A fresh glass of orange juice and oranges are seen here in this undated photo. (Getty Images)

Natalya Murakhver, a New York food writer and mother of an 18-month year old daughter, loved her premium brand orange juice — the “100 percent pure” and “not from concentrate” kind that comes in the colorful carton and tastes consistently delicious.

That is, until she said she learned from her first-time moms group that there’s a “secret ingredient” in all premium orange juices that companies are not required to put on their labeling.

Now, after writing Whole Foods, she refuses to buy her favorite, “365″ juice, amid uncertainty about its contents.

“One of the moms said she had read about [how the juice is made] and they held it in tanks for up to a year and it pretty much lost all of its flavor and had to be reinvigorated with these flavor packs, which are essentially chemicals,” said Murakhver, 40, and co-author of “They Eat What?: A Cultural Encyclopedia of Weird and Exotic Food from around the World.”

For the last 30 years, the citrus industry has used flavor packs to process what the Food and Drug Administration identifies as “pasteurized” orange juice. That includes top brands such as Tropicana, Minute Maid, Simply Orange and Florida Natural, among others.

Murakhver said the addition of the flavor packs long after orange juice is stored actually makes those premium juices more like a concentrate, and consumers need to know that.

Experts estimate two-thirds of all Americans drink Florida orange juice for breakfast, and companies spend millions on their marketing campaigns touting its health benefits.

The “not from concentrate” brands appeared on store shelves sometime in the 1980s to differentiate them from frozen juice and other bottled concentrates. Despite its high price tag — now up to $4 a carton — sales of the premium brands have soared.

But those juices don’t just jump from the grove to the breakfast table.

After oranges are picked, they are shipped off to be processed. They are squeezed and pasteurized and, if they are not bound for frozen concentrate, are kept in aseptic storage, which involves stripping the juice of oxygen in a process called “deaeration,” and kept in million-gallon tanks for up to a year.

Before packaging and shipping, the juice is then jazzed up with an added flavor pack, gleaned from orange byproducts such as the peel and pulp, to compensate for the loss of taste and aroma during the heating process.

Different brands use different flavor packs to give their product its unique and always consistent taste. Minute Maid, for example, has a distinctive candy-sweet flavor.

Kristen Gunter, executive director of the Florida Citrus Processors Association, confirmed that juices are blended and stored and that flavor packs are added to pasteurized juice before shipping to stores.

Flavor packs are created from the volatile compounds that escape from the orange during the pasteurization step.

But, she said, “It’s not made in a lab or made in a chemical process, but comes through the physical process of boiling and capturing the [orange essence].”

The pasteurization process not only makes the food safe, but stabilizes the juice, which in its fresh state separates. Adding the flavor packs ensures a consistent flavor.

The United States Department of Agriculture (USDA) grades the quality of the juice based on color, flavor and defects.

“To get grade A, we have to blend it,” she said. “Because oranges and their growing seasons vary, both the Valencia — ‘king of the oranges’ — and its lesser cousin, the Hamlin, are combined in the process.

“A processor is faced with harvesting the crop and giving the consumer some sense of what [he or she] might be getting,” she said. “You buy branded orange juice, you kind of want it to taste, generally, the same. That expectation is met by blending different varieties and, in order to blend, storage is involved.”

The Food and Drug Administration does not require adding flavor packs to the labeling of pasteurized juice (which includes the from-concentrate as well as the not-from-concentrate versions), because, “it is the orange,” said Gunter.

Non-pasteurized juice must be labeled as such, with warnings about potential pathogens. These regulations have been in place since 1963, she said.

As for the New York City mothers, Gunter said, “I don’t think there has been a large outcry.”

“If consumers have the false impression that pasteurized orange juice is not heated or treated because they have a picture of an orange on the carton, then they are not informed,” said Gunter.

“There’s a lot of literature and movies taking the food manufacturers to task on food preparation,” she said. “We have left the farms and it’s just not possible to feed everybody. I love the raw-food crowd, but we cannot get that many oranges out to that many people before they go bad in refrigeration.”

But Alissa Hamilton, a former food and policy fellow at the Institute of Agriculture and Trade, said that modern technology is so “sophisticated” that these flavor pack mixtures “don’t exist in nature.”

“They break it down into individual chemicals,” she said. “The flavor of orange is one of the most complex and is made up of thousands of chemicals.”

“They are fine-tuned so each company has its trademark flavor,” said Hamilton, who is author of the 2009 book, “Squeezed: What You Don’t Know About Orange Juice.”

One that is used in a variety of foods, including alcoholic beverages, chewing gum and as a solvent in perfumes, is ethyl butyrate.

According to Doug Kara, a spokesman for the FDA’s food safety division, the chemical is “generally recognized as safe as a food additive for flavoring.”

“The orange juice companies market their premium brands as fresh-squeezed and better than concentrated,” said Hamilton. “But it’s a heavily processed product.”

She advises on the blog, Civil Eats, that the freshest orange juice can be bought in May when the bright and flavorful Valencia oranges are harvested and have “not spent months in storage.”

She adds that consumers can eat a whole Florida orange, which is higher in vitamin C than processed juice and much tastier.

As for health risks, Hamilton said, “I don’t know,” but many of the oranges used for juice come from mega-producer Brazil, where regulation of pesticides is not as stringent as in the U.S.

Still, according to the FDA’s Karas, “We do screening of imports, and imported foods need to meet the same standards as do foods grown or produced domestically.” Mothers such as 36-year-old Yujin Kim, who has a 3-year-old and a 4-month-old, said she is concerned about what is in her orange juice.

“It’s not arsenic but still something I didn’t know I was drinking, so I ended up researching juice machines and bought one today,” said Kim, who lives in New York City. “I definitely will not be buying any juice from now on.”

“It makes sense that they would need to add chemicals for it to last through the transit time and for the consumers to buy and store at home,” she said. “It’s just wrong that they aren’t being transparent about it. We as a consumer have a right to know exactly what’s in the foods we are buying.”

Her friend, Murakhver, said she has been buying “365″ from Whole Foods “for years” and was under the impression that “all the ingredients were disclosed.”

“It’s arguable if it’s bad for you or not. Still, it’s a secret ingredient and no one seems to know about it,” she said. “‘Oranges’ is all it says on the label — a perfect product.”

Concerned, Murakhver wrote to Whole Foods and got an email response, which she shared with ABCNews.com.

Whole Foods spokesman Julie Campbell wrote that she was unable to disclose the name of the company that makes its orange juice, “as that information is proprietary.”

“Flavor Packs are typically made by fractional distilling the oil from orange peel; essentially concentrating the components,” she wrote. “Flavor packs are used by other brands to standardize their products. We accomplish the same thing by blending orange juice from different varieties and parts of the season together.”

“I don’t know what that means,” said Murakhver.

“There hasn’t been a day in the last three years that we’ve not had it in the fridge and at the top of the shopping list with the milk,” she said. “We are going to get a juicer and eat fresh fruit every morning and try to get our sugar high from fresh fruit.

“I like vintage champagne, not vintage orange juice,” said Murakhver.