The safe storage of expressed breast milk is important for maintaining its nutritional integrity and minimizing the risk of bacterial proliferation. Human milk contains active components, such as antibodies and living cells, that can be preserved when stored correctly at consistent, cold temperatures. The method and duration of storage depend entirely on how low and steady the temperature remains.
Standard Guidelines for Breast Milk Storage
The temperature of the storage environment is the primary determinant of how long breast milk remains at its highest quality. Freshly expressed milk can be kept in the refrigerator at 40°F (4°C) or colder for up to four days. If the milk will not be used within that window, it should be moved to the freezer right away to protect its quality.
A standard freezer compartment typically maintains 0°F (-18°C) and holds milk for about six months. The longest storage duration is achieved in a deep freezer, a standalone unit that reaches -4°F (-20°C) or colder.
Milk stored in a deep freezer is considered safe for up to 12 months, providing maximum flexibility for long-term storage. While it remains safe for a full year, many experts suggest using the milk within six to nine months for the best retention of quality and nutrients. Placing milk toward the back of any freezer unit, rather than in the door, helps shield it from temperature fluctuations.
Preparing Milk for Freezing
Proper preparation of milk before freezing helps to maximize its storage life and minimize waste. Containers used for freezing should be clean, food-grade, and either hard plastic that is BPA-free, glass, or specialized breast milk storage bags. Avoid using disposable bottle liners or general-purpose plastic bags, as these are not designed for safe, long-term milk storage.
It is most practical to freeze milk in small, usable portions of about two to four ounces, which helps prevent waste if the baby does not finish a larger bottle. When pouring milk into the storage container, it is necessary to leave about an inch of empty space at the top. This headspace accounts for the natural expansion of the milk as it freezes solid.
Before placing the milk into the freezer, every container must be clearly labeled with the date it was expressed. Dating the milk ensures that the oldest portions are used first, following the “first in, first out” principle. If freshly pumped milk is combined with already-chilled milk, it must be cooled to the same temperature before mixing.
Thawing and Using Previously Frozen Milk
Once breast milk is removed from the deep freezer, it should be thawed using a method that preserves its safety and nutritional value. The safest method is a slow thaw, which involves moving the frozen container to the refrigerator overnight. For a faster thaw, the sealed container can be held under lukewarm running water or placed in a bowl of warm water.
Any method involving high heat is strongly discouraged. Never thaw or warm breast milk in a microwave or on a stovetop. Microwaving creates uneven hot spots that can burn a baby’s mouth and can destroy some of the beneficial components of the milk. Once milk is fully thawed in the refrigerator, it must be used within 24 hours.
Thawed milk cannot be refrozen after it has completely turned to liquid. It is common for frozen and thawed milk to appear separated, with a layer of fat rising to the top. This separation is normal, and the milk should be gently swirled to recombine the fat, rather than being vigorously shaken. If the baby prefers warm milk, the container can be placed in a warm water bath just before feeding.