Breast milk is widely recognized as the optimal source of nutrition and immune support for infants, offering a complex blend of fats, proteins, and antibodies. Many parents rely on pumping and freezing to ensure a steady supply. Freezing does not sterilize the milk; instead, it puts the microbial life within it into a state of deep sleep. Understanding this distinction is fundamental to following safe storage and handling procedures.
How Freezing Affects Bacteria and Microbes
Freezing milk does not kill most bacteria; it stops their ability to multiply and significantly slows their metabolic processes. The effect of extremely low temperatures is bacteriostatic, meaning it inhibits the growth of bacteria, as opposed to bactericidal. When the temperature drops below the freezing point, bacterial replication ceases.
The bacteria become dormant, pausing the spoilage process that occurs rapidly at room temperature. Once the frozen milk is thawed, these dormant bacteria can become metabolically active again. Strict guidelines must be followed for storage duration and post-thaw use. Storing milk in a deep freezer, typically -4°F (-20°C) or colder, is more effective than standard freezer compartments that may fluctuate in temperature.
Understanding Normal Bacterial Presence in Breast Milk
It is a misconception that human milk is sterile, as it naturally contains a diverse and beneficial community of microorganisms. This natural population, known as the human milk microbiota, includes hundreds of bacterial species, such as Staphylococcus, Streptococcus, and Bifidobacterium. This bacteria is transferred to the infant, helping to colonize and shape the baby’s developing gut microbiome.
The presence of these microbes promotes a healthy immune system and reduces the risk of certain infections. Bacteria can also be introduced through external contact, such as from the mother’s skin or improperly cleaned pump parts. Freezing merely suspends the activity of these external microbes, making cleanliness during expression paramount.
Essential Guidelines for Safe Milk Storage
Maintaining hygienic practices when collecting milk is the first step in safe storage. Always wash hands thoroughly with soap and water before expressing or handling pump parts and storage containers. Pump components and bottles must be cleaned and inspected to ensure no residue remains that could contaminate the milk.
Expressed milk should be stored in containers specifically designed for this purpose, such as hard-sided glass or BPA-free plastic bottles, or specialized storage bags. When filling containers, leave about an inch of space at the top, because breast milk expands as it freezes. Labeling each container with the date of expression is essential to ensure the oldest milk is used first.
Freshly expressed milk can be kept in a refrigerator at 40°F (4°C) or colder for up to four days. For longer-term storage, expressed milk can be kept in a standard freezer at 0°F (-18°C) or colder for about six months, though it is acceptable for up to twelve months. Storing the milk toward the back of the freezer, away from the door, helps to maintain a consistent, colder temperature.
Handling and Using Thawed Breast Milk
Frozen milk must be thawed carefully to preserve its nutritional and immunological quality. The safest method is to thaw the milk slowly in the refrigerator overnight, which typically takes about 12 hours. Thawing can also be done more quickly by placing the sealed container in a bowl of warm water or under lukewarm running water.
Never thaw or heat breast milk using a microwave, as this can create dangerous hot spots that could burn a baby’s mouth and destroy beneficial components. After breast milk has fully thawed in the refrigerator, it must be used within 24 hours, starting from the moment it is completely liquid. Thawed milk that has been brought to room temperature or warmed should be used within two hours.
Thawed breast milk should never be refrozen, as the repeated freeze-thaw cycle increases the risk of bacterial proliferation and degrades the milk’s quality. Thawed milk may have a soapy or metallic smell, usually due to the enzyme lipase, which is not an indication of spoilage. If the milk smells sour, it should be discarded, but separating fat layers or a slight odor is often normal.