What they may not have known is that ammonia - often associated with cleaning products - was cleared by U.S. health officials nearly 40 years ago and is used in making many foods, including cheese. Related compounds have a role in baked goods and chocolate products.

Using small amounts of ammonia to make food is not unusual to those expert in high-tech food production. Now that little known world is coming under increasing pressure from concerned consumers who want to know more about what they are eating.

"I think we're seeing a sea change today in consumers' concerns about the presence of ingredients in foods, and this is just one example," said Michael Doyle, director of the University of Georgia's Center for Food Safety.

Ammonia, known for its noxious odor, became a hot topic last month with the uproar over what the meat industry calls "finely textured beef" and what a former U.S. government scientist first called "pink slime".

Used as a filler for ground beef, it is made from fatty trimmings that are more susceptible to contamination than other cuts of beef, and are therefore sprayed with ammonium hydroxide - ammonia mixed with water - to remove pathogens such as salmonella and E.coli.

After critics highlighted the product on social media websites and showed unappetizing photos on television, calling it "pink slime," the nation's leading fast-food chains and supermarkets spurned the product, even though U.S. public health officials deem it safe to eat. Hundreds of U.S. school districts also demanded it be removed from school lunch programs.

One producer, Beef Products Inc, has since idled three factories. Another, AFA Foods, filed for bankruptcy protection.

The outrage, which many experts say has been fueled by the term "pink slime," seems more about the unsavoriness of the product rather than its safety.

"This is not a health issue," said Bill Marler, a prominent food safety lawyer. "This is an 'I'm grossed out by this' issue."

Still, critics of so-called "Big Food" point out that while "pink slime" and the ammonia in it may not be harmful, consumer shock over their presence points to a wider issue.

"The food supply is full of all sorts of chemical additives that people don't know about," said Michele Simon, a public health lawyer and president of industry watchdog consulting firm Eat Drink Politics.

NOT AS BAD AS IT SOUNDS?

The meat industry has been trying to raise awareness of other foods that contain ammonia, in response to what it has characterized as an unfair attack on a safe and healthy product.

For example, ammonia compounds are used as leavening agents in baked goods and as an acidity controller in cheese and sometimes chocolate.

"Ammonia's not an unusual product to find added to food," Gary Acuff, director of Texas A&M University's Center for Food Safety, told a recent press conference hosted by Beef Products Inc. "We use ammonia in all kinds of foods in the food industry."

Kraft Foods Inc, whose brands include Chips Ahoy cookies and Velveeta cheese, is one company that uses very small amounts of ammonium compounds in some of its products. It declined to specify which products.

"Sometimes ingredient names sound more complicated than they are," said Kraft spokeswoman Angela Wiggins. She also pointed out that ammonia, made up of nitrogen and hydrogen, occurs naturally in plants, animals, water, air and in some foods, including milk.

Wiggins said that in turning milk to cheese, a tiny amount of ammonium hydroxide is added to a starter dairy culture to reduce the culture's acidity and encourage cheese cultures to grow.

"It is somewhat similar to activating yeast for dough by adding warm water, sugar and salt to create the proper environment for yeast growth," Wiggins said.

In the case of ammonium phosphate, used as a leavening agent in baking, she said the heat during baking causes the gas to evaporate so no ammonia is left in the product. "It is quite similar to adding wine to a sauce and cooking away the alcohol."

DON'T ALWAYS COUNT ON LABELS

Compounds such as ammonium hydroxide, ammonium phosphate and ammonium chloride are considered safe in small amounts.

The U.S. Food and Drug Administration granted ammonium hydroxide status as a GRAS, or Generally Recognized as Safe, substance in 1974.

Ammonium hydroxide is also an acceptable ingredient under the conditions of "good manufacturing practices" in dozens of foods, from soft drinks to soups to canned vegetables, according to the General Standards for Food Additives set forth by the Codex Alimentarius Commission, a group funded by the World Health Organization and the United Nations' Food and Agricultural Organization.

A trip to the grocery store revealed ammonium chloride - a salt - present in Wonder Bread and Chef Boyardee Mini Ravioli, made by ConAgra Foods. Ammonium phosphate, another type of salt, is listed on Chips Ahoy cookies.

But ammonium hydroxide, the chemical often used to sanitize the "pink slime," was harder to find.

That is because it is often considered a "processing aid," which is not required by U.S. regulators to be included on food labels.

"If it helps facilitate a process, it's not required and (if) it's used at a percent less than 1 percent, it doesn't have to be declared on the label," said Roger Clemens, president of the Institute of Food Technologists and chief scientific officer of E.T. Horn Co, a private chemical and ingredient company.

He said ammonia in food is now being used less than before, as replacement products gain popularity.

When asked if their products were made with ammonium hydroxide, Sara Lee Corp, Hormel Foods, Kellogg and ConAgra said they were not.

Hershey said it uses "natural cocoa" in most of its chocolates, but in the few products that use "alkalized cocoa," it uses potassium carbonate, not ammonium hydroxide.

General Mills said the company does not discuss its production processes. Campbell Soup Co did not respond to repeated requests for comment.

(Additional reporting by Lisa Baertlein in Los Angeles and Ernest Scheyder in New York; Editing by Tim Dobbyn)

• A Permanent Economic Emergency