Storing and preparing breast milk

How do I store and prepare my milk?

If expressed breast milk is to be stored, mothers should use appropriate, clean containers. They may be glass or plastic bags, containers, or bottles designed for breast milk storage. The container should be labelled with the date the milk was expressed. The length of time breast milk can be stored safely depends on temperature and other factors. In general, use freshly expressed milk first, then refrigerated milk, then frozen milk. In each case, use the oldest first. Thaw frozen milk in a refrigerator or in warm water, not in a microwave. Cooled milk is also warmed in warm water. The temperature of the milk should always be tested before use by shaking a few drops onto the wrist. If a baby doesn’t finish all the milk in a bottle, throw out the leftover milk within two hours.

A) Describing breast milk storage

Mothers may express and store breast milk for a number of reasons. Most mothers, for example, keep expressed milk in the freezer for unexpected periods of separation from the baby or other emergencies. Others will build up a stash in the freezer in anticipation of returning to work or will express in the morning for use later in the day. In each case the milk needs to be safely expressed and stored to prevent contaminating the milk.

Expressing and storing breast milk will cause it to change slightly. It may also smell and taste different after being stored.

Different storage guidelines may apply for mothers of very small premature babies or other vulnerable ones who are hospitalized and for mothers who express for milk bank donation.

B) Storage containers

1) Routine storage

If your baby is healthy and you are not told otherwise by your health-care providers, to store milk, use clean, appropriate containers including:

  • Glass bottles
  • Plastic bottles or containers designed for breast milk storage
  • Single-use sturdy plastic bags designed for breast milk storage

Ensure that any plastic containers do not contain bisphenol A (BPA).

Ensure that the containers seal well and won’t leak.

Consider choosing clean re-usable containers as they are generally more cost-effective and create less waste.

2) Colostrum

Mothers collecting colostrum may choose to collect it in small needle-free syringes. Colostrum is only present in small amounts and may be wasted when collected in regular containers. Syringes can be used to collect colostrum, store it, and then deliver it right to the baby.

3) Storing milk for premature or vulnerable babies

Very premature and vulnerable babies are often at an increased risk of infection from milk that was not stored properly (Smith and Serke 2016). To prevent contamination of the milk thereby increasing the risk of infection, extra care needs to be taken to ensure colostrum and breast milk is safely expressed and stored.

Some hospitals will give mothers sterile containers to use for collection. They may also have special expressing, collecting, and labelling requirements (Picaud 2018). Mothers who are using reusable storage containers may wish to wash them in the dishwasher or sterilize them by boiling as is done with bottle-feeding supplies.

Please discuss this with your health-care providers.

4) Labelling

Clearly label the milk with the date it was expressed so that you can use the oldest milk first.

The levels of some breast milk components change during the day and night. These variations may be providing nutrients when they are most needed by the baby, teaching and supporting the baby’s internal clock, and synchronizing the mother’s and baby’s body rhythms.

Babies who receive significant amounts of stored milk may benefit from receiving milk that was expressed at a similar time of day to the baby’s feed. For example, a baby fed around 10 a.m. would be given milk that was expressed around this time on a previous day. This requires the milk to be labelled with the time at which it was expressed.

C) Safe storage of breast milk

1) General storage guidelines

Ensure you are storing your milk safely (AAP 2021; CDC 2019):

  • Wash your hands before expressing or handling breast milk.
  • Thoroughly clean pumps and pump parts and then store them in a dry, clean area after use.
  • Use appropriate containers for storage.
  • Warm and cold milk can be combined and milk can be pooled.
  • Do not add fresh milk to already frozen milk within a storage container.
  • Store milk at the back of the fridge or freezer, where it is colder.

2) Preparing milk for freezing

When adding milk to containers for freezing:

  • To avoid throwing out milk, don’t freeze it in amounts larger than what the baby can take in at one feed.
  • Freeze some milk in smaller amounts so that you have enough for a small top-up if the baby needs more milk than expected.
  • Leave a little room at the top of the container for the milk to expand as it freezes.

3) How long to store breast milk

The length of time you can safely store milk depends on a number of factors, including how it was stored and how long it was stored. Once the milk is past the recommended times, it should be thrown out. 

