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Onion Finding Won't Leave a Wet Eye in the House

Scientists find gene that makes you cry

WEDNESDAY, Oct. 16, 2002 (HealthDayNews) -- People who cry like babies when they take the knife to onions are in for a slice of good news.

Japanese food scientists have discovered a new gene that seems to be the tearjerker in the pungent herb. Removing the gene, they say, could lead to a tear-free onion with all the flavor and health benefits of the natural version without the drama in preparing it.

Scientists have long believed the substance that causes onion choppers to weep -- a molecule called propanthial S-oxide -- is merely a by-product of the chemical reaction that gives the bulbous vegetable its distinctive flavor. But because these chemicals are so volatile, they've been difficult to observe in action.

The new work, appearing in tomorrow's issue of Nature, shows that in fact propanthial is generated by an enzyme, which the researchers dubbed "lachrymatory-factor synthase."

The team, led by scientists at House Foods Corp. in Chiba, isolated three proteins from alliinase, a sulfur-rich enzyme in onions and garlic at the top of the vegetables' flavor chemistry flow chart. They traced the three proteins to the previously undescribed gene. When they added the new gene to E. coli bacteria, the bugs began producing the teary enzyme.

When the researchers removed the lachrymose enzyme from a mixture of onion chemicals, they were able to extract more of a substance called thiosulphinate, which gives fresh onions their flavor. Thiosulphinate is also believed to lower cholesterol and prevent blood cells from sticking together.

Deleting the gene for lachrymatory-factor synthase could therefore lead to tear-free onions that are even more flavorful and salutory than conventional bulbs, the researchers say.

"Not necessarily," warns Eric Block, a chemist at the State University of New York at Albany and one of the world's leading onion experts. "It's a more complicated story than it looks to the eye."

Although it's true that a modified onion might be richer in thiosulphinate, there are other chemicals that confer its flavor and health benefits, and these are generated in the reaction that makes the tear factor, Block says. What's more, thiosulphinates aren't totally benign. They cause bad breath and heartburn, the latter problem by apparently forcing the valve dividing the esophagus and stomach to flutter. So eating onions with more of these molecules wouldn't be an unalloyed good.

Then there's the gastronomic wild card: Zing. Food scientists have found the tear stimulator in onions warm the tongue and give it a tingle. "You would lose that in an engineered onion," Block says.

Irwin Goldman, an onion expert at the University of Wisconsin-Madison who was familiar with the Nature paper, called the work "amazing." But Goldman adds the brief research letter was too shy on detail and left several key questions unanswered.

Most important, Goldman says, was the researchers' contention that suppressing the tear gene led to an increase in flavor chemicals, a claim for which they offer no data.

"There's a lot of potential there, but I would like to know more about how it was done and what the results were," Goldman says.

Americans, it seems, think a few tears are a fair trade for their onions. Each of us eats an average of 18.1 pounds of the vegetables every year, according to the National Onion Association. That's up 68 percent from two decades ago, says Tanya Fell, a spokeswoman for the Greeley, Colo.-based growers group. "I don't think people really do complain about" the crying problem, Fell claims.

Still, Fell notes onion producers are trying to appeal to consumers with delicate eyes by making their produce sweeter and milder and by introducing new products like pre-diced onions.

One Japanese firm, Amu Corp., has been selling what it calls a "tear-free" onion since 1998. The secret to that vegetable is in the fertilizer, however, and not the bulb itself.

What To Do

If pre-chopped produce isn't for you, there are a couple of things you can do to reduce the irritation of slicing onions, Goldman says. Cutting them under water helps, as does chilling the vegetables first, by retarding the enzymes that generate the noxious chemicals.

For more on onions, try the National Onion Association. This WholeHealthMD site has a rundown on the purported health benefits of onions.

SOURCES: Irwin Goldman, Ph.D., associate professor, horticulture, chairman, plant breeding and plant genetics program, University of Wisconsin-Madison; Eric Block, Ph.D., chemist, State University of New York, Albany; Tanya Fell, spokeswoman, National Onion Association, Greeley, Colo.; Oct. 17, 2002, Nature
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