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As onions are sliced, cells are broken, allowing enzymes called
alliinases to break down sulfides and generate sulfenic acids
(amino acid sulfoxides). Sulphenic acids are unstable and decompose
into a volatile gas called syn-propanethial-S-oxide. The gas
dissipates through the air and eventually reaches the eye, where it
reacts with the water to form a dilute solution of sulfuric acid.
This acid irritates the nerve endings in the eye, making them
sting. Tear glands produce tears to dilute and flush out the
irritant.
Supplying ample water to the reaction prevents the gas from
reaching the eyes. Eye irritation can, therefore, be avoided by
cutting onions under running water or
submerged in a basin of water. Rinsing the onion and leaving it wet
while chopping may also be effective. Chilling or freezing onions
prevents the enzymes from activating, limiting the amount of gas
generated. Using a sharp blade to chop onions will limit the cell
damage and the release of enzymes that drive the irritation
response.
