You can safely store breast milk in:
•Glass or BPA-free plastic bottles with a one-piece, tight sealing lid
•Thick plastic storage bags made especially for storing human milk
For short-term storage, it may be okay to use:
•Plastic bags that are used to line bottles (These should be double-bagged or placed in a larger freezer bag and aren't recommended for long-term storage.)
The benefit of breast milk storage bags is that the milk can be frozen flat, saving freezer space and making for quicker thawing. However, bags can be contaminated more easily since it's easy to get small tears in the bags or for water to get in during the thawing process.