With proper care and timely use, donated breastmilk can be reused as follows:

Fresh milk
Fresh human milk can safely stand at room temperature for 6 to 8 hours and need not be discarded if the first feeding attempt is incomplete.¹

After feeding
Regarding reusing milk after a feeding, Dr. Ruth Lawrence writes that “whether you can use the remaining milk really depends on how long it sits around. Once the baby takes the bottle, there is a certain amount of saliva that gets in the bottle that creates bacteria and gets saliva enzymes in the milk. This is why it isn’t good to give milk that has been sitting around. But, if it sits for only a half-hour or so, this is fine.”² There can be confusion as to what ‘sits around’ means. Eats on Feets assumes that in this context ‘sits around’ means ‘stand at room temperature.³

See How do I feed the milk to my baby? for efficient, cleaner, and thus safer feeding options.

After refrigeration
Doctors recommend unfinished portions of expressed milk be thrown away when infants do not finish a bottle.⁴ A small study that examined bacterial levels in expressed, partially consumed breastmilk that was stored for 48 hours at 4-6°Cº showed no significant difference between bottles that were partially consumed and those that were not exposed to the baby’s mouth for 5 out of 6 participants. All milk samples had colony counts in the acceptable range.⁵

Most caretakers reheat leftover and refrigerated milk. If the milk is warmed up but not used, it is OK to reheat the milk once. But the more you reheat the more you decrease some of the valuable immunologic properties of the milk.⁶

After freezing
Regarding previously frozen raw milk, it is generally accepted that thawed milk can be kept in the refrigerator for up to 24 hours. See How can expressed milk be thawed? 

Most caretakers reheat (warm) previously frozen raw milk as well (after a first feeding). It is not advisable to leave previously heat-treated milk out for longer than necessary because heat-treating affects the anti-bacterial properties of breastmilk.

In general, when food has thawed in the refrigerator, it is safe to refreeze it without cooking⁷ There is however little information on refreezing thawed human milk. Bacterial growth and loss of antibacterial activity in thawed milk will vary depending on the technique of milk thawing, duration of the thaw, and the number of bacteria in the milk at the time of expression.⁸

Breastmilk, previously frozen or not, can be re-frozen after it has been heat-treated. This is the process that milk banks follow: frozen donated breastmilk is pasteurized, after which it is stored in the freezer.

Reusing heat-treated milk
Research shows that after cooling, flash-heat was successful in completely eliminating bacteria in the majority of samples, and prevented substantial growth for up to 8 h when stored at room temperature.⁹

To date, the safety of reheating previously heat-treated milk¹⁰ has not, to our knowledge, been studied. It should be noted that most literature speaks in terms of heating expressed milk for feeding but that caretakers generally warm the (thawed) milk under running warm water or set it in a container in warm water for a short while to get the chill out of it. Gently warming should not be considered heat-treating. For optimal properties of breastmilk, reheating (warming) milk should not be done directly in a pan on the stovetop nor in a container in boiling water on the stovetop (which would be heat-treating it unnecessarily). However, it is suggested that after a first feeding, milk that was heat-treated should not be refrigerated and reheated again.

Next: How do I clean or sterilize supplies and pump parts?

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1. Lawrence, Ruth. Breastfeeding, A Guide for the Medical Profession. 6th Edition. 2005 p. 438. Print. ↩︎
2. Ruth Lawrence, MD. The Bump. ↩︎
3. This reference is for a room temperature of 10-29ºC/50-85ºF. Studies suggest different optimal times for room temperature storage because conditions vary greatly in the cleanliness of milk expression technique and the room temperature. Anne Eglash, et al. 2017. Breastfeeding Medicine Vol. 12, Number 7. ABM Clinical Protocol #8: Human Milk Storage Information for Home Use for Full-Term Infants ↩︎
4. Anne Eglash, et al. 2017. Breastfeeding Medicine Vol. 12, Number 7. ABM Clinical Protocol #8: Human Milk Storage Information for Home Use for Full-Term Infants ↩︎
5. Brusseau, R. Bacterial Analysis of Refrigerated Human Milk Following Infant Feeding. May 1998. Web. When infants do not finish a bottle of expressed breastmilk, doctors recommend unfinished portions be thrown away. This study examined bacterial levels in expressed, partially consumed breastmilk that was stored for 48 hours at 4-6°C. A portion of unconsumed milk was examined as a control. Samples were taken every 12 hours for bacterial analysis. Tests were performed to identify total colony counts, pathogenic Staphylococci, coliforms, and b-hemolytic Streptococci. This study showed no significant difference between bottles that were partially consumