Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears. Scientists used to blame the enzyme allinase for the instability of substances in a cut onion.
The process goes as follows:
- Lachrymatory-factor synthase is released into the air when we cut an onion.
- The synthase enzyme converts the amino acids sulfoxides of the onion into sulfenic acid.
- The unstable sulfenic acid rearranges itself into syn-ropanethial-S-oxide.
- Syn-propanethial-S-oxide gets into the air and comes in contact with our eyes. The lachrymal glands become irritated and produces the tears!
When an onion goes under the knife, cells are ruptured, which causes a chemical reaction. The resulting gas is highly irritating to most eyes. When the eye processes irritation, neurons send up all kinds of flares to the brain asking for help flushing out the invaders. Reflexive tears to the rescue!
What do onions contain?
Onions contain amino acid sulfoxides that form sulfenic acids in the onion cells. Both the enzymes and the sulfenic acids are kept separately in the cells. When you cut the onion, the otherwise separate enzymes start mixing and produce propanethiol S-oxide, which is a volatile sulphur compound that starts wafting towards your eyes. The gas that is emitted reacts with the water of your eyes and forms sulphuric acid. The sulphuric acid thus produced causes burning sensation in your eyes and this in turn leads to the tear glands secreting tears. Thus you end up with watery eyes every time you cut onions at home.
It is the sulphur compound in the onions that also leave a typical odour of onions on your hands and utensils even after washing.
Tips to avoid the tears:
- Make sure that you keep your fans off while cutting onions. If it is running, the air is on constant circulation, which will lead to the spread of gas and increase the burning sensation in your eyes.
- Refrigerate the onion before cutting. By refrigerating, you freeze the enzymes from mixing with the sulfenic acids and the production of propanethiol S-oxide is stopped. As a result, sulphuric acid is not formed when you chop the onions. Wear safety goggles while chopping onions. It is also a good idea to keep away the gas from reaching your eyes.
- Cooking the onion inactivates the enzyme. So cook the onion after you de-skin and before letting the knife touch the onion. Soak onions in water before cutting. Water absorbs the gas and you can avoid the tears.
- Lighting a candle or a lamp near the chopping board is another effective way to avoid the tears. When a candle is lit near the chopping board the gas that is emitted from the onions get drawn towards the flame of the burning candle or lamp and it does not reach your eyes
- Avoid cutting the onion till the root. This will prevent the mixing of enzymes with the sulfenic acids and you can spare your eyes of the burning sensation and tears.