Everyday Chemistry - Why do onions make us cry?

The mere thought of chopping onions can literally make you cry. But the moment your knife touches the onion, your eyes start burning and you have tears in your eyes. Here’s why it happens...

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.
• 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.

CAN you now answer the questions:

What do onions contain?

How can you avoid the tears?

Why do onions make us cry?