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Less Crying in the Kitchen: Tasty, tearfree onions on the horizon

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11:09am, October 16, 2002
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A cook cutting onions can look very sad. There's no shortage of folk remedies to prevent the tears, but none works very well. Now, food scientists see the possibility of an onion that retains a full flavor but avoids the discomfort.

Japanese researchers have discovered an enzyme that the onion uses specifically to create the tear-jerking chemicals. Shinsuke Imai of the House Foods Corp. in Chiba, Japan, and his team report their finding in the Oct. 17 Nature.

Until this discovery, if scientists had used genetic modification to yield a tearfree onion, they probably would also have compromised the flavor, says Imai. Propanthial S-oxide–the onion irritant behind all the culinary sobbing–was considered a by-product of the reactions that produce the onion's characteristic flavor compounds, Imai says.

Instead, Imai's group found a new enzyme that works specifically to produce the irritant. This enzyme doesn't contribute to the reactions that lead to the onion-flavor compounds.

Therefore, genetic engineering might modify an onion so that it lacks only this particular enzyme–and so remains full-flavored.

Imai and his colleagues made their finding accidentally when they tried to produce propanthial S-oxide with precursor compounds from onion and a crude preparation of the enzyme alliinase derived from garlic. Although the plants are closely related, garlic doesn't produce tears. Scientists had previously considered alliinase to be the only enzyme that onions need to produce propanthial S-oxide and their flavor compounds.

Further analysis showed that to make propanthial S-oxide, onions employ a second enzyme. Imai and his team call it lachrymatory-factor synthase.

"If they could knock out this synthase enzyme, then they could minimize the amount of tearing agent" in full-flavored onions, says Kirk L. Parkin, a food scientist at the University of Wisconsin–Madison. However, it's possible that some irritant might form spontaneously, he says.

Onions grown commercially on low-sulfur soils already induce fewer tears than ordinary onions do, but their flavor is weaker, notes Parkin.

The Japanese finding may "open up a new era of onion science and horticulture," says Eric Block, an organic chemist at the State University of New York at Albany. However, adds Block, "it's reasonable to assume that Mother Nature incorporated the [tear-inducing chemical] to afford some protection." An onion stripped of this defense may be more prone to attack by insects and microorganisms, he says.

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