William Strang, of St Patrick's College, asks :-

Eve Crossan, of Stirling, asked:-

When you cut an onion what makes you cry?

Conor Delahunty, of Otago University's Sensory Science Research Centre in the Department of Food Science, responded.

The onion is a member of the Allium (lily) family of vegetables, a family that also contains the shallot, leek, chive, scallion, and garlic.

Allium vegetables contain organic molecules called amino acid sulfoxides. When you peel, cut or crush an onion, you damage its tissue and release an enzyme called lachrymatory-factor synthase. This enzyme starts reactions that convert the amino acid sulfoxides into a compound called syn-propanethial-S-oxide (this is the compound that causes the tears) and thiosulfinates (these compounds give the familiar and desirable flavour of onion).

In other Allium vegetables, such as garlic, the reactions take a different pathway, and syn-propanethial-S-oxide is not produced, and so these do not cause tears.

The syn-propanethial-S-oxide compound is volatile, meaning that it can easily travel through the air and enter your eye, where it is perceived as a chemical irritant.

This compound is thought to protect the onion from being eaten by herbivores.

When syn-propanethial-S-oxide enters your eye it is perceived as a burning sensation by the common chemical sense, and this sensation in turn causes a reflex that orders the lachrymal glands around the eye to produce tears to wash away the irritant compound. The tears that we cry when sad or happy are called emotional tears, and are in fact different in composition to reflex tears (emotional tears contain endorphins, which are natural pain relief substances).

It is difficult to avoid crying when cutting onions, but several solutions can be suggested. You could use a very sharp knife to minimise onion tissue damage, store the onions in the fridge before cutting (the syn-propanethial-S-oxide compound is less volatile when it is cold). You could also heat the onions before chopping to denature the enzymes. Alternatively try ways to limit contact with the onion vapours: chop in a breezy room, chop under water, chop mechanically in a sealed container, or wear goggles. Recently, scientists in Japan proposed growing an onion without the enzyme lachrymatory-factor synthase. They suggested that it would not cause tears, but still taste just as good.