The length of time breast milk can be stored depends whether it is kept at room temperature, in a fridge or frozen. This article shares the recommendations for storing breast milk at different temperatures, and answers frequently asked questions about collecting and using expressed breast milk.
Breast milk storage
The Academy of Breastfeeding Medicine’s (ABM) Clinical Protocol #8: Human Milk Storage Information for Home Use for Full-Term Infants is a useful guide to storage times last revised in 2017. Their information and the thoughts of others are summarised below:
How long does breast milk last at room temperature?
- For temperatures 16–29°C (60–85 °F) the optimal storage time is 4 hours, but 6–8 hours may be acceptable for breast milk collected under very clean conditions at lower temperatures in the range (ABM, 2017)
- For temperatures 27–32°C a storage time of up to 4 hours is recommended (ABM, 2017).
Note: Historically, researchers have speculated that breast milk can be stored longer than four hours. Nwanko et al1 say six hours for storage of mature milk and 12 hours for colostrum at 27-32ºC; Ajusi et al2 say eight hours at room temperature and Igumbor et al3 say breast milk may be acceptable for up to eight hours at 15-27ºC or four hours at 30-38ºC.
How long does breast milk last in an insulated cool bag with ice packs?
Based on a single study by Hamosh et al in 1996 human milk may be safe to use for 24 hours in an insulated cool bag at an average temperature of 15 °C.4
How long does breast milk last in a fridge?
At 4 °C (39 °F) or below, the optimal storage time is 4 days, but 5–8 days may be acceptable for breast milk collected under very clean conditions and stored towards the back of the fridge (ABM, 2017).
How long does breast milk last in the freezer?
The Academy of Breastfeeding Medicine Protocol has slightly different information in two places of their protocol. A summary in Table 1 says at temperatures below -4 °C (24.8°F) the optimal storage time is 6 months however 12 months is acceptable (ABM, 2017). The main text says:
Freezing expressed human milk (-4°C to -20°C = 24.8°F to -4°F) has been demonstrated to be safe for at least 3 months. Evidence indicates that thawed human milk, previously frozen for at least 6 weeks at -20°C (-4°F), has the same bacterial viability and diversity as it did when it was freshly expressed.
Other sources state that breast milk frozen at -18 °C can be stored for six7 or twelve months8. Note that little freezer “ice compartments” within an old style fridge only have a recommended storage time of two weeks (NHS, 2023).
Can I keep milk longer than 12 months in the freezer?
More research is needed to determine whether frozen milk is safe to use after 12 months. The Academy of Breastfeeding Medicine explains that as a general guide foods frozen at -18°C are indefinitely safe from bacterial contamination; however there could be possible changes in milk quality due to the enzymes present (ABM, 2017).
Collecting breast milk for storage
- Wash hands with soap and water
- Clean pump parts as per manufacturer recommendations
- Containers can be washed in hot soapy water and rinsed well.
Cleaning your pump equipment after use
The UK’s National Health Service recommends sterilising all feeding equipment after use until a baby is 12 months old, see Sterilising Baby Bottles (NHS website) for comprehensive information on cold water sterilising, steam sterilising and boiling.
However the American based Academy of Breastfeeding Medicine and US Food and Drug Administration state sterilising is not necessary for milk storage containers and pump parts:
Containers for human milk storage and breast pump milk collection kits must be completely dismantled, washed in hot soapy water and rinsed or washed in a dishwasher, and should always be thoroughly air dried or dried with paper towels. They do not need to be sterilized. If soap is not available, then boiling water is preferable.
All breast pump parts that come in contact with breast milk, such as bottles, valves and breast shields, should be cleaned after each use. It is not possible to completely sterilize breast pump parts at home, even if you boil them. However, sterilization is not necessary to keep these parts safe and sanitary. You can do that by thoroughly washing away germs and bacteria with liquid dishwashing soap and warm water.
