Lipase is an enzyme naturally present in all breast milk, playing an important part in the digestion of fats for infants. This enzyme breaks down fat molecules, a process called lipolysis, allowing the baby to absorb essential nutrients more easily. For some individuals, the lipase enzyme is highly active, causing fat breakdown to happen quickly once the milk is expressed and stored.
The rapid breakdown of fats creates free fatty acids, which cause the characteristic soapy, metallic, or rancid smell and taste that develops in stored milk. This altered flavor is not a sign of spoilage, and the milk remains safe and nutritious. The main concern is that some babies may refuse to drink the milk due to the altered taste, requiring strategies to manage the enzyme’s activity.
Deactivating Lipase Through Heat Treatment
Heat treatment is the most effective method to stop lipase activity and prevent the off-taste from developing in stored milk. This process, called scalding, uses heat to denature and permanently inactivate the lipase enzyme. Scalding must be performed on freshly expressed milk, typically within a few hours of pumping, before the undesirable taste begins to form.
To scald the milk, gently heat it in a clean saucepan over medium heat, or use a bottle warmer capable of reaching the necessary temperature. The goal is to bring the milk to approximately 180°F (82°C), indicated by small bubbles forming around the edges of the pan. Monitor the temperature with a thermometer to avoid boiling, as excessive heat can reduce beneficial heat-sensitive nutrients.
Once the target temperature is reached, remove the milk from the heat immediately. Rapidly cooling the milk is necessary to halt the scalding process and prevent bacterial growth. This is best achieved by placing the container of warm milk into an ice bath until the temperature drops completely. The milk can then be stored in the refrigerator or freezer for long-term use without the taste changing.
Optimizing Storage Time to Minimize Activity
While heat treatment stops lipase activity, managing storage time and temperature can slow the process, allowing for use before the taste becomes noticeable. Lipase continues to break down fats even in cold temperatures, though at a reduced rate. For individuals with highly active lipase, the off-taste may appear after only 24 hours of refrigeration or after a week in the freezer.
A simple “taste test” is the best way to determine the milk’s specific lipase threshold. This involves tasting a small sample of freshly expressed milk, then tasting refrigerated samples after 12 and 24 hours, and a frozen sample after a few days. This testing establishes how quickly the milk changes flavor and provides a precise window for use or heat treatment.
For short-term storage, high-lipase milk should be prioritized for use within the determined safe window, often 24 to 48 hours for refrigeration. For longer storage, freezing the milk quickly after expression minimizes the time the enzyme works in the liquid state. While freezing does not stop the enzyme, it slows the activity significantly, sometimes allowing for several weeks of storage before the taste is affected.
Salvaging Affected Milk and Addressing Common Myths
If stored milk has already developed the soapy or metallic taste, it remains safe to feed to the baby. If the baby rejects the flavor, methods exist to make the milk more palatable and ensure it is not wasted. One common technique is to mix the affected milk with a batch of fresh, un-scalded milk or formula.
Starting with a small ratio of high-lipase milk to fresh milk and gradually increasing the proportion can help the baby become accustomed to the flavor change. For babies who have started solids, the high-lipase milk can be mixed into purees, oatmeal, or other foods where the altered flavor is less noticeable. Adding a drop of alcohol-free vanilla extract to the bottle can also help mask the taste.
A common misconception is that changing the maternal diet can lower lipase levels in the milk. Lipase activity is a natural variation and a biological component of the milk, not a result of short-term dietary choices or health status. Therefore, attempting to reduce fat intake or eliminate specific foods will not prevent the lipase-driven flavor change from occurring.