Freezing breast milk is straightforward: express it into a clean, food-grade container, leave room for expansion, label it with the date, and place it in the back of your freezer. Frozen breast milk stays at its best quality for about 6 months, though it’s considered acceptable for up to 12 months. The details below will help you store it safely and avoid wasting a single ounce.
Choosing the Right Container
You have two good options: breast milk storage bags designed specifically for freezing, or rigid food-grade containers made of glass or BPA-free plastic with tight-fitting lids. Storage bags are popular because they lie flat in the freezer and stack easily once frozen, saving space. Glass or hard plastic containers work just as well and can be washed and reused.
Don’t use disposable bottle liners or regular plastic bags. They’re thinner, more likely to leak or burst during freezing, and aren’t designed for safe food storage at freezer temperatures.
How to Fill and Label Each Bag
Breast milk expands as it freezes, just like water. Leave about an inch of space at the top of each bag or container so it doesn’t split open. Storing in small portions of 2 to 4 ounces helps in two ways: smaller amounts thaw faster, and you’re less likely to waste milk your baby doesn’t finish in one feeding.
Write the date you expressed the milk on every container before it goes in the freezer. If your baby is in daycare, add their name too. Using a waterproof marker or a label that won’t peel off in the cold keeps things legible months later. Organize your freezer so the oldest milk is in front, and use that first.
Where to Store It in the Freezer
Place milk toward the back of the freezer, not in the door. The door is the warmest spot and experiences the biggest temperature swings every time you open it. A chest freezer or deep freezer maintains a more consistent temperature than the freezer compartment attached to your refrigerator, which makes it a better choice for longer-term storage.
The storage timeline looks like this:
- Room temperature (77°F or cooler): up to 4 hours
- Refrigerator: up to 4 days
- Freezer: about 6 months at best quality, up to 12 months acceptable
Milk stored beyond 12 months isn’t necessarily dangerous, but it loses some nutritional quality and can pick up off-flavors from the freezer over time.
Cooling Milk Before Freezing
Freshly expressed milk is warm, and putting it directly next to already-frozen bags can partially thaw the older milk. The safest approach is to chill freshly pumped milk in the refrigerator first, then move it to the freezer once it’s cold. This is especially important if you plan to combine milk from multiple pumping sessions into one bag. Cool each session’s milk in the fridge before adding it to previously chilled milk, then freeze the combined amount within 24 hours.
How to Thaw Frozen Breast Milk
The gentlest method is moving a bag from the freezer to the refrigerator the night before you need it. It’ll be thawed and ready by morning. If you need it sooner, hold the sealed bag under warm running water or set it in a bowl of warm water. Swirl the container gently as it thaws to remix the fat, which naturally separates during storage.
Never microwave breast milk. Microwaves heat unevenly, creating hot spots that can burn your baby’s mouth. They also break down some of the protective proteins that make breast milk valuable. Stovetop boiling causes the same problems.
Once breast milk is fully thawed, use it within 24 hours. Keep it refrigerated during that window. Don’t leave thawed milk sitting at room temperature for more than 2 hours.
Can You Refreeze Thawed Milk?
It depends on how far the thawing went. If the milk still contains ice crystals, you can safely refreeze it. This sometimes happens during power outages or if a bag was accidentally left in the fridge for a few hours. Once the milk is completely thawed with no ice crystals remaining, don’t refreeze it. The freeze-thaw cycle degrades some of the milk’s immune and nutritional components, and a second full cycle pushes that too far.
Spotting Spoiled Milk vs. High-Lipase Milk
Some parents thaw a bag and notice a soapy or slightly metallic smell. This is usually caused by lipase, an enzyme naturally present in breast milk that starts breaking down fats during storage. High-lipase milk is safe to drink. Most babies don’t mind it, though some refuse the taste.
Spoiled milk is different. It smells distinctly sour or fishy, similar to cow’s milk that’s gone bad. The taste follows the smell: sharp, sour, clearly unpleasant. If your baby suddenly rejects thawed milk they previously accepted, spoilage is a likely explanation. When in doubt, trust your nose. A mild change in smell after freezing is normal. A strong, foul odor means the milk should be discarded.
If you consistently notice the soapy lipase flavor and your baby won’t take the milk, you can scald freshly expressed milk before freezing. Heat it in a pot until you see tiny bubbles around the edges (about 180°F), then cool it quickly and freeze. Scalding deactivates the lipase enzyme and prevents the flavor change, though it does reduce some of the milk’s immune benefits.