic, it destroys many cells and increases the mixing. The same thing happens when we cut an onion.
The chemical reactions that ensure result in sulfur compounds. Initially, one or more sulfur atoms were already contained in amino acids, but when the cell walls break down, the amino acids react with enzymes. The products of the latter are excellent insecticides, repelling insects and giving the plant an unpleasant taste (we are talking about raw onions). Such a protective reaction can sometimes be very effective.
The chemical composition of the “weapon” varies depending on the type of plant. Meanwhile, in ordinary onions, the sulfuric acids are poisonous. They are unstable and easily react with the enzyme, turning into another substance - thio propanol-S-oxide. Its molecule has the chemical formula C3H6SO. Thio propanol-S-oxide is an excellent insect repellent, even better than sulfuric acids - because it is not them, but this strong compound that makes cooks weep.
Well, since the substance is very volatile, it easily rises to our eyes, which defend themselves as best they can: secret tears and wash the foreign mixture off the eye membrane. Most unpleasant of all, when it comes into contact with water (including tears), a small portion of thio propanol-S-oxide turns into sulfuric acid H2SO4.Fortunately, this dose is negligible, even so, precautions should not be disregarded, because it is the strongest acid in existence.