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The onion’s tearful enzyme

Weeping over onions at the chopping board may one day be a culinary nuisance of the past. Scientists have discovered an enzyme in onions largely responsible for making our eyes water. The researchers are already trying to develop a tear-free onion by suppressing the gene that produces the enzyme.

The onion enzyme helps produce a chemical that causes burning sensations in the eyes of cooks everywhere. Known as lachrymatory factor, the irritant is released when the onion's tissue is crushed or ruptured during chopping. Tears then flush the chemical from the eyes.

Researchers at House Foods Corporations in Japan, a leading maker of spices and tofu internationally, discovered the enzyme unexpectedly. They had been investigating blue pigment created by mixing onion and garlic purees.

Getting rid of the irritant will not come at a cost to flavor, the researchers say. A tear-free onion would still have its pungent and delicious taste because the enzyme, lachrymatory factor synthase, is not involved in producing flavor.

The researchers suspect that suppressing the tearing enzyme may actually increase the levels of thiosulfphinates, which add flavor and may have health benefits, they write in Nature.

It may take decades to develop modified onions, and no one really knows how this product will taste. "At this point, we can't precisely predict how the flavor of tear-free onions will change," says Shinsuke Imai, who led the study at House Foods.

For now, chefs can reduce tears by chilling onions, removing the top, and peeling the outer layers. Running the onions under a steady stream of water while chopping also works.

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Imai, S. et al. An onion enzyme that makes the eyes water. Nature 419, 685 (October 17, 2002).

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