Women who are breastfeeding their babies often pump their breast milk so that it can be used to feed the babies when they are not available to nurse them. Breast milk is put into a sterile breast milk bag and then frozen or refrigerated for later use. These breast milk storage bags are different from the bags sold for use as bottle liners and are not interchangeable.
Breast milk bags are made from a very thick plastic or medical-grade nylon. Some bags can be attached directly to the breast pump, and others are strictly for storage. Most breast milk bags have pre-marked measurements on the outside of the bag as well as a place to note the date that the milk was pumped, or expressed. A few brands have a handle and built-in pour spout to make it easier to transfer the milk to a bottle. Bags typically come with either a twist-tie or double-zip closure.
One of the advantages of storing breast milk in bags versus plastic containers or glass bottles is that the breast milk bags take up much less space. After they are filled, they can be stacked and kept in the freezer or refrigerator. The better bags for saving pumped breast milk will resist puncturing and leaking, both when the milk is frozen and in when it is in liquid form.
To avoid waste, two to four ounces (about 56 to 113 g) of breast milk should be stored in each bag. All of the air should be removed from the bag before it is sealed, especially if the bag will be frozen. A small amount of space should be left at the top of the bag if it will be frozen, because the breast milk will expand as it freezes.
The thick breast milk bags protect the milk from degrading and losing its beneficial components. The anti-bacterial nature of breast milk allows storage bags to be left on the counter at room temperature for four to six hours with no ill effects. The bags also can be kept in a refrigerator for as long as eight days after pumping. If breast milk bags are frozen and kept in a deep freeze, the breast milk will last about six months. In a self-contained freezer, milk will last three to four months, or if it is stored in the freezer compartment inside a refrigerator, the breast milk will keep safely for as long as two weeks.
Milk stored in breast milk freezer bags can be thawed simply by running warm water over the bag of milk. If the bag was stored flat, it will thaw very quickly. Breast milk bags should not be thawed in the microwave or on the stove. Like other non-homogenized milk, breast milk will separate, so it should be shaken gently to remix it before it is used.