The appearance of layers in refrigerated breast milk is a common observation that can cause concern for new parents. When expressed milk is chilled, a thick, creamy layer often rises to the top, leaving a thinner, more watery substance below it. This visual change is normal stratification for human milk and is not an indication that the milk has spoiled or is unsafe to consume.
Understanding Breast Milk as an Emulsion
Breast milk is scientifically classified as an emulsion, which means it is a mixture of two liquids that do not naturally dissolve into one another, with one suspended in the other. In this case, microscopic fat molecules, known as fat globules, are suspended in a water-based solution that contains lactose, proteins, and other nutrients. This state is maintained at body temperature, but cold temperatures destabilize the mixture.
When the milk is cooled in the refrigerator, the fat globules begin to coalesce and separate from the watery components. Fat has a lower density than the rest of the milk, causing it to rise and accumulate at the top of the container. Unlike commercial cow’s milk, which is processed through homogenization to prevent this separation, expressed human milk retains its natural structure.
Practical Steps for Safe Handling and Re-Mixing
The separated breast milk is safe for the baby to consume, provided it has been stored according to safety guidelines. Before feeding, the layers must be gently recombined to ensure the baby receives the full nutritional content. Re-emulsifying the milk restores the even distribution of fat and other components.
The recommended method for re-mixing is to gently swirl or roll the container between your hands for about 30 seconds. This action is sufficient to blend the layers without disrupting the milk’s structure. Avoid shaking the milk vigorously, as this force can potentially damage beneficial protein structures and fat globules. If warming the milk, place the sealed container in a bowl of warm water or hold it under warm running water, which will also aid in the gentle re-mixing process.
Distinguishing Separation Caused by High Lipase
While physical separation is normal, some parents notice that their stored milk develops a distinctly soapy, metallic, or sometimes rancid odor or taste, even when following all storage guidelines. This chemical change is due to high levels of the enzyme lipase, which is naturally present in all human milk to help infants digest fats. Lipase works by breaking down the fat molecules into smaller components, such as free fatty acids and glycerol.
In milk with elevated lipase activity, this breakdown occurs more quickly during storage, leading to the noticeable change in flavor and scent. Although this milk may be rejected by the baby due to the altered taste, it is not spoiled. The most effective way to manage milk affected by high lipase is to deactivate the enzyme before the flavor change occurs.
This is accomplished by scalding the milk immediately after pumping, which involves heating the milk to approximately 180°F (82°C) until small bubbles appear around the edges, without allowing it to boil. Scalding stops the lipase activity, after which the milk must be rapidly cooled in an ice bath before being refrigerated or frozen. This simple heat treatment preserves the milk’s original, fresh taste for storage.