Health organizations uniformly advise against refreezing breast milk that has fully thawed due to concerns about safety and quality. Once the milk leaves the deep-freeze environment, its composition begins to change, and the risk of microbial growth increases significantly. Understanding the science behind this recommendation—specifically how thawing affects the milk’s microbial load and delicate components—is important for safe handling. The accepted guidelines aim to protect the infant and ensure the milk retains its nutritional benefits.
The Primary Safety Concern: Bacterial Proliferation
Thawing breast milk introduces a significant risk of bacterial growth, which is the main reason health experts advise against refreezing it. Freezing temperatures halt the reproduction of most bacteria, putting them into a dormant state. When the milk is thawed, these microorganisms become active again and begin to multiply rapidly.
The thawing process exposes the milk to temperatures within the “danger zone,” typically between 40°F and 140°F (4°C and 60°C), where bacteria thrive. This range allows for quick proliferation of any bacteria present. While the initial bacterial load in freshly expressed milk is low, multiple thawing cycles can dramatically increase the overall microbial count.
Refreezing the milk does not kill the bacteria that have multiplied; it only makes them dormant again. A subsequent thaw would start with a much higher initial bacterial load, posing a greater risk to an infant, especially one with a compromised immune system. Consuming milk with an elevated microbial count increases the likelihood of gastrointestinal illness, making the milk unsuitable for consumption.
Compromise of Nutritional and Immunological Integrity
Repeated freezing and thawing cycles diminish the quality of breast milk by degrading its delicate components. Breast milk contains numerous proteins, enzymes, and immune factors that can be damaged by temperature fluctuations.
One notable change involves the natural enzyme lipase, which helps infants digest fats. Thawing activates this enzyme, causing it to break down the milk’s fats (lipids) into free fatty acids. If the milk is refrozen and thawed again, this process accelerates, potentially leading to an unpleasant soapy or rancid taste or smell that may cause the baby to refuse the milk.
Immune factors, such as secretory immunoglobulin A (IgA) and lactoferrin, are also susceptible to damage. These protective proteins can be compromised by the physical stress of ice crystal formation and temperature changes associated with a second freeze-thaw cycle. Although the milk retains most of its macronutrients, the reduction in these fragile, disease-fighting properties means the milk loses some of its unique protective qualities.
Guidelines for Using Thawed Breast Milk
Since refreezing is not recommended, parents need clear, practical guidelines to safely use thawed breast milk and prevent waste. The safest method for thawing is to place the frozen milk in the refrigerator overnight, where it can be stored for up to 24 hours after it has completely thawed. The 24-hour countdown begins once all ice crystals are gone, not from the moment the container is removed from the freezer.
If milk needs to be thawed more quickly, it can be placed in a container of warm water or held under lukewarm running water. The use of a microwave should be avoided as it can destroy nutrients and create dangerous hot spots. Once thawed milk is brought to room temperature, it should be used within two hours. This short window is necessary because room temperature significantly accelerates bacterial growth, even with the milk’s natural antimicrobial properties.
After an infant has partially consumed a bottle of thawed milk, the remaining milk should be treated with extra caution because the baby’s saliva introduces new bacteria into the fluid. To ensure safety, any leftover milk should be used within two hours of the feeding starting, or it must be discarded completely. Storing milk in small, usable quantities, typically two to four ounces, can help maximize usage and minimize the amount that needs to be discarded.