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Orange juice from Tropicana, Simply Orange, Minute Maid, Florida's Natural, etc. all claim to be 100% juice, but there's something they don't mention: after the juice is produced, it's sent to large vats, where all of the oxygen is removed. No big deal, you might think - but why do they do that? They do it because oxygen drives spoilage, and they want their cartons to have a long shelf-life; preferably, up to a year.
But this creates another problem: removing the oxygen also removes the orange flavor. To get around that, they turn to companies that specialize in creating perfumes and flavor enhancers. These companies custom-make flavor packs, which are added to the largely oxygen-free juice to make it taste like oranges again. Oh, and they're not listed as an ingredient.