Q: Does milk in plastic bottles lose more vitamins than milk in cardboard cartons?
A: Yes, but as long as the plastic is opaque, not clear, the difference is minimal under normal circumstances. With increasing transparency, however, more light can get through to the milk, and that can reduce levels of vitamins A and C, as well as some B vitamins.
When exposed to sunlight or very strong artificial light for long periods, even milk in opaque containers loses some vitamins, though less than milk in clear glass bottles or clear plastic containers. This can also impart an off-flavor.
The milk industry is usually careful to protect milk from the adverse effects of light during processing and storage, and consumers keep milk in dark refrigerators. Even if milk loses some vitamins because of exposure to light or air, it’s still very nutritious. Calcium and other minerals won’t be affected, in any case.
Also see 8 Key Facts About Milk.