When expressed breast milk is stored in the refrigerator, it often undergoes visual changes, such as a distinct, chunky layer. This is a common and usually non-alarming phenomenon, indicating a natural shift in the milk’s composition. Understanding the difference between this normal separation and actual spoilage is key to safely managing stored milk.
Normal Reasons Why Milk Separates
The primary reason refrigerated breast milk appears chunky is that it is a natural emulsion that is not homogenized, unlike commercial cow’s milk. When the milk rests, the components naturally separate based on their density. The fat, which is the lightest component, rises to the top of the container.
This cream layer can look thick, clumpy, or curdled, sometimes resembling cottage cheese. The watery, bluish-white layer remains at the bottom, containing proteins, carbohydrates, and other nutrients. This separation is a physical process and does not mean the milk is spoiled or less nutritious.
To restore the milk’s uniform consistency, gently swirl the container to mix the fat layer back into the watery layer. Gentle swirling is preferable to vigorous shaking, which can introduce air bubbles that may contribute to gassiness for the baby. Successful reintegration confirms the change in appearance was merely physical separation, not bacterial contamination.
When Lipase is the Cause of Clumps
Another common cause of altered appearance or taste in refrigerated or thawed milk is the natural enzyme lipase. Lipase is present in all breast milk and serves the function of breaking down milk fats, or triglycerides, into smaller components called free fatty acids, which aids in the baby’s digestion.
In some mothers, this enzyme is highly active, continuing to break down the fats even after the milk is stored (hyperlipase activity). This process can cause the milk to develop a distinctly soapy, metallic, or rancid smell or taste over time. Although the flavor may be off-putting, the milk is still safe and nutritionally sound for the baby.
If the baby rejects the milk due to the altered flavor, scalding can be used to deactivate the lipase. Immediately after pumping, the milk is heated to approximately 180°F (82°C)—just until small bubbles form around the edges—before being rapidly cooled and stored. This heat treatment stops the enzyme activity, preserving the original flavor profile.
How to Determine if the Milk is Spoiled
While separation is normal, it is important to distinguish this from true bacterial spoilage, which renders the milk unsafe. The most reliable way to check for spoilage is the smell test, as spoiled milk will smell distinctly sour, rancid, or fishy, similar to spoiled cow’s milk. This putrid odor is a result of bacterial growth and is different from the soapy or metallic smell associated with high lipase.
If you gently swirl the container and the clumps do not dissolve back into the liquid, remaining stringy or stubbornly separated, this indicates spoilage. Spoiled milk resists reintegration, maintaining a curdy consistency even after warming.
Visual inspection can also reveal signs of spoilage, such as unusual pink, green, or black discoloration, indicating mold or harmful bacteria. If there is any doubt about the milk’s safety, especially if stored beyond recommended guidelines, it should be discarded immediately.
Best Practices for Storing Breast Milk
To maintain the quality of expressed milk and minimize spoilage, follow proper storage guidelines. Store freshly expressed milk in clean, food-grade containers, such as hard plastic or glass bottles with tight-fitting lids, or specialized storage bags. Place the milk in the back of the refrigerator or freezer, as this area experiences the least temperature fluctuation.
The Centers for Disease Control and Prevention (CDC) guidelines suggest that freshly expressed milk can be stored in the refrigerator at 40°F (4°C) for up to four days. If the milk will not be used within this timeframe, it should be frozen immediately to preserve quality. Labeling each container with the date of expression is important for tracking duration and ensuring the oldest milk is used first.