Thousands of types of processed foods -- including many varieties of soups, chips, frozen dinners, hot dogs and salad dressings -- may pose a health threat because they contain a flavor enhancer that could be contaminated with salmonella, the Food and Drug Administration said Thursday.

Officials believe the public health risk is low, and no one is known to have fallen ill as a result of the contamination. But manufacturers voluntarily recalled 56 products Thursday, and that number is expected to balloon in the coming weeks into what could be one of the largest food recalls in U.S. history.

"We don't know precisely how large this recall will get," said Jeff Farrar, associate commissioner for food protection at the FDA. "The potential amount of products . . . is very large."

Salmonella was detected early last month in one lot of the flavor enhancer -- hydrolyzed vegetable protein, or HVP -- made by Basic Food Flavors, as well as inside the company's Nevada manufacturing facility, the FDA said. The company, one of only a handful that make HVP, has an extensive customer list. The additive, which comes as a powder or a paste and is mixed into foods to give them a meaty or savory flavor, is similar to monosodium glutamate, or MSG.

The contamination is believed to date to September 2009, meaning millions of pounds of potentially tainted HVP -- all of which the company has recalled -- was shipped in bulk to foodmakers over five months. Many of those companies then sold their products to other clients, complicating the chain and making it hard for federal officials to gauge the scope of the problem.

"This can potentially be in over 10,000 products," said Michael Hansen, senior scientist at Consumers Union.

The FDA has posted on its Web site a searchable list of products being recalled by manufacturers. It can be found at http:/ / www.accessdata.fda.gov/ scripts/ HVPCP/ .

Federal officials said the public health threat is low because most products containing HVP are cooked during processing or carry cooking instructions for consumers, so any salmonella would be destroyed before the food was eaten. Ready-to-eat products, such as chips and other snack foods, would carry greater risks.

In recent days, FDA officials have told foodmakers that their products do not need to be recalled if they can document that foods containing HVP were heated to appropriate temperatures.

Because of the number of products involved and the uncertainty of the risk, officials have been struggling to find the balance between protection and alarm.

"They're trying to come to some reasonable decision about how to protect the public health but not be so cautious as to be ridiculous and throw out tons and tons of product that may be fine," said Don Schaffner, a professor of microbiology professor and food-safety expert at Rutgers University, who has been advising several foodmakers that bought HVP from Basic Food Flavors.

FDA officials declined to say Thursday when they or state health officials last inspected the Nevada plant, or whether the company had a history of sanitation problems. The FDA was still attempting to determine what caused the contamination.

The company did not return calls seeking comment.

The salmonella bacterium is usually found in animal or human feces. Most healthy people infected with salmonella recover without treatment but experience fever, diarrhea, nausea, vomiting and abdominal pain. Salmonella infections can cause serious problems and even death in the young, the elderly and those with compromised immune systems.

The agency learned about the problem after a foodmaker found the bacterium in a shipment of HVP and reported it under a new law that requires companies to notify the federal government if they detect contamination in a product or ingredient. Before September, the food industry was not required to alert the government to contamination.

"The FDA identified this before any major outbreak [of illness], and I think that is very good news," FDA Commissioner Margaret A. Hamburg said.

Joshua Sharfstein, deputy commissioner of the FDA, said the outbreak underlines the need for the Senate to pass a food-safety bill that has stalled since the House approved it last year. The bill would require companies such as Basic Food Flavors to take actions aimed at preventing contamination in the first place.

"We want to be able to set up preventive standards, so we don't have situations like this at all," Sharfstein said.