When expressed breast milk is refrigerated or frozen, some parents notice a distinct change in its appearance, texture, or odor, which can be alarming. The milk might look separated, slightly foamy, or even take on a soapy or metallic smell that was not present when it was fresh. This common occurrence is not a sign of spoilage but rather a result of a natural biological process taking place in the stored milk.
The alteration is due to the continued activity of an enzyme that is naturally present in all breast milk. Understanding this process can provide reassurance and help you manage your stored milk effectively.
The Primary Culprit: Lipase Activity
The change in stored breast milk is generally caused by the enzyme lipase, which is responsible for breaking down fats. Lipase is a component of all human milk, and its primary function is to hydrolyze the triglycerides (fats) into their smaller components: free fatty acids and glycerol. This process is beneficial because it pre-digests the fats, making them easier for a newborn’s immature digestive system to absorb and utilize.
In some individuals, the lipase enzyme is more active than usual, a condition often called “high lipase milk.” When the milk is expressed and cooled, this enzyme continues its work, accelerating the breakdown of milk fats over time. This enhanced activity causes the sensory change in the milk, particularly when it is stored for several hours or days.
The resulting free fatty acids produce the noticeable change in flavor and scent, often described as soapy, metallic, or sometimes rancid. The intensity of this change can vary greatly between individuals, with some noticing it after only 24 hours of refrigeration and others only after milk has been frozen for a long period. This is a natural variation in the composition of human milk.
Determining If the Milk is Safe to Use
The most common concern for parents is whether milk that tastes or smells different is still safe for their baby to consume. Milk affected by high lipase activity remains completely safe and retains all of its nutritional benefits. The enzyme’s activity does not compromise the milk’s quality or introduce any harmful bacteria; it simply changes the flavor profile.
The milk is still nutrient-dense and provides the same antibodies and growth factors as fresh milk. The primary drawback of high lipase milk is that some babies may refuse to drink it due to the altered taste. A baby’s rejection of the milk is typically a matter of taste preference, not a sign that the milk is unsafe.
It is important to distinguish high lipase milk from milk that is genuinely spoiled from bacterial contamination. Spoiled milk will typically have a sour smell and taste, a chunky texture, and an appearance that is visibly different from the metallic or soapy scent of high lipase milk. If you are unsure, the best way to confirm is by tasting a small amount of the stored milk yourself.
Techniques for Managing High Lipase Milk
To prevent the development of the soapy or metallic flavor, the most effective technique is to deactivate the lipase enzyme shortly after expression. This is accomplished through a process called scalding, which involves gently heating the milk to a specific temperature before cooling and storage. This heat treatment halts the enzyme’s activity, preserving the milk’s original taste.
Scalding Procedure
To scald milk, pour the freshly expressed milk into a clean saucepan and heat it while monitoring the temperature with a thermometer. The milk needs to reach approximately 180°F (82°C), at which point small bubbles will begin to form around the edges of the pan. It is important not to let the milk come to a full boil, as excessive heat can reduce some of the milk’s beneficial properties.
Once the target temperature is reached, the milk must be immediately removed from the heat source. The next step is to cool the milk rapidly to stop the heating process and minimize any potential nutrient loss. Placing the container of hot milk into an ice bath for a few minutes is an efficient way to achieve this quick cooling. Once cooled, the milk can be transferred to storage bags or containers and frozen or refrigerated according to standard guidelines.
Storage and Blending
If scalding is not a viable option, a simpler approach is to minimize the amount of time the lipase has to work before the milk is used. This can be achieved by using the milk within the first 24 to 48 hours of refrigeration, before the taste change becomes significant. You can test small amounts of your milk at different time intervals to determine when the flavor begins to change.
Another strategy is to freeze the expressed milk as soon as possible, as the cold temperature significantly slows the enzyme’s activity. If your baby rejects the stored milk, you may try mixing it with a smaller amount of freshly expressed milk to dilute the altered flavor. This blending method can sometimes mask the soapy taste enough for the baby to accept the bottle.