An artificial fat once embraced as a cheap and seemingly healthy alternative to saturated fats like butter or tropical oils, partially hydrogenated oil has been the food industry's favorite cooking medium for decades. It makes French fries crisp and sweets creamy, and keeps packaged pastries fresh for months.

But scientists contend that trans fat, a component of the oil, is more dangerous than the fat it replaced. Studies show trans fat has the same heart-clogging properties as saturated fat, but unlike saturated fat, it reduces the good cholesterol that can clear arteries. A small but growing body of research has connected it to metabolic problems.

The Food and Drug Administration has declared that there is no healthy level in the diet and has ordered food companies to disclose trans fat amounts on food labels by January 2006.

That has sent dozens of companies on an expensive and frustrating race to this popular oil.

So far, only the most health conscious consumers are shopping to avoid trans fat. But food companies are betting that will change when the labeling law takes effect, and they have already spent tens of millions of dollars trying to get rid of trans fat without changing the taste of processed and fast foods.

"Whoever's on that list of products with trans fats is going to be sweating bullets," said Harry Balzer, vice president for the NPD Group, a consumer research company based in Port Washington, N.Y.

At least 30,000 and as many as 100,000 cardiac deaths a year in the United States could be prevented if people replaced trans fat with healthier nonhydrogenated polyunsaturated or monounsaturated oils, according to a 1999 joint report by researchers at the Harvard School of Public Health and the Brigham and Women's Hospital in Boston.

This and other studies led the government's top medical advisers for the Institute of Medicine at the National Academy of Sciences to declare in 2002 that they could not determine a healthful limit of trans fat, as they had for other dietary fats. The following year the government approved the labeling law.

The $500 billion food processing industry has long defended trans fat, starting in the 1970's when scientists first raised concerns. But with the new labeling requirement looming and lawmakers searching for ways to hold food companies responsible for their customers' health, getting rid of it has become an obsession.

"It's the perfect storm for these companies: concern over litigation and legislation, as well as a market opportunity of baby boomers getting older and being more concerned with their health," said Dean Ornish, the director for the Preventive Medicine Research Institute in Sausalito, Calif., and a consultant to PepsiCo, McDonald's and ConAgra Foods.

PepsiCo has already scrubbed trans fats from its Frito-Lay brand chips. Health-oriented grocery stores like Whole Foods and Wild Oats refuse to sell any processed food that contains it. Last month, Gorton's removed trans fat from its fish sticks, and Tyson Foods introduced frozen fried chicken products without it. Executives at Kraft Foods, ConAgra, Kellogg and Campbell Soup want to get trans fat out of most or all of their products by the beginning of next year.

Unlike diet-driven trends that filled store shelves with low-fat products in the 1990's and, more recently, low-carb foods, the removal of trans fats does not have a strong consumer constituency. Although some market research shows that more than 80 percent of consumers have heard that trans fat is unhealthy, few shop to avoid it. Most seem to be like Joan Nicholson, 57, a New Yorker who retired to Boise, Idaho. "I read about cholesterol and trans fats and fatty acids and I try to keep it all straight," she said, "but I'm afraid I don't do a great job of it."

Unsatisfying Alternatives

Finding a substitute for partially hydrogenated oil is more daunting and considerably more expensive than food companies first imagined. That is because it is the perfect fat for modern food manufacturers. Produced by pumping liquid vegetable oil full of hydrogen with a metal catalyst at high heat, the fat stays solid at room temperature - an essential trait for mass-produced baked goods like crackers or cakes. But that is the very process that creates the dangerous trans fat.

The shortening-like oil is an industry workhorse. Its smoothness and high melting point make it a great medium for the creamy filling in an Oreo. In the deep-fat fryer, partially hydrogenated oil can take repeated heatings without breaking down.

It also helps products stay fresh longer on supermarket shelves. Small amounts keep peanut butter from separating. It is even found in products promoted as healthful, like Nutri-Grain yogurt bars and Quaker granola bars.

According to one survey on trans fat issued by the Food and Drug Administration in 1999, partially hydrogenated oil was in 95 percent of the cookies, 100 percent of crackers and 80 percent of frozen breakfast foods on supermarket shelves.

