Finding expressed breast milk has a noticeably sour, soapy, or metallic flavor after storage can be unsettling. While fresh breast milk is typically sweet and mild, a flavor change after refrigeration or freezing is common and often causes alarm. This shift does not mean the milk has spoiled or is unsafe. The difference between palatable fresh milk and milk that develops an off-flavor is usually related to a natural, internal enzymatic process.
The Action of High Lipase in Stored Milk
The most frequent cause of a soapy or metallic taste in stored milk is the enzyme lipase. Lipase is a natural component of human milk, and its primary function is to begin breaking down the milk’s fat into smaller, more digestible components, such as free fatty acids. This process is beneficial, helping the baby absorb fat-soluble vitamins and essential fatty acids needed for growth. In some individuals, the lipase enzyme is naturally more active, a condition called high lipase. When this highly active milk is expressed and stored, the enzyme continues its work at an accelerated rate, and the rapid release of free fatty acids causes the characteristic soapy, sour, or metallic flavor that some babies reject. Despite the altered taste, high lipase milk remains completely safe and nutritionally sound. The issue is purely one of palatability, which can be frustrating if a baby refuses the milk.
Managing Lipase Activity Before Storage
Since the flavor change cannot be reversed once it occurs, the solution for high lipase milk is to deactivate the enzyme before storage. The most effective method is scalding, which involves gently heating the milk immediately after expression. The goal is to heat the milk to approximately 180°F (82°C), which is hot enough to neutralize the lipase enzyme without boiling. Heating the milk beyond this point can degrade some of its beneficial anti-infective components. Use a cooking thermometer to monitor the temperature, removing the milk once small bubbles appear around the edges of the pan. Following heating, the milk must be cooled rapidly, often using an ice bath, before refrigeration or freezing. This quick cooling preserves the milk’s quality. A less time-intensive technique is to mix the older, high-lipase milk with a greater volume of fresh, unheated milk to dilute or mask the altered flavor.
Non-Enzymatic Causes of Flavor Changes
While lipase is the most common cause of flavor changes, other non-enzymatic factors can also alter the taste. A mother’s diet can directly influence the flavor of her milk, as volatile compounds from foods and supplements pass into the milk. Strong-tasting items, such as garlic, mint, or certain spices, can be detectable by infants within a few hours of maternal ingestion. These diet-related changes are transient and do not affect the milk’s safety. Another potential cause is oxidation, where oxygen reacts with the fats in the milk. This process is distinct from lipase activity and can create a sour or rancid odor, though it is less common. Contamination or improper handling can lead to spoilage, resulting in a truly sour taste. Storing milk in unclean containers or exceeding recommended storage times and temperatures can cause bacterial growth. Exposure to strong odors in the refrigerator or freezer can also be absorbed by the milk.
When Flavor Changes Signal a Maternal Health Issue
In rare instances, a sudden change in the milk’s flavor may signal a maternal health issue requiring attention.
Mastitis
The most distinct change occurs with mastitis, an inflammation of the breast tissue, which may or may not include a bacterial infection. Inflammation increases the permeability of the breast, causing a higher concentration of sodium and chloride to enter the milk. This influx of salts gives the milk a noticeably salty taste, which can lead to a baby fussing or refusing the affected breast. Mastitis is typically accompanied by maternal symptoms such as fever, a painful, hot, or red area on the breast, and general malaise. While the salty milk is safe, the mother should seek medical advice to manage the infection or inflammation.
Thrush
A yeast infection, or thrush, on the nipples or in the baby’s mouth, has been anecdotally associated with a change in milk flavor, sometimes described as yeasty or sour. Thrush is primarily characterized by maternal symptoms like burning or itchy nipple pain and visible white patches in the baby’s mouth. If a flavor change is accompanied by pain or other physical symptoms, medical consultation can help confirm the cause.