ATLANTA – Coca-Cola keeps the recipe for its 127-year-old soda inside an imposing steel vault that's bathed in red security lights. Several cameras monitor the area to make sure the fizzy formula stays a secret.

But in one of the many signs that the surveillance is as much about theater as reality, the images that pop up on video screens are of smiling tourists waving at themselves.

"It's a little bit for show," concedes a guard at the World of Coca-Cola museum in downtown Atlanta, where the vault is revealed at the end of an exhibit in a puff of smoke.

The ability to push a quaint narrative about a product's origins and fuel a sense of nostalgia can help drive billions of dollars in sales. That's invaluable at a time when food makers face greater competition from smaller players and cheaper supermarket store brands that appeal to cash-strapped Americans.

It's why companies such as Coca-Cola and Twinkies' owner Hostess play up the notion that their recipes are sacred, unchanging documents that need to be closely guarded. As it turns out, some recipes have changed over time, while others may not have. Either way, they all stick to the same script that their formulas have remained the same.

John Ruff, who formerly headed research & development at Kraft Foods, said companies often recalibrate ingredients for various reasons, including new regulations, fluctuations in commodity costs and other issues that impact mass food production.

"It's almost this mythological thing, the secret formula," said the president of the Institute of Food Technologists, which studies the science of food. "I would be amazed if formulas [for big brands] haven't changed."

Twinkies made a comeback this summer after being off shelves for about nine months following the bankruptcy of Hostess Brands. At the time, the new owners promised the spongy yellow cakes would taste just like people remember.

A representative for Hostess, Hannah Arnold, said in an email that Twinkies today are "remarkably close to the original recipe," noting that the first three ingredients are still enriched flour, water and sugar.

Yet a box of Twinkies now lists more than 25 ingredients and has a shelf-life of 45 days, almost three weeks longer than the 26 days from just a year ago. That suggests the ingredients have been tinkered with, to say the least, since they were created in 1930.

"When Twinkies first came out they were largely made from fresh ingredients," notes Steve Ettlinger, author of "Twinkie, Deconstructed," which traced the roots of the cake's many modern-day industrial ingredients.

Coca-Cola and PepsiCo, the nation's No. 1 and 2 soda makers, respectively, also are known for touting the roots of their recipes.

In the 1980s, Coca-Cola and PepsiCo both switched to high-fructose corn syrup, a less expensive sweetener. The companies last year also said they'd change the way they make the caramel coloring to avoid having to put a cancer warning label on their drinks in California, where a new law required such labels for foods containing a certain level of carcinogens.

Both Coca-Cola and PepsiCo say the sweetener and caramel sources do not alter the basic formulas or taste for their sodas.

This past spring, for example, Coca-Cola welcomed the widespread news coverage of a Georgia man who claimed to have found a copy of the soda's formula and tried to sell it on eBay. The company saw the fanfare as evidence of the public's fascination with its formula, and eagerly offered to make its corporate historian available for interviews to fuel the media attention.

Likewise, the company is happy to reminisce about the backlash provoked by the introduction of New Coke in 1985. The sweeter formula was a marketed as an improved replacement for the flagship soda, and the company points to the outrage that ensued as proof of how much people love the original. According to the emailed statement from Coca-Cola, that's the only time the company ever tried changing its formula.