Breast milk storage containers
Breast milk can be stored in glass or food grade plastic containers with secure lids, or special freezer bags for breast milk. There are concerns about chemicals leaching from plastic containers into foods or liquids they contain.9 Although moves to use BPA free (bisphenol A) plastics such as polypropylene are helpful there are still many other chemicals in plastics.
Some parents choose to avoid the potential dangers of plastic altogether by using glass containers. Glass is said to be the least damaging to human milk.10 If freezing glass, use special glass containers designed to withstand freezing, leave space for expansion and thaw milk slowly e.g. overnight in a refrigerator.
If using plastic storage bags, use those that are strong and sturdy and specifically made for storing breast milk. Take care the bags of milk are in a suitable container and can’t get punctured by other food packages. As with the hard sided containers, leave space for expansion and squeeze air out of the top of the bag before sealing.
- Use a label to mark all stored milk with the date it was pumped. If it is to be used at day care you can add your child’s name.
- To prevent waste, store breast milk in small amounts equivalent to a typical feed e.g. 60–120ml (2–4 oz).
- Store milk in the coldest part of the fridge, usually at the back rather than in the door compartment.
- If adding newly pumped milk to previously pumped milk—cool down the new milk in the fridge first before combining them, and label with the date of the older milk (ABM, 2017). One study found that pooling expressed milk over a 24 hour period averaged out the differences in calories and nutrients that can be seen with individual expressions—without increasing bacterial counts.11
- Freeze expressed milk within three days of being left in the fridge.1213
- Leave space (e.g. 2.5cm or 1”) at the top of a container for breast milk to expand in the freezing process.
Breast milk doesn’t need any special handling and can sit in a work place refrigerator that is used to store food (ABM, 2017).
Handling breast milk, mixing layers
It’s normal for expressed breast milk to separate into a milk layer and a cream layer and to look blueish, yellow or even brownish.14 Simply mix the layers back together before feeding the milk to your baby .
Gently swirl or give a good shake?
You may have heard that breast milk that has separated into layers should be gently swirled together and never vigorously shaken in case it damages the milk. As breast milk is quite robust, damage seems unlikely by the force of an average hand shaking a container of milk.15 In some situations breast milk is centrifuged at high speed to separate the layers—apparently without concerns about damaging the milk.1617
Using frozen breast milk
What is the best way to thaw frozen breast milk?
The Academy of Breastfeeding Medicine recommends that breast milk can be defrosted:
- Overnight in a refrigerator
- By running the container under warm water
- By standing the container of frozen milk in another container of warm water.
A microwave is not a good way to thaw milk. It can create hot spots in the milk and decreases the milk’s anti infective and health qualities. And heating milk above 40°C can damage enzymes and the immunologically active factors in breast milk (ABM, 2017).
How long does thawed breast milk last?
There are a mix of opinions ranging from advice to use thawed milk immediately18 to within a few hours if left at room temperature (ABM, 2017) to use within 24 hrs in a fridge 1920. Frozen milk is thought to be less able to inhibit bacterial growth compared to fresh refrigerated milk as it loses some of its immune properties (Sears, 2020). However Fogelman et al21 found that human milk might be stable at longer storage times than these quoted in two small pilot studies. They call for more research on previously frozen leftover milk before making new recommendations.
Can thawed milk be refrozen?
The Academy of Breastfeeding Medicine states that it cannot make any recommendations regarding refreezing thawed breast milk due to insufficient research. However Rechtman et al22 found that breast milk was very robust and suggested that human milk that has thawed in a fridge for up to eight hours can safely be refrozen. Kelly Bonyata concludes from their research that if breast milk still contains ice crystals it can be refrozen:
If there are still ice crystals in the milk, then it is still considered frozen– it is safe to either use the milk or return it to a working freezer.
How does freezing affect the quality of milk?
