You’re actually being subjected to an acid attack. Onions take up sulphur from the soil while growing. This is combined with amino acids within the onion’s cells to form amino acid sulfoxides. In 2002, Japanese researchers identified an enzyme, also present in onions that unlocks the sulfoxides when the onion is cut. The resulting reaction produces sulfenic acid that rapidly reforms into a gas called syn-Propanethial-S-oxide. When this stuff hits the sensitive surface of our eyes, it causes the stinging that leads to tears. My top tip is to store your onions in the fridge before cutting into them. Cooling them slows down the chemistry and curbs the tear-jerking reaction.
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