Margarine, which was very high in trans fat, was one of the first foods to change. ConAgra Foods in Omaha spent about a year creating trans fat-free versions of soft tub margarines like Parkay and Fleichmann's. But the company is having a tougher time cracking the code on stick margarines, frozen dinners and microwave popcorn.

The company tested liquid soybean oil in its Marie Callender's frozen dinners, but the oil puddled under the roasted potatoes and the sauce slipped right off the meat, leaving it barren and dry.

"It wasn't very appealing," recalled Pat Verduin, senior vice president for product quality and development at ConAgra, which owns dozens of household brands, including La Choy, Hunt's and Peter Pan.

At the Pepperidge Farm division of Campbell Soup, in Norwalk, Conn., puff pastry sheets and pot pies are causing the most trouble. Concoctions tested over the last year have made the crusts unpalatably dense and breadlike.

"We can't get the flakiness and layering with these softer fats," said Scott Gantwerker, its quality assurance chief.

The company had more success with its Goldfish snack crackers, which after two years of tinkering are made with a sunflower oil blend and are free of trans fat. The oil, called NuSun, resists oxidation and spoilage. But it will not solve every company's problem. Only 2 million acres of the sunflowers are planted each year, compared with 75 million acres of soybeans. As a result, the sunflower oil can cost 20 percent to 25 percent more, said Larry Kleingartner, executive director of the National Sunflower Association.

Feeding the Fast Food Giants

Finding a way to have businesses change the oil they use is even more problematic for the fast-food industry, which uses partially hydrogenated oil in deep-fat fryers and on griddles. Some chains, like Legal Seafood and Ruby Tuesday, replaced their oil with healthier versions, but they are the exceptions. Restaurants face no government labeling requirement.

"We're not into knee-jerk reactions," said Yum Brands' chief executive, David C. Novak, whose company owns KFC, Taco Bell and Pizza Hut. "We've seen things come and go." Yum Brands, Mr. Novak said, "is at the early stages" of trans fat replacement.

McDonald's replaced beef tallow with partially hydrogenated soybean oil in 1990. In September 2002, the company vowed it would use healthier oil in its 13,000 stores in the United States by February 2003. Two years later, it is still serving up six grams of trans fat in a large order of fries and has given no indication of when that will change. Last week, the company agreed to a $8.5 million settlement of a lawsuit accusing it of misleading the public about its efforts to remove trans fat.

During a conference call in December, McDonald's chief executive, James A. Skinner, offered few specifics on the company's progress in eliminating trans fat. He would say only that levels had been reduced in fried chicken products by 15 percent. "We remain committed to reduce trans fats," he said.

McDonald's problem, like that of many other giant food companies, is one of supply and demand. There simply is not enough reasonably priced replacement oil that is capable of retaining the signature flavor of a McDonald's fry, said John Jansen, senior vice president for sales and marketing at Bunge, the world's largest processor of oilseeds like soybean and canola.

Among the options McDonald's considered is a new breed of oil called high-oleic canola, which can withstand repeated heating in a deep-fat fryer without compromising taste. But it is in short supply and expensive. The annual production of the oil this year will be about a billion pounds and McDonald's would require about a third of that, Mr. Jansen said. At roughly 20 cents more a pound, the switch would cost the company an additional $70 million a year, according to figures offered by Mr. Jansen.

And until large users like McDonald's commit themselves to it, oil-seed growers will not produce more. The scale of the problem becomes clear at the J. R. Simplot French fry and hash brown plant in Caldwell, Idaho, where Burbank russet potatoes become McDonald's fries.

Before being frozen and shipped to restaurants and supermarkets, all frozen fries are given an initial light frying, usually in cheap partially hydrogenated soybean oil. Simplot food scientists recently developed the Infinity fry, cooked in a high-oleic canola blend. The fry takes well to baking in the school cafeteria, where it has found a market. It can also be fried in trans-fat-free oil.

The Infinity can cost up to 50 percent more than the average fast-food fry. As a result, it is expected to make up only 1 percent to 2 percent of food sales this year for Simplot, a privately held company with $3 billion in annual sales that was the first to sell frozen fries to McDonald's.

