Nothing is more disheartening than tossing out breast milk, no matter how large or small the quantity. There are mothers who follow all the collection and storage guidelines, only to find that their milk, when thawed, tastes or smells bad. (They 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.

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. How long the milk can be safely stored before it starts to smell varies from hours to days to weeks. Some babies refuse to drink the milk once the flavor changes. Others don’t seem to mind the taste. Unfortunately, there is no way to salvage the milk once it smells or tastes bad. If the baby refuses to drink the milk, the only option is to discard the milk or donate it to the nearest milk bank.

To prevent newly expressed milk from “souring,” mothers can heat (scald) the milk before storing it, to reduce lipase activity. 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 unless all of the milk a baby receives is heat-treated.

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 container. More than one container can be thawed if larger amounts are needed. Small amounts of breast milk can be combined in the refrigerator to make a single serving.

Recommended storage times vary from study to study. Some studies recommend longer time periods, but to be extra safe, store your milk at room temperature (25°C or 77°F) for up to five hours, in the refrigerator (4°C or 39°F) for up to five days, in the freezer section of a refrigerator/freezer (-5°C or 23°F) for up to five months, or in an upright or chest freezer (-20°C or -4°F) for up to 12 months.

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.