How do you store, thaw, and reheat breast milk?
Updated: 6 hours ago
Breast milk is special: it's living food. It contains not only fats, carbohydrates, and proteins, but also antibodies, enzymes, and living cells that protect your baby. These precious nutrients are sensitive to heat, light, and time. That is why you store, freeze, thaw and warm expressed milk the right way. It keeps the quality as good as possible, so your baby gets the best every time.
In short
Subject | Short guideline |
What to store in? | Breast milk bags: space-saving, but pour over after thawing and often not in the warmer. Plastic bottles (BPA-free): directly in the warmer, take up more freezer space. Glass bottles: easy to keep clean, directly in the warmer, but fragile. |
Labels | Always note the date and time (day/night milk if necessary) before filling. |
How to fill? | Bottles/jars: up to ¾ full Bags: 150-180 ml max or below the filling line. Always leave 1-2 cm space and squeeze out air |
Freeze portions | Preferably 60-120 ml per portion (for newborns also 30-60 ml) |
Room temperature (16-25 °C) | Fresh milk: 6-8 hours Colostrum: up to 12 hours Source: ABM and CJG Rijnmond. The Netherlands Nutrition Centre keeps to 4 hours. |
Refrigerator (0-4 °C) | Up to 8 days: store at the back/bottom, not in the door Source: ABM. Netherlands Nutrition Centre: 3 days. JGZ guideline: 72 hours. |
Freezer | Freezer compartment refrigerator (≈ -6 °C): up to 2 weeks Small freezer (≈ -12 °C): up to 4 months Deep freezer -18 °C or colder: safe for up to 12 months. Preferably use within 6 months Source: ABM and CJG Rijnmond. Netherlands Nutrition Centre and JGZ guideline: 6 months in the deep freezer. |
Combining milk | Only combine milk of the same temperature. Cool fresh milk before adding it to frozen milk. |
Defrost | Preferably slowly in the refrigerator (12-24 hours) Or under lukewarm running water Preferably no microwave and no hot water |
After thawing | Use within 24 hours (stored in refrigerator). Do not refreeze. Source: CJG Rijnmond. |
Warm up | In a bain-marie or milk warmer on low setting. Until lukewarm (~37 °C). Do not shake, but swirl gently. |
Why not too hot? | High temperatures break down protective substances and can cause hot spots |
Leftovers after feeding | Baby has drunk from it: use within 1-2 hours, then discard (ABM). Not yet drunk: use straight away, discard after 1 hour (Jeugdgezondheidszorg Utrecht). Do not cool down again or refreeze. |
Recognizing spoiled milk | Discard if there is a sour/rancid odor, a foul taste, or a gritty texture that doesn't mix. |
Soapy smell? | Often lipase, not spoiled. Sometimes babies refuse it. Scalding can stop lipase, but it also costs antibodies (only if necessary). |
Important to remember | Clean working Always note the date Never refreeze thawed milk Preferably small portions → fewer leftovers |
Contents
Where do you store breast milk?
First of all: think about where you are going to store your breast milk.
That really makes a difference to the quality.
Preferably choose
Sturdy, special breast milk bags: You can freeze these flat. That is very handy, because they take up little space in your freezer. Note: after thawing you pour the milk into a bottle. Most bags are not suitable for a milk warmer.
Well-sealed hard plastic bottles: Big advantage: you can simply place them in the milk warmer. Less convenient: they take up more space in the freezer. And heating plastic can release substances into the milk. So always choose BPA-free.
Well-sealed glass bottles: These are sturdy and easy to clean. They can go in a milk warmer without the risk of substances from plastic. They are more fragile, though, and take up more space in the freezer.
Regular freezer bags? They leak more easily, so preferably not.
Which bottle works best at feeding time is covered in the article on choosing a baby bottle.
Which one you choose depends mainly on what you find practical. Do you want to save freezer space? Choose bags. Do you want to avoid pouring over? Choose bottles or glass.
Smart labeling
Always write the date and time on the pump bag before filling and freezing it.
This is the easiest way to write it down. And later you will know exactly how old the milk is. You also see which portions to use first.
Some parents also like to note whether it is day or night milk (for example with a sun or moon).
Breast milk changes with the daily rhythm. Morning milk generally contains slightly more "wake-up" substances like cortisol. Evening and night milk contains more melatonin. In theory it can help to feed your baby around the same time as you pumped the milk. The evidence that this makes a real difference in practice is still limited. See it as a pleasant bonus, not a requirement.
How full can you fill it?
