Depending on the type of orange juice you buy and when you drink it, you might not be getting as much vitamin C as you think, the Journal of the American Dietetic Association reports. Compared with ready-to-drink (packaged) OJ, frozen concentrates contained significantly more vitamin C -- about 86 milligrams per cup vs. 27-65 milligrams. The reason: Pasteurization and packaging processes destroy vitamin C. What's more, once a container of any type was opened, vitamin C in both types of OJ plummeted by about 2 percent a day. After four weeks, frozen OJ contained 39-46 milligrams per cup, while the ready-to-drink contained 0-25 milligrams. Drink frozen-concentrate OJ within the first week of mixing it or buy ready-to-drink orange juice three to four weeks before its expiration date and drink it within the first week of opening it.