Which Chemical in Onion Causes Tears When it is Cut?


The chemical in onions that causes tears when the vegetable is cut is called syn-propanethial-S-oxide. This compound is released when an onion’s cells are damaged, such as when you cut into it. When syn-propanethial-S-oxide comes into contact with the moisture in your eyes, it forms sulfuric acid. This sulfuric acid irritates the nerve endings in your eyes, causing them to produce tears as a protective mechanism to flush out the irritant.

To reduce the tearing effect when cutting onions, some strategies include cutting them under running water, chilling the onions in the freezer before cutting, or wearing protective eyewear like goggles to shield your eyes from the irritant. Additionally, using a sharp knife and cutting the onion as quickly as possible can help minimize the release of syn-propanethial-S-oxide and reduce tearing.

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