Table: Storage Duration of Fresh Human Milk for Use With Healthy Full-Term Babies (CDC 2019; NHMRC 2012) 

Storage Location and Temperatures

Time

Countertop (room temperature): 25°C (77°F) or colder

4 hours

Refrigerator: less than 4°C (39°F)

Up to 4 days

Refrigerator/freezer compartment: less than -4°C (24°F)

2 weeks

Freezer with separate door: less than -4°C (24°F)

3 months

Deep freezer: less than -18C (0°F)

Within 6 months is best.

Up to 12 months is acceptable

Once the milk is past the recommended times it should be thrown out.

Thawed and previously frozen milk can be kept for 1-2 hours at room temperature 25°C (77°F) or colder. It can be kept up to one day (24 hours) in a refrigerator.  It should not be refrozen. 

Milk that was left over from a feeding (baby did not finish the bottle) should be thrown out within 2 hours after the baby is finished feeding.

Unfortunately, there is not a lot of research for the safe handling and storage of breast milk, and guidelines can also be vague or confusing (Scott 2019). The length of time you can safely store milk depends on a number of factors and it is also possible that storage guidelines will change in the future (Fogleman 2017). 

Breast milk expressed before or during a nipple yeast infection is perfectly acceptable to use. It should not be thrown out.

D) Travelling with expressed milk

If you are travelling, liquid expressed breast milk can be safely stored in an ice-filled cooler for 24 hours (Hamosh 1996).

Regular ice will keep milk cold but not frozen. Rather, frozen milk should be placed in a separate cooler with dry ice or frozen gel packs and dead space should be filled with items such as blankets or insulating clothing.

Dry ice is frozen carbon dioxide and has a surface temperature of about -80 ° C (-100 °F). It will keep milk frozen for longer. This can be helpful if you are shipping breast milk or are travelling for longer periods. Dry ice must be handled with more care than regular ice:

  • It is much colder than regular ice and should never be touched with bare skin. Gloves and tongs can protect the hands.
  • Avoid eye contact by using protective eyewear when handling dry ice.
  • As dry ice warms, it returns to the gas form of carbon dioxide. It can take the place of oxygen in a small room. Always open containers of dry ice in large, well-ventilated rooms.
  • As dry ice warms, it takes more space and can cause containers to explode. Store dry ice in a foam cooler which allows carbon dioxide to escape safely.
  • Airplanes have limits on the amount of dry ice.

Some companies offer cold shipping, however, this will be more expensive than taking the milk with you.

If you are flying without a baby, you may not be allowed to take more than 100 millilitres (3 1/2 U.S. fluid ounces) of liquid expressed breast milk onto the plane in your hand luggage. Regulations vary between countries. Frozen breast milk can be placed in checked luggage.

E) Using expressed milk

In general, expressed milk should be used in the following order:

  1. Freshly expressed milk
  2. Milk in the refrigerator
  3. Frozen

In each case, the oldest milk should be used first.

As freezing will kill the health-promoting bacteria found in breast milk, it is best to use fresh or refrigerator-stored milk first (Pandya 2020).

Babies who receive significant amounts of stored milk may benefit from receiving milk that was expressed at a similar time of day to the baby’s feed.

If delivering breast milk to a child-care provider, clearly label the container with the child’s name and the date the milk was delivered.

F) Thawing and warming expressed milk

1) Thawing frozen expressed milk

Frozen breast milk can be thawed:

  • In the refrigerator overnight.
  • By setting it in a container of warm water right before the baby feeds.

To minimize fat loss from thawed breast milk that was stored in plastic bags, mothers can fold and squeeze the bags before use (Friend 2020).

2) Warming expressed milk

a) Reasons for warming milk

The baby may drink the breast milk cool, at room temperature, or warmed to body temperature. In general, we recommend warming milk to body temperature whenever possible to make it similar in temperature to the milk received while breastfeeding. Babies take in large amounts of milk relative to their size in the first year. Just like adults, quickly taking in large amounts of a cold liquid may not feel all that good.

