MILWAUKEE - Under pressure to reduce sodium and saturated fats in American diets - especially those of children - the cheese industry has tried to make products with less salt or fat that consumers will like.

It has not had great success.

"We've made some progress in that arena," said Gregory D. Miller, president of the Dairy Research Institute. "But we have not been able to crack the code."

Dr. Miller, whose group is financed by the dairy industry, was referring to efforts to reduce salt, but he had a similar appraisal of the challenges of low-fat cheese. "When you take a lot of the fat out, essentially cheese will turn into an eraser," he said.

The trouble is that salt and fat are critical components.

Salt helps control moisture content and bacterial activity - the starter culture that is added to the milk and naturally occurring strains. They flavor the cheese.

"Salt serves as a preservative, as a director of flavor development," said Mark Johnson, senior scientist with the Wisconsin Center for Dairy Research at the University of Wisconsin-Madison. "If I remove it, my flavor goes in a different direction."

Fat affects moisture levels, too - less fat generally means more water, which can speed spoilage - and helps govern texture, balancing out proteins so a cheese slices properly.

"If you really want to make bad cheese, make a low-fat, low-sodium one," said Lloyd Metzger, a professor of dairy science at South Dakota State University.

Few people are talking about tinkering with specialty cheeses - making a low-fat Camembert, say, or a low-salt Roquefort. But what the cheese industry calls "American type" (natural Cheddar, Colby and similar varieties) and "Italian type" (mozzarella, provolone and others) account for about four-fifths of the more than 4.5 billion kilograms of cheese made in the United States each year. So producing good-tasting, good-textured versions with "reduced" fat or salt (defined by the United States government as at least 25 percent less than typical) or "low" fat or salt (containing a specific small amount) could have a large effect on Americans' diets.

There are some cheeses, like Swiss and mozzarella, that are naturally lower in salt than others, and cheese companies have had success marketing reduced-fat mozzarellas, particularly for school-lunch foods like pizza. But food shoppers have not flocked to most other lower-fat or lower-salt cheeses.

Cheese making is a straightforward process. Bacteria is added to milk, converting lactose (milk sugar) to acid and starting the curdling process. Enzymes are added that break down the proteins in the milk and help the curdling, and salt is usually added to limit the bacterial action and draw out more of the liquid whey from the curds. The curds then settle or are formed into a block of cheese.

But there are countless variations in the process, which account for the hundreds of cheese varieties made around the world: the source of the milk (cow, sheep, goat and even reindeer), its fat content, whether it is pasteurized, the strain of starter bacteria used, when and how the salt is added, whether the curds are "cheddared" (cut up, allowed to set, then cut and stacked several times, a process developed in Cheddar, England), whether the block is pressed or molded, how long and where it is ripened .

A common salt-reduction technique is to replace a portion of the sodium chloride with potassium chloride, which has a similar ability to control bacteria and provides a similar salty taste. But that presents other problems.

"Potassium salt by itself also gives a bitter note," Dr. Johnson said. "If you get it too high, then you taste that as an off flavor." So some cheese manufacturers add compounds that bind with the taste buds so the potassium salt does not.

When it comes to fat, there are different challenges, said Tonya C. Schoenfuss, a professor at the University of Minnesota who has worked on several varieties of cheese, including blue. "For something like blue, fat is really important for flavor," she said. "Will we ever have a low-fat blue? I kind of doubt it."

Texture is a big issue, too. The structure of many cheeses can be thought of as a network of casein and other proteins, interspersed with balls of fat. Remove a lot of the balls and the cheese is denser. And an inadequate amount of fat can leave the proteins exposed when the cheese is heated, allowing them to burn.

One approach, Dr. Schoenfuss said, is to put something else in to break up the protein network.

Most lower-fat cheeses are made from lower-fat milk. But David M. Barbano, a professor at Cornell University in Ithaca, New York, developed a process for removing fat from full-fat Cheddar. The cheese is shredded and warmed, and much of the fat is spun off in a centrifuge, said Michael Adams, a doctoral student in Dr. Barbano's lab. The defatted shreds are once again formed into a block.

But the technique would require an investment in costly equipment. Researchers are working on improvements.

"We use that centrifuged cheese as a flavoring component, but remake the cheese," Mr. Adams said. The result is softer than most low-fat Cheddars, although the researchers still have to do some work on the texture, he said.

Because it no longer conforms to strict federal standards, it is a "Cheddar cheese product."

"But the flavor's actually excellent," Mr. Adams said.

The New York Times