You may be preparing to pump in myriad situations—from the office, during work hours, to at home, before an evening date with your partner. If you aren’t immediately bottle-feeding your freshly pumped breast milk to your baby, you’ll need to store it.

In general, “if it’s a very clean collection and storage method,” says Dr. Laurie Jones, a pediatrician, you may find it liberating to know that your freshly pumped milk is “good for eight hours on the counter and up to eight days in the fridge¹.”

If you are on the go, and need to store breast milk until you get home, you can safely store the milk in a portable cooler or cooler bag with a frozen gel pack for up to 24 hours. If you don’t manage to feed it to your baby within that period, throw it out. 

If you’re pumping multiple times a day at work, Dr. Jones says you can safely consolidate the pumped milk (freshly pumped milk with refrigerated milk from a pumping session earlier that day) into bags or bottles to feed your baby the next day.

To accommodate other needs, such as adding breast milk to baby cereal or other solids (once your baby is ready for solids, that is), try freezing some of the milk in an ice cube tray.

And if your baby doesn’t finish all the milk from a bottle feeding within a two-hour period—regardless of whether it’s been freshly pumped, refrigerated, or defrosted—you’ll need to discard the remaining milk.

Here’s a basic rundown of how to keep your pumped milk safe to feed your baby.

Keep everything clean

Wash your hands before and after pumping. Between each session, wash your pump parts in warm, soapy water and let dry completely. You can save time by keeping extra sets for your Willow Go and Willow 360 on hand. Just rinse after each use, and then batch clean and dry them once a day.

Store it safely

Pump directly into milk bags or pour your pumped milk into a cooler, glass bottles or plastic bags designed specifically to store breast milk. Willow's Portable Milk Cooler keeps up to 16 ounces of milk at a safe temperature for up to 24 hours.

If you use bags, label them with the date and number of ounces pumped, then store on the counter top, the refrigerator, or the freezer. (Let this handy CDC chart be your milk storage safety guide.) 

Store in serving sizes

Get the most out of what you pump by keeping it in serving sizes that make sense for your use. If you’ll be away from your baby for a single daytime feeding session, for example, you might simply fill a 4-ounce bottle with pumped milk (and freeze whatever remains from your pumping session). 

Label and organize it

Label your pumped milk with dates and ounces. For short-term storage in the refrigerator, you’ll want to ensure that the milk you serve is as fresh as possible. For long-term storage, the CDC says you can freeze breast milk for up to 12 months, though thawing and feeding it to your baby within 6 months is optimal. 

Freeze it like you mean it 

Try to freeze your milk bags flat and away from the high-traffic areas of your freezer (they can break easily), ideally in a colder section to ensure longevity. Place them in chronological order so you can easily use them in that order. And if for some reason the bags should thaw (i.e., during a power outage), use them or lose them. 

Minimize milk transfer

With milk fat clinging to bags and bottles, the more you transfer milk between containers, the less fat your baby is likely to get. “Ideally, you pump into containers you will feed from, or you transfer the milk when it’s warm and freshly pumped—which means you take your bottles to work with you, if you don’t pump directly into them,” Dr. Jones says. 

Carefully manage formula feeding

If your baby has been in the NICU or has a complex medical condition, a physician may instruct you to add formula powder to your breast milk. Because you must throw away any milk left over from a bottle feeding within an hour², Dr. Jones advises asking the physician whether the total recommended daily formula amount can be fed in one bottle (mixed with water, based on package directions) rather than split across multiple bottles.

If you are combination-feeding your baby (routinely feeding your baby both breast milk and formula), Dr. Jones advises against combining the substances—keeping bottles of breast milk completely separate from bottles of formula. The reason goes beyond reducing waste.

“The formula deactivates some of the value of the breast milk,” Dr. Jones says³. Specifically, mixing formula and breast milk can affect protein intake, as well as the milk’s ability to retain calcium, phosphorus, and zinc.

Safely thaw and feed

Because thawed breast milk must be consumed or thrown out within a few hours or day, depending on how you thaw it, you should be judicious about both how you thaw and how much you thaw.

There are two ways to safely thaw your frozen breast milk: in the refrigerator overnight, for use the next day; or in or under warm or lukewarm water, for more immediate use. 

If you defrost a bag of breast milk in the refrigerator, it’s safe to keep it refrigerated for up to 24 hours and at room temperature for up to 2 hours. If you defrost the bag using warm water, you must feed or toss it within 2 hours. 

Do not use hot water to thaw breast milk, as the excessive heat may destroy some of its nutrients and antibodies⁴. And never use a microwave for this purpose, either, because in addition to destroying protective properties, it can create hot spots that could burn your baby’s mouth.

Check for freshness

There are a couple ways to tell whether your stored breast milk is no longer safe to feed your baby. 

If it smells rancid or excessively sour, that’s an indication that your milk has “gone bad,” or spoiled due to chemical oxidation. There are two common reasons this happens: 

Your pumped breast milk can also spoil if you are eating polyunsaturated fats or rancid fats, or drinking water with too many copper or iron ions. You can address these dietary issues by reducing your intake of polyunsaturated fatty acids (PUFAs) such as canola, vegetable, sesame, cottonseed, peanut, walnut, and flaxseed oils, along with margarine, and by drinking bottled water instead of tap water. 

Regardless of the reason, when the milk becomes spoiled, it’s not safe for your baby to drink, and it must be discarded.

Troubleshoot an off smell

For some moms, however, breast milk develops a somewhat sour, soapy scent when stored because of the presence of lipase, an enzyme that helps break down fats. The lipase scent tends to increase in intensity with the amount of time stored. 

Unlike oxidation, lipase doesn’t present a safety issue to feed your baby, though your little one may find the scent uninviting and refuse the milk. Instead of throwing out this milk, Dr. Jones suggests tasting it to confirm that it isn't rancid, then mixing it with freshly pumped milk or adding a drop of vanilla extract to it before feeding it to your baby.

For a deeper dive into distinguishing whether your breast milk has undergone chemical oxidation or has become impacted by lipase, see La Leche League’s article on milk issues.

Think about whether you want a modest freezer stash

Maybe you’re heading out for a date with your partner. Or maybe you’re preparing for a multiday work trip away from your baby. In either case, you likely want to leave behind some extra milk. But be careful not to create a massive stockpile of it, warns Dr. Jones.

“No one needs to pump for [their older] baby while feeding their current baby at the same time,” she says. “It is almost impossible and creates a cascade of oversupply problems for mother and baby.” (See our article on managing your supply for more on this topic.)