Salt has been quietly slipping out of dozens of the most familiar foods in brand-name America, from Butterball turkeys to Uncle Ben's flavored rice dishes to Goya canned beans.

A Kraft American cheese single has 18 percent less salt than it did three years ago. The salt in a dollop of Ragu Old World Style pasta sauce is down by 20 percent. A handful of honey Teddy Grahams has 33 percent less salt.

Their manufacturers are among 21 companies that have met targets so far in a voluntary, New York City-led effort to get food manufacturers and restaurateurs to lighten up on salt to improve Americans' heart health, Mayor Michael Bloomberg announced Monday. While it's unclear whether consumers have noticed the changes, campaigns aim to get more salt out of the national diet in the coming years - a challenge for an ingredient that plays a role in the taste, preservation and even texture of food.

Salt reduction has become a recent focus of public health campaigns in the city and elsewhere. Salt, or sodium chloride, is the main source of sodium for most people.

Sodium increases the risk of high blood pressure, a major cause of heart disease and stroke. Dietary guidelines recommend no more than 2,300 milligrams of sodium a day, equal to about a teaspoon of salt; the American Heart Association suggests 1,500 milligrams or less. Average sodium consumption in the U.S. is around 3,300 milligrams, the federal Centers for Disease Control and Prevention have found.

Officials said the first step was a meaningful one.

"The products they're making healthier are some of America's most beloved and iconic foods," noted Bloomberg, a fan of Subway's meaty Italian BMT sandwiches, which are now 27 percent less salty.

At a Manhattan grocery store, shoppers said they hadn't noticed the salt falloff, either because they didn't taste the difference or because they eschew prepared foods to begin with.

"A decrease is good - not putting anything in there is even better. People should add their own salt," shopper Lynne Davis said.

Health officials say Americans get the vast majority of their salt from processed and prepared foods, and not necessarily the foods they'd imagine: Bread and rolls are the No. 1 source.

"The problem is not the salt on the table. The problem is the salt on the label," city Health Commissioner Dr. Thomas Farley said.

The amount of salt in any given food item can vary widely. A slice of white bread can have 80 to 230 milligrams of sodium, for example. A cup of canned chicken noodle soup has 100 to 940 milligrams.

Boston-based cafe chain Au Bon Pain lowered salt in sandwiches and breads by getting suppliers to use fresh vegetables, whole grains and herbs, CEO Sue Morelli said in a release.

Kraft Foods Inc. squeezed salt out of products ranging from steak sauce to bacon partly by substituting potassium chloride, research Vice President Russ Moroz said. It's also salty-tasting, but potassium lowers blood pressure, and most Americans don't get enough of it, Farley said.

The switch works up to a point - generally, about 10 to 15 percent of the sodium content - before potassium chloride causes a bitter or metallic taste, Moroz said. Northfield, Ill.-based Kraft can use other flavors to mask that, but maintaining the taste is "really the challenge in continuing to reduce sodium," he said.

Some companies, meanwhile, have embarked on their own salt-reduction plans.

ConAgra, which makes Chef Boyardee and Marie Callender's products, is following its own 2009 commitment to shave the amount of sodium in its foods by 20 percent by 2015.

Salt was simply reduced in some recipes; others have swapped some table salt with potassium chloride or sea salt, which has lower sodium levels, said Mark Andon, vice president of nutrition at Omaha, Neb.-based ConAgra Foods Inc. Another technique is using finer salt particles, which spread the taste over more surface area; that approach has reduced salt in its Orville Redenbacher and Act II popcorn by 25 percent.

PepsiCo Inc., which makes Frito-Lay products, announced in 2010 that it would cut sodium in key brands by one-fourth in five years. Spokesman Christopher Wyse said Monday that the Purchase, N.Y.-based company was looking for alternatives after a plan to use smaller salt crystals didn't work. The company does offer "lightly salted" chips that have half the sodium of the regular ones.

The Campbell Soup Co. announced in 2009 that it was lowering salt in half its soups, including its famous condensed tomato soup. But two years later, the Camden, N.J.-based company said it was bringing back some higher-sodium soups out of concern about the taste.

Associated Press food industry writer Candice Choi and AP writers Stephanie Nano and Jake Pearson contributed to this report.