Science News Archive

Shed a tear for the humble onion

Scientists in New Zealand have used GM technology to develop onions that can't make you cry.

Working with colleagues in Japan, Colin Eady, who is based at New Zealand Crop & Food Research, has successfully silenced the gene responsible for producing the lacrimatory (tear-jerking) factor, which is a volatile chemical called syn-propanethial-S-oxide and is emitted by freshly cut onions.  Originally scientists thought that the chemical was pre-made within the vegetable, so preventing its production would probably interfere with the flavour of the onion.  But in 2002 scientists in Japan successfully unpicked the chemical pathway responsible for its formation.

Cutting into an onion releases the enzyme allinase, which breaks down a family of pungent chemicals in the onion called amino acid sulphoxides.  Two further chemical steps, one of them achieved with the help of a second enzyme discovered by the Japanese and known as "lacrimatory factor synthase", yields the
syn-propanethial-S-oxide.  It's this final step in the pathway that has been targeted by the researchers who have used a technique called "gene silencing" to deactivate the enzyme.  The result is an onion that tastes great but doesn't reduce you to tears.

In fact, according to Eady, preventing the sulphur compounds from being converted into the tearing agent redirects them into compounds responsible for flavour and health, so the process could even improve the taste of the onion.  "We anticipate that the health and flavour profiles will actually be enhanced by what we've done," he said.

But cooks hungry to embrace the new eye-friendly foodstuff will have to wait for at least ten years before it is commercially available, says Eady.  So in the meantime why not resort to folklore, which says that burning a candle close to where you are chopping an onion can help to burn of the irritant vapours.  Or trying chopping your onions underwater - so long as you can hold your breath for long enough!

3rd Feb 2008