Why does my breast milk smell sour?

Nothing is more disheartening than tossing out breast milk, no matter how large or small the quantity. A small number of mothers find that their expressed breast milk smells and even tastes bad after refrigeration or thawing, even if they follow proper milk collection and storage guidelines. In these cases, mothers typically describe the milk as having a sour, metallic, or soapy taste.

In the absence of other causes, it is usually assumed that lipase is the culprit. Lipase is an important enzyme found in human milk. Lipase breaks down the milk fats into small particles that babies can easily digest. Although it’s never been proven, it’s thought that high levels of lipase might explain why the milk of some mothers begins to smell bad, refrigerated or frozen. 

Laboratory testing is the only way to confirm the level of lipase in human milk. To avoid the added expense, some mothers simply test their milk at home by placing small amounts of expressed milk on the kitchen counter at room temperature and in the refrigerator or freezer and checking it periodically for changes in taste or smell. How long the milk can be stored before it starts to smell varies from hours to days to weeks.

Fortunately, only a small number of mothers report noticeable changes in the taste of their milk as a result of storage, but if you are one of them, it can be extremely frustrating. While the milk is still safe to consume, some babies may refuse to drink it if there is a strong change in flavor, though others don't seem to mind a mild change. If your baby refuses to drink the milk, the only option is to discard the milk or donate it to the nearest milk bank

If you notice odor changes in your expressed breast milk after storage, consult your health care provider and/or a lactation consultant. Mothers can heat (scald) the milk before storing it, to reduce lipase activity and prevent odor changes. Scalding requires that the milk be heated until tiny bubbles form around the edge of the pan (approximately 180°F or 82°C). Do not heat to a full, rolling boil (212°F or 100°C). Remove the milk from the heat as soon as the bubbles appear, then cool and store. Scalding will destroy some of the milk's anti-infective properties and may alter nutrient levels, but this is seldom a concern since babies will still accept milk during breastfeeding.

Storage guidelines 

Make milk storage easy and safe by storing your milk in any glass or BPA-free plastic container made for food storage—including plastic bags specifically designed for storing human milk. Label the container with the date and time. Allow room for expansion if you plan to freeze the milk. 

Place a single serving in each storage container—2 ounces per container for younger infants, 4 ounces for older infants. More than one container can be thawed if larger amounts are needed. Storing milk in small volumes causes less waste, since any unconsumed served milk would need to be discarded about an hour after each feeding.

Human milk stored in the refrigerator or freezer should be placed in the middle of the compartment away from the door to avoid temperature changes. Do not store milk in the refrigerator or freezer door. Make sure that all packages in your refrigerator or freezer are sealed well, so that your milk cannot absorb odors from other foods. A box of baking soda placed in the refrigerator or freezer may help to absorb odors. 

To thaw, place the unopened container in the refrigerator or in a pan of warm water. Do not thaw or warm any milk for your baby in a microwave oven. A microwave oven destroys live cells and heats the milk unevenly, which increases the risk of burning your baby. Breast milk can be served chilled from the refrigerator or at room temperature. No heating is necessary. If your baby prefers milk at room temperature, simply place the unopened container in a pan of warm water for several minutes. 

Milk that has been thawed in the refrigerator should be used within four hours once it is removed from the refrigerator or within 24 hours if it is kept in the refrigerator. Milk that has been thawed in a pan of warm water should be used right away or stored in the refrigerator for up to four hours. Fresh milk left in the feeding container should be stored in the refrigerator and used within one hour to complete the feeding. Previously frozen milk left in the feeding container should be discarded. 

For guidelines on how long you can store your breast milk, read this. And you can download our free PDF guide, Breast Milk Storage & Thawing Guidelines for Healthy, Full-term Babies.

Last updated April 12, 2020

Suggested Reads