Why Do Onions Make You Cry
The tears we shed when chopping onions are caused by an amazing chemical reaction. Slicing through an onion initiates a protective mechanism that the vegetable naturally possesses.
An onion’s interior contains multiple chemical compounds kept separately in different cell compartments. When the onion is cut these compounds mix together and produce Syn-propanethial-S-oxide. It is a gas responsible for the irritation.
This gas is extremely volatile and quickly rises up towards your eyes. As the gas reaches your eyes, it comes into contact with the sensitive nerve endings.
Your eyes immediately recognize this as an irritant and seek to respond by immediately producing tears. This is a natural way your body uses to try to clean away the irritating substance from the eyes in an attempt to avoid potential damage.
The tearful reaction is actually the onion's evolutionary defense mechanism. In nature, this helps prevent animals and insects from eating the onion.
The sulfur compounds that create this reaction are the same ones that give onions their distinctive sharp flavor and strong smell.
The good news is that this defense mechanism isn't super powerful. Cooking the onion completely neutralizes these tear-inducing compounds. So while raw onions might make you cry, a quick saute turns them into a tear-free ingredient.