There may be some small changes to the nutritional value of frozen stored breast milk e.g. a reduction in vitamin C content, and a reduction in fat, protein, calories and some of the anti-infective properties.23
Fat, protein, and calories decrease in human milk when frozen for 90 days compared to fresh human milk. Frozen human milk has a significant increase in acidity by 3 months, likely due to ongoing lipase activity, that increases free fatty acids in the milk. Based on a few studies with very small samples sizes, vitamin E appears stable in frozen milk over time, and vitamin C levels decrease significantly after 1–5 months of storage. There is a paucity of research on how freezer storage affects nearly all vitamins and minerals in human milk.
Bioactive factors in human milk variably diminish with freezing. Lactoferrin levels and bioactivity are significantly lower in human milk frozen at -20°C for 3 months.
Taste and smell
Some mothers notice that their previously frozen breast milk tastes or smells soapy, sour or rancid. For more information to explain this and tips to avoid it see My Breast Milk Smells Soapy or Sour.
Milk storage for premature or poorly babies
Because premature or poorly babies are more vulnerable to infection, greater care over milk handling and sterilisation is usually recommended. Check with your doctors what they advise.
Breastfeeding and Human Lactation, Sixth Edition p 409/410 in the chapter “The Use of Human Milk and Breastfeeding in the Neonatal Intensive Care Unit” recommends:
- Wash hands and nails thoroughly before expressing. After pumping, wash pump equipment that has been in contact with breast milk throughly with hot, soapy water and rinse after every use. Sterilise equipment once a day.
- Use fresh milk within four hours of expression if left at room temperature
- Put expressed milk in the fridge straight after expressing as long as the milk will be used within 96 hours (four days) and if not then freeze the milk straight away
- Store each session’s milk separately unless pumping both breasts at the same time i.e. don’t combine sessions
- Transport pumped milk to hospital on ice in an insulated container
- Milk that has been frozen and then thawed should not be refrozen.
FAQs about stored milk
If my baby doesn’t finish their bottle, how long can I keep the leftovers?
General advice varies slightly for the handling of a partly drunk bottle of breast milk. Once the milk has been in contact with baby’s saliva there is a risk of bacterial contamination but few studies are available. The UK’s National Health Service recommends partly drunk breast milk be used within an hour and then discarded24 and The Academy of Breastfeeding Medicine recommends discarding milk within 1-2 hours after baby has finished feeding (ABM, 2017). For further discussion see Kelly Bonyata’s Reusing Expressed Breastmilk (2018). Fogleman et al share a small study with 12 women that indicated the bacteriological and immunological characteristics of left over milk were stable when stored in a fridge at 4ºC for six days. They say more research is needed to determine safe storage times of leftover milk.25
Why does my breast milk smell soapy or sour?
If you are worried that your breast milk smells or tastes like soap, smells sour or like garlic, onion or other food see My Breastmilk Smells Soapy or Sour for further information. Milk that seems to have gone off should not be fed to your baby.
Can I use milk collected in breast shells (drip milk)?
Drip milk collected from one breast in a breast shell while feeding from the other is said to be lower in fat with more skin bacteria and rather than storing this, current HMBANA guidelines advise discarding it.26
Can I store breast milk if I have thrush (fungal infection)?
If a mother has breast or nipple pain from what is considered to be a bacterial or yeast infection, there is no evidence that her stored expressed milk needs to be discarded. Human milk that appears stringy, foul, or purulent should however be discarded not be fed to the infant.
In Breastfeeding Answers the author says that if parents are concerned about potential reinfection, expressed milk can be warmed to 50 ºC (122 ºF) to kill the yeast prior to calling and feeding to baby (Mohrbacher, 2020 p 516).
Opinions vary slightly but most sources agree that the ideal storage times for breast milk are four hours in a sealed container at room temperature, four days in the back of a cold fridge, and six months frozen in a stand alone deep freezer. Although these are the “ideal times”, they err on the side of caution because actually breast milk is so full of antibacterial agents that it can often last longer than these times when collected under clean conditions and fed to healthy term babies.