Premature or small babies in particular may benefit from warming milk (Dumm 2013; Uygur 2019).

b) How to warm milk

Milk can be warmed by setting it in a container of warm water right before the baby feeds. Avoid using water that is too hot as it can overheat the milk. Hot water can also spill and burn other children present (Jeffery 2000; Möhrenschlager 2003).

Do not microwave milk. Microwaving can:

  • Heat milk unevenly and burn the baby.
  • Reduce some of the quality of breast milk.
  • Cause bottles to explode if left in the microwave too long.

Electric bottle warmers can overheat breast milk, possibly damaging some of its components (Bransburg-Zabary 2015).

Always make sure the milk is not too hot before giving it to the baby. This is done by shaking a few drops on the inside of your wrist.

G) Mixing breast milk and infant formula in one bottle

Some mothers have some expressed breast milk for the baby’s supplement but also need to give the baby infant formula. In this case, the breast milk should be given first and the infant formula second, if the baby is still hungry.

Our clinic does not recommend combining breast milk and infant formula in the same bottle. It is impossible to predict how much milk the baby will take and occasionally babies leave milk in a bottle. This combination of breast milk and infant formula should be treated like infant formula and thrown out after the baby has drank from the bottle. If the breast milk is given first, it is only infant formula that is thrown out.

References

Bransburg-Zabary S, Virozub A, Mimouni FB. Human Milk Warming Temperatures Using a Simulation of Currently Available Storage and Warming Methods. PLoS One. 2015 Jun 10;10(6):e0128806
 
Centers for Disease Control and Prevention (CDC). Proper Handling and Storage of Human Milk. Atlanta. Centers for Disease Control and Prevention; 2019 Mar 26 [cited 2019 Mar 31]
 
Dumm M, Hamms M, Sutton J, et al. NICU breast milk warming practices and the physiological effects of breast milk feeding temperatures on preterm infants. Adv Neonatal Care. 2013 Aug;13(4):279-87

Friend LL, Gutierrez Dos Santos B, et al. Method for Removing Thawed Human Milk from a Plastic Storage Bag Impacts Fat Retention [published online ahead of print, 2020 Aug 20]. J Pediatr Gastroenterol Nutr. 2020;10.1097/MPG.0000000000002918 

Fogleman AD, Meng T, Osborne J, et al. Storage of Unfed and Leftover Mothers' Own Milk. Breastfeed Med. 2017 Dec 13

Hamosh M, Ellis LA, Pollock DR, et al. Breastfeeding and the working mother: effect of time and temperature of short-term storage on proteolysis, lipolysis, and bacterial growth in milk. Pediatrics. 1996 Apr;97(4):492-8

Jeffery SL, Cubison TC, Greenaway C, et al. Lesson of the week. Warming milk-a preventable cause of scalds in children. BMJ. 2000;320(7229):235
 
Möhrenschlager M, Weigl LB, Haug S, et al. Iatrogenic burns by warming bottles in the neonatal period: report of two cases and review of the literature. J Burn Care Rehabil. 2003 Jan-Feb;24(1):52-5; discussion 49

National Health and Medical Research Council (NHMRC). Infant Feeding Guidelines. Canberra: National Health and Medical Research Council: 2012 Dec

Pandya SP, Doshi H, Codipilly CN, et al. Bacterial stability with freezer storage of human milk. J Perinat Med. 2020 Sep 14;49(2):225-228

Picaud JC, Buffin R, Gremmo-Feger G, et al. Review concludes that specific recommendations are needed to harmonise the provision of fresh mother's milk to their preterm infants. Acta Paediatr. 2018;107(7):1145-1155
 
Scott H, Sweet L, Strauch L, et al. Expressed breastmilk handling and storage guidelines available to mothers in the community: A scoping review. Women Birth. 2019 Oct 20:S1871-5192(19)30859-5 Smith SL, Serke L. Case Report of Sepsis in Neonates Fed Expressed Mother's Milk. J Obstet Gynecol Neonatal Nurs. 2016 Sep-Oct;45(5):699-705
 
Uygur O, Yalaz M, Can N, et al. Preterm Infants May Better Tolerate Feeds at Temperatures Closer to Freshly Expressed Breast Milk: A Randomized Controlled Trial. Breastfeed Med. 2019 Apr;14(3):154-158