When you chop, cut, crush or smash any vegetable, its cells break open and release bitter-tasting chemicals called polyphenols as part of its defence mechanism. This will prevent animals and birds from eating the vegetable. The same goes for the onion. However, the effect is a bit severe, as onions, when cut, release an irritating chemical, propanethial S-oxide. This chemical is highly volatile, meaning it evaporates quickly. When it finds its way into our eyes, it dissolves in the water covering the surface of our eyes and forms sulphenic acid. This acid irritates the lacrimal gland or the tear gland. The gland produces tears to wash off the irritation. Since the acid produced is small, it is not harmful.
Is there a way to avoid crying while chopping onions? Scientists suggest two options-put the onions in fridge or soak them in water before cutting. The chemical propanethial S-oxide will not escape into the air as easily when the onion is cold. And in water, the chemicals will dilute, becoming less potent. These tactics can, however, alter the flavour of the onion.
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