Simplot's real profit center is the huge fry factory just across a muddy parking lot from the test kitchen where the Infinity fry was born. There, 720,000 pounds of frozen fries made with partially hydrogenated vegetable oil tumble off the line every day and are shipped to restaurants like McDonald's.

"Logistically, trying to turn the restaurant industry on its head is essentially impossible on a 'let's do it by May' sort of basis," said Kevin Storms, president of Simplot's food group. And then there is the matter of cost.

"Most restaurant customers," Mr. Storms said, "want a specific taste at a specific price."

Medical Advice Changes

Balancing health with taste has long been a challenge for food manufacturers. In the 1980's, on scientists' advice, the industry replaced saturated fats like coconut oil and butter with oil containing trans fat. Now nutritionists have changed their edict.

"There was a lot of resistance from the scientific community because a lot of people had made their careers telling people to eat margarine instead of butter," said Walter Willett, chairman of the Department of Nutrition at the Harvard School of Public Health and one of a handful of medical researchers who have led the fight against trans fat. "When I was a physician in the 1980's, that's what I was telling people to do and unfortunately we were often sending them to their graves prematurely."

He and other researchers say that cells rely on natural fatty acids to function. Trans fat is artificial, and acts in the body like grains of sand do in the workings of a clock.

The strongest argument against trans fat is its role in heart disease. Like lard, beef fat or butter, trans fat increases low-density lipoprotein, or LDL, the so-called bad cholesterol. But it also decreases HDL, the good cholesterol that helps clean arteries, several studies have shown.

Food companies have, for the most part, accepted the word of scientists and are working to remove trans fat, even though they know finding a new oil is going to cost them. Not only does equipment need to be retooled, budgets must be re-examined.

Taste and Technology

Food companies argue that completely eliminating trans fat might be impossible given the cost and the fact that consumers do not want the taste of favorite foods to change. That is why a coalition of edible oil producers and food manufacturers persuaded both the Agriculture and Health and Human Services Departments to soften the federal government's stance on trans fat consumption in the latest version of the dietary guidelines released in January.

The scientific advisory committee that created the guidelines originally warned that trans fat consumption should be "limited to less than 1 percent of total calories," or about the amount in half a doughnut. But the numeric value was replaced with the phrase "keep trans fatty acid consumption as low as possible."

Food companies are also fighting a campaign by the Center for Science in the Public Interest, which frequently criticizes the industry, and a group of cardiologists and researchers to ban trans fat altogether, a proposal similar to one snaking through Canada's legislative system.

Faced with the lack of trans fat free vegetable oil alternatives, some companies are gingerly turning back to palm oil, a saturated fat that was taken out of many products in the late 1980's after an effective campaign waged in part by the American Soybean Association and the Center for Science in the Public Interest helped turn people away from all forms of "tropical grease."

Kraft is using a combination of palm fruit oil and high-oleic canola for the filling in its three trans-fat-free Oreo varieties - a reduced-fat version and two with yellow, rather than chocolate, wafers. Without the firmness of palm oil, getting the consistency that Oreo lovers expect would have been nearly impossible, said Jean Spence, Kraft's executive vice president for technical quality.

The trade-off was an extra half-gram of saturated fat per serving. The company still has not figured out how to make the traditional Oreo taste the same without trans fat or significantly higher saturated fat levels. So far the new versions make up 9 percent of Oreo sales, according to data from Information Resources, an industry research firm.

Some companies are experimenting with new blends of liquid oil and fully hydrogenated oil, which does not contain trans fat. Others are using an enzyme method called intersterification to blend the oils.

Critics say that these offerings are still artificial, highly processed ingredients that may not be much safer than oils produced by partial hydrogenation. And nutritionists wonder whether consumers know enough to distinguish good fat from bad, and natural oils from artificial.

"I don't know that they will look at a label that has low trans fat and high saturated fat and be able to figure out if it is healthy or not," Joanne Ikeda, a nutrition professor at the Center of Health and Weight at the University of California, Berkeley.

And consumers might not even care.

"I know there are healthy fats and there are unhealthy fats and that trans fats are the unhealthy ones, but I don't know what they are supposed to do to you," said Thai Bu, 32, who was buying whole-grain bread and eggs recently at a West Seattle grocery store. "If I want a cookie and it has it in it, I'll still eat one or two."