Because breast milk expands during freezing, fill in:
Bottles or jars: up to approximately ¾ full
Breast milk bags: maximum 150-180 ml per bag, or fill them to just below the fill line if indicated
Rule of thumb: always leave at least 1-2 cm of space at the top.
Always squeeze as much air as possible out of breast milk bags before closing them.
Portion sizes for freezing
It's helpful to freeze breast milk in 60-120 ml portions. This is roughly an average feeding, so you won't have to throw away leftovers as often.
It's better to freeze several small 60 ml portions than one large one. You can always thaw two portions, but once thawed, you can't refreeze.
Do you pump regularly? The article on choosing a breast pump explains which pump suits your situation.
How long can you store breast milk?
Room temperature (16-25 °C)
Fresh breast milk: 6 to 8 hours. That is the line of the ABM and CJG Rijnmond, and it only applies if you work hygienically. The Netherlands Nutrition Centre keeps to 4 hours.
Colostrum: up to 12 hours, according to CJG Rijnmond, at 19 to 22 °C.
Breast milk contains substances that inhibit bacterial growth. If you don't expect to use the milk within a few hours, it's best to refrigerate it.
Refrigerator (0-4 °C)
Fresh breast milk: up to 8 days. That is the wider line of the ABM, for a healthy full-term baby and hygienic handling. The Netherlands Nutrition Centre keeps to 3 days, the JGZ guideline to 72 hours.
Store the milk at the back and bottom of the refrigerator. Don't put it in the door, as the temperature fluctuates there. You can combine milk of similar temperatures. The date of the oldest milk counts towards the entire batch.
Tip: Going out, going to work, or want to keep expressed milk cool overnight without having to constantly run to the refrigerator? Then you can use a special bottle or milk cooler.
This is ideal if you want to keep your milk cold for longer. Or if you want to leave the bottle on your bedside table after pumping. Place a milk warmer next to it, and you won't even have to get out of bed.
Freezing breast milk
How long you can store breast milk in the freezer depends on the type of freezer. As a rule, the colder and more stable the temperature, the better the quality is preserved.
Freezer compartment in refrigerator (approximately -6 °C): can be kept for up to 2 weeks. The Netherlands Nutrition Centre gives the same period.
Small freezer/table model (approximately -12 °C, opened and closed frequently): shelf life up to 4 months. This step only appears with La Leche League, not in the Dutch guidelines.
Large freezer with separate door (-18 °C or colder): safe for up to 12 months according to the ABM and CJG Rijnmond. Preferably use the milk within 6 months. The Netherlands Nutrition Centre and the JGZ guideline keep to 6 months.
Do you plan to breastfeed for longer, and do you have the space? Then a separate freezer might be worthwhile. It helps if you can easily reach the oldest bags, so a freezer with drawers or compartments often works better than a chest freezer. You have to dig less to reach the oldest milk.
First cool the fresh milk in the refrigerator and preferably freeze it within 4 days
Don't add fresh, warm milk to fully frozen milk. The temperature difference can reduce the quality and cause partial thawing. If you do want to combine the milk, cool it completely in the refrigerator first. Then both are equally cold before you freeze them together.
Thawing breast milk
When thawing, always choose the oldest date first. As long as that milk is still within its shelf life. If the milk has been stored longer than recommended, it is best to discard it. Milk that has been frozen for too long can lose quality and is then less safe for your baby.
Safe ways to thaw
Slowly in the refrigerator (recommended): Place the milk in the refrigerator for 12-24 hours until it
is completely thawed.
Thaw faster: Hold the bottle or bag under lukewarm (not hot!) running water and move it gently back and forth.
Using a milk warmer: This is only permitted if the milk is already (partially) thawed or has come straight from the refrigerator. Use a low or standard setting and avoid high temperatures.
In a bain-marie (in a bowl of warm water): Place the bottle in a bowl or pan of lukewarm water. Do not boil or use boiling water.
It is best to use thawed milk within 24 hours (stored in the refrigerator). That period comes from CJG Rijnmond.
How to warm breast milk safely
First of all, only heat the milk just before feeding. Place the bottle or jar of (thawed or refrigerator-cold) milk in a bowl or container of warm water. This is called a bain-marie. You can also use a milk warmer on a low/standard setting. Let the milk warm gently, swirling the bottle occasionally to mix the fat layer. Do not shake.
How warm should it get?
Start at lukewarm, about body temperature (approximately 37 °C). Test a drop on your wrist. It should feel neutral/warm, but not hot.
