For moms returning to work or just ready for a bit of flexibility in their breast-feeding routine, understanding how to safely store and reheat pumped breast milk is important.

With all the work that goes into building a stockpile of breast milk, you’ll want to make sure that all those nutrients and antibacterial properties are properly preserved.

You can do that by following best practices for storing and reheating breast milk.

Storing Breast Milk

According to the La Leche League, pumped breast milk should be frozen or refrigerated immediately after expressing. Store your expressed breast milk in 2- to 4-ounce amounts in milk storage bags, glass, or stiff plastic containers with tops that fit tightly.

Note that milk storage bags are specially designed for expressed breast milk. Don’t substitute standard kitchen storage bags or disposable bottle liners. Not only are these bags less durable and prone to leaking, the risk of contamination is higher. Some types of plastics can destroy the nutrients in breast milk, too. Before sealing, squeeze out the air in the bag.

If you use plastic bottles, be sure to avoid containers that have BPA (bisphenol A). These containers can be identified with a 3 or a 7 in the recycling symbol. Instead, opt for those made with polypropylene, which will have a 5 in the recycling symbol, or the letters PP. If you’re worried about the leeching potential of chemicals from any plastic container, opt for glass.

Before putting breast milk into any container, make sure to wash it with hot, soapy water. Rinse well, and leave to air dry before using it. Or, use a dishwasher. Take a moment to inspect your containers before adding milk. Never use a bottle that looks damaged in any way, and discard any milk that has been stored in a damaged container. Make sure you also always wash your hands before expressing or handling breast milk.

When filling containers, leave space at the top. Breast milk expands as it freezes, so leaving about an inch at the top will allow for this expansion.

Label your bags or containers with the date expressed and the amount of milk. Also write your child’s name if you may be giving it to a child care provider. Store your bags or containers with expressed breast milk at the back of the refrigerator or freezer. That’s where the air will stay the most consistently cool. If you are using bags, put these into another sealed container for storage.

If you have freshly expressed milk, the Mayo Clinic advises that you can add it to refrigerated or frozen milk if you expressed it earlier in the same day. However, the American Academy of Pediatrics recommends against this. If you do so, make sure to allow the newly expressed milk to cool in the fridge before adding it to the already chilled or frozen milk. Adding warm breast milk to frozen milk can cause the frozen milk to thaw slightly, which can increase chances of contamination.

Reheating Breast Milk

Choose the oldest milk to thaw first. La Leche League advises that frozen milk should be thawed overnight in the refrigerator. You can also place it under a slow, steady stream of cool running water. To heat the milk, slowly increase the temperature of the running water to bring it to feeding temperature.

If you’re reheating milk that’s been refrigerated, use warm running water to take off the chill. You can also heat a pot of water on the stovetop, and place the bottle or bag into the water. Don’t heat the breast milk directly on the stovetop, and never make breast milk hot enough to boil. If you’re using refrigerated milk, you might try offering it to your baby before warming. Some babies are fine with cool milk.

Never use a microwave to heat breast milk. Microwaves can destroy some of the nutritional content of breast milk. There’s also a risk of scalding because microwaves heat liquids unevenly, which can cause hot spots within the container. These hot spots could burn your baby as you’re feeding them.

Note that refrigerated breast milk may look separated, with a thin cream layer on milk and a watery milk layer beneath. This doesn’t mean that the milk has spoiled or gone bad. Just gently swirl the container or massage the bag to redistribute the cream before feeding your baby.

Thawed milk may sometimes have a soapy odor or taste, which is caused by the milk fats breaking down. This milk is still safe to feed to your baby, though there is a possibility that they won’t drink it. If that’s the case, try reducing the length of time you store your expressed milk.

Storing Guidelines

If you’ve thawed milk that your baby isn’t ready to eat, there’s no need to throw it away.

According to the La Leche League, frozen milk that has been thawed can be safely stored in the refrigerator for up to 24 hours. However, it’s generally recommended not to refreeze milk that has been thawed.

The Mayo Clinic shares the following guidelines for how long to keep expressed breast milk.

  • Freshly expressed breast milk can keep at room temperature for up to six hours, though it’s considered optimal to use it or properly store it within four hours. Note that if a room is exceptionally warm, four hours should be the limit.
  • Breast milk that has just been expressed can be kept in an insulated cooler with ice packs for up to 24 hours.
  • Freshly expressed breast milk can be stored in the back of the refrigerator for up to five days. However, it’s considered optimal to use or freeze appropriately within three days.
  • Breast milk that has just been expressed can be stored in a deep freezer for up to one year. Use within six months is considered optimal (you can store breast milk in a normal freezer for 3 to 6 months).

The Takeaway

There are a few things to keep in mind when storing breast milk.

First, the longer it’s stored in either the fridge or the freezer, the more vitamin C is lost from the milk. Second, breast milk that you expressed when your baby was a newborn won’t meet their needs in the same way when they’re even a few months older.

Note that storage and reheating guidelines for breast milk can vary if you have a baby who is preterm, sick, or in the hospital. In these instances, speak with a lactation consultant and your doctor.