When parents discover pumped milk has developed an unusual odor or a distinct taste after thawing, such as soapy, rancid, or metallic, it raises concern. This sensory change is common in expressed milk and is a natural chemical process that occurs during storage. Understanding the cause of this flavor alteration is the first step in managing a freezer stash of milk.
The Primary Cause: High Lipase Activity
The metallic or soapy taste is primarily caused by the activity of an enzyme called lipase, which is naturally present in all human milk. Lipase plays a beneficial role in digestion by breaking down milk fats, or lipids, into smaller components, making it easier for an infant to absorb nutrients. This process begins immediately after milk is expressed from the breast.
In some individuals, the lipase enzyme is highly active, causing the fat breakdown process, known as lipolysis, to accelerate rapidly. This accelerated chemical reaction releases an increased amount of free fatty acids and glycerol. It is these newly liberated free fatty acids that the human palate perceives as a soapy, metallic, or sometimes fishy flavor.
Freezing does not completely halt the action of lipase, but rather slows it down significantly. The enzyme continues to work slowly in cold storage, and the resulting change in flavor becomes noticeable once the milk is thawed. The degree of this flavor change varies widely, depending on the individual’s natural level of lipase activity.
Safety and Acceptance of High-Lipase Milk
The primary concern for parents is whether this milk remains safe for their baby to consume. Milk with high lipase activity is perfectly safe and retains its full nutritional value, despite the change in flavor. The process affects only the taste and smell, not the quality or safety of the milk itself.
The main challenge associated with high-lipase milk is infant refusal. Many babies will reject the milk due to the strong, altered flavor, leading to wasted milk and parental frustration.
If a baby refuses the thawed milk, a practical strategy is to mix it with freshly expressed milk to dilute the strong flavor. The milk can also be blended into solid foods such as purees or baked goods, where the altered flavor will be masked. Offering the milk slightly chilled is another option, as a cooler temperature can sometimes make the taste less noticeable to the baby.
Eliminating the Metallic Taste Before Freezing
The most effective way to prevent the development of a metallic or soapy taste is to deactivate the lipase enzyme before storing the milk. This is achieved through a process called scalding, which uses heat to denature the enzyme. Scalding must be done with freshly expressed milk, as the flavor change is difficult to reverse once it has developed in storage.
Heating the Milk
The process involves gently heating the milk on a stovetop until it reaches a temperature of approximately 180°F (82°C). This temperature is reached when small bubbles begin to form around the edges of the pan, but the milk is not yet boiling. Using a food thermometer is recommended to ensure the temperature is precise, as overheating can reduce the milk’s beneficial properties.
Rapid Cooling
Immediately after reaching the target temperature, the milk must be rapidly cooled, typically by placing the container in an ice bath. This step quickly stops the heating process, minimizing the impact on the milk’s nutritional components. Once cooled, the milk can be transferred to storage containers and frozen for later use without the risk of developing the soapy or metallic taste.
Optimizing Storage to Minimize Flavor Changes
Proper storage practices can help minimize flavor alterations and maintain the quality of the milk. Use clean, food-grade containers, such as glass or BPA-free plastic bags designed for breast milk storage. These containers should be sealed tightly and labeled with the date of expression.
Temperature stability is important for preserving the milk’s quality and is best achieved by storing the milk in the back of the freezer or refrigerator. Placing milk in the door exposes it to frequent temperature fluctuations every time the appliance is opened, which can hasten any flavor changes. Freezing milk within four days of expression is recommended, and using it within six months is optimal to preserve its quality.
While less common than lipase activity, oxidation can cause an off-flavor where fats in the milk react with oxygen. This typically results in a sour or rancid taste, rather than a soapy or metallic one. Ensuring containers are completely sealed and minimizing the amount of air inside the storage bag can help reduce the possibility of this type of flavor change.