Why onions make you cry

As Australia’s fourth-largest vegetable crop, onions account for 9% of the country’s total vegetable production. In 2015, 231,484 tonnes of onions were produced in Australia and the average consumption was 7.8 kg per person. Belgium imports more Australian onions than any other country.

But the downside of so many onions is eye-watering. While they add a savoury richness to our meals, when we cut them they release lachrymatory factor (LF), a chemical compound that makes our eyes sting and water.

For a long time it was assumed that the enzyme alliinase, which operates in the biochemical pathway that produces the compounds responsible for the onion’s characteristic flavour, was behind the synthesis of LF but this wasn’t so. A completely different enzyme, Lachrymatory-factor synthase, was responsible.

If this enzyme is downregulated or turned off you get non-lachrymatory onions that maintain their flavour and nutritional properties. ‘Tearless’ onions, sold exclusively in Japan for a hefty price, don’t make LFS so they also don’t produce the irritant LF.

Having established which enzyme was responsible for making LF, scientists still had no idea how this was achieved. Marcin Golczak and colleagues have solved this mystery and published their findings in ACS Chemical Biology.

The team determined the crystal structure of LFS and analysed it. With the crystal structure, they could finally see the architecture of the enzyme as a whole and its active site as it bound to another compound. By combining this information with known information about similar proteins, the group developed a detailed chemical mechanism that could explain the precise steps involved in LF synthesis — and hence, why people wind up crying when they chop an onion.