Do not make it too hot
At high temperatures, protective substances break down, such as antibodies, enzymes and living cells. These are the very “magical” components that help protect your baby from infection. The composition of the milk can also change, as fats and proteins are damaged by heat. Furthermore, if milk is heated unevenly, especially in the microwave, hot spots can develop that can burn your baby. And sometimes the flavor changes, making babies less likely to drink it. The Dutch JGZ guideline does allow the microwave, as long as you stir the milk halfway through. Even so, it is better not to use it: the milk heats unevenly and antibodies are lost.
Therefore, always warm your breast milk gently until it is lukewarm (approximately body temperature) and not warmer.
Leftovers after a feeding
Milk your baby has drunk from
Use within 1-2 hours. Then discard. That is the advice of the ABM.
As soon as your baby drinks from the bottle, bacteria are introduced into the milk. These bacteria multiply faster in warm milk, so don't save leftovers for later or reheat them.
Warmed but not yet drunk
Use warmed milk straight away. If it has not been finished within one hour, throw it away. Do not reheat breast milk. That is the advice of the Dutch child health service Jeugdgezondheidszorg Utrecht.
Do not cool down again or refreeze.
Important to remember
Never refreeze thawed milk
Do not reheat breast milk
Always warm breast milk gently to lukewarm, not hot
How do you recognize spoiled milk?
Normal breast milk can vary in appearance. There might be a layer of fat floating on top that mixes nicely when you gently swirl the bottle. The colour can range from yellow or cream to light blue or even a little greenish. This is all normal.
But just like regular milk, breast milk can also spoil. Fortunately, this is usually easy to recognize:
smells sour or rancid
tastes distinctly unpleasant
remains grainy and crumbly (even after you gently stir it)
If you see or smell this, throw the milk away.
Lipase: a “soapy” smell is not spoiled milk
Sometimes breast milk smells sharp or slightly soapy after cooling or freezing. This is due to the enzyme lipase, which breaks down fats. This is not a spoilage, and the milk is perfectly safe to feed.
So the milk is good, but some babies dislike the taste and refuse the bottle. If this happens, you can heat the milk briefly right after pumping, until small bubbles appear around the edge. This is called scalding: heating to just below boiling point, around 82 °C. It is not the same as pasteurising, which keeps milk at 62.5 °C for 30 minutes. Scalding stops the lipase, but it also costs antibodies and enzymes. No Dutch guideline recommends it, so discuss it with a lactation consultant first.
Longer shelf life than you think
Breast milk is full of antibodies and therefore often keeps longer than many people think.
However, official guidelines are sometimes cautious, as relatively little research has been conducted on daily use in home settings. Therefore, storage times have been deliberately kept well to the safe side.
Breast milk is robust and precious, even after cooling or freezing, it remains a wonderful food. With clean materials, good labels, and the correct temperatures, you can go a long way. And remember: these are guidelines, not stress-inducing rules.
If something accidentally falls outside the guideline, use your senses: look, smell, and even taste a drop. If you're unsure about the smell or quality, it's best to throw the milk away. Safety first.
How calm and safety help your milk flow is covered in the article on the invisible side of breastfeeding.
Colostrum you collect before birth is stored in the same way.
Frequently asked questions
How long can you keep breast milk outside the fridge?
Fresh breast milk can be kept at room temperature (16-25 °C) for about 6-8 hours. Colostrum up to 12 hours. That is the line of the ABM and CJG Rijnmond. The Voedingscentrum keeps 4 hours.
How long does breast milk stay good in the fridge?
Up to 8 days at 0-4 °C according to the ABM, stored at the back or the bottom of the fridge. The Voedingscentrum keeps 3 days.
How long can you freeze breast milk?
In a freezer at -18 °C or colder up to 12 months according to the ABM, preferably used within 6 months. The Voedingscentrum and the JGZ guideline keep 6 months.
Can you freeze breast milk again after thawing?
No. Thawed breast milk should not be frozen again.
What is the best way to warm breast milk?
In a water bath or with a bottle warmer on a low setting, until it is hand warm. Preferably not in the microwave, because it heats unevenly.
How do you recognise spoiled breast milk?
Spoiled milk smells sour or rancid and stays grainy. A soapy smell caused by lipase is not spoilage.
Love,
Bobby
Sources
Written by Bobby, maternity nurse at Bobby Kraamzorg. Last updated: September 2026.
Bobby believes in being transparent. Some links in this article are affiliate links. This information is free and stays free, for everyone. Bobby only recommends products she would recommend without a fee, and works with no sponsors. If you order through such a link, a small commission goes to Bobby. It is a way to support her, if you want to.
