ONTE DE SOR, Portugal — "A cork!" exclaimed Marta Sá Pinto, a biotechnologist who has a doctorate in the manufacture of wine corks. "It seemed like such a simple thing, but there really is a lot going on with it."

In a previously sleepy corner of the global economy, a fierce battle has been joined: natural cork versus synthetic material to keep the world's wines at their finest.

For Portugal, synthetic corks have become a bane in the neck of the wine bottle, for they threaten an industry that represents 3 percent of gross domestic product in a country that accounts for half of the world's cork production, and 85 percent of all wine corks.

Ms. Sá Pinto is in charge of research for Suberus Group, one of Portugal's largest cork manufacturers, and she can detail exhaustively the ozone baths, microwave treatments and boiling processes that go into the production of the apparently simple modern cork.

While wine collectors and columnists debate the philosophical desirability of living in a world of plastic- stoppered wine, cork manufacturers here have scrambled to create a new breed of cork able to compete with the taint-free, consistent performance offered by synthetics.

In recent years, many even felt the growing market share of synthetic corks — currently estimated at 5 to 10 percent of the global market — was threatening even more than the loss of Portuguese jobs. Environmental groups warned that a crash in cork prices would cause the loss of the huge cork forests, which in turn would hasten the demise of already marginal animal life and encourage desertification.

In the heart of cork country, Ms. Sá Pinto is tweaking equipment at a new factory that incorporates the results of her research and should produce up to three million state-of- the-art corks a day when it starts up next month. "A lot of it is in the ozone," she said. "One of our big secrets is how we mix our ozone with special ingredients to remove spores and dangerous compounds."

The aim is to keep both the cork industry growing and to preserve the cork forests.

The manufacturers of plastic corks are not going to take the challenge lying down. "We can print a U.P.C. code on the side of our product with high-resolution inks," said Robert Anderson, the president of Supreme Corq, a leading synthetic cork manufacturer. "Using the closure as a marketing tool provides a huge opportunity to build a brand."

Mr. Anderson said the growth of synthetics was a result of their ability "to create a product that lets wine taste the way the winemaker intended it to." "Wine is a trinity," counters João Posser de Andrade, the owner of a cork farm in southern Portugal. "The bottle is the skeleton, the wine is the blood and the cork is the lung. Can you imagine a lung made of the byproducts of the petroleum industry?"

Those who have to decide between the two products wish each side would see the benefits of the other. "It's a bit childish," said Patrick Mahaney, vice president for winemaking at Robert Mondavi wineries in Napa Valley, Calif. "Proponents of plastic don't want to hear about the benefits of cork, and those entrenched in naturals don't want to hear about what plastic has to offer."

The Australian Wine Research Institute added fuel to the fire when it released preliminary results last summer of a long-term study that indicated that synthetic corks provided a lot of benefits over natural corks. Each side saw what they wanted to see in the results — synthetics scored highly, but the natural cork people pointed to the "rubberlike aroma" lent the wine sealed with synthetics.

Seen from Mr. Andrade's bucolic cork farm, cork indeed seems essential to the fine taste of wine. This year's cork bark harvest stood carefully stacked among the trees as it began the curing process and the long road to the bottle.

Even here, however, change is palpable. "There has been a revolution in our cork," Mr. Andrade said, reviewing recent changes to the business, in which his family has worked since 1840. In a few years, he conceded, harvesting may even come to be done by a machine, not — as for centuries — by hand. Mr. Andrade held up a piece of his carefully produced cork. "Plastic has no chance," he said. "I am confident in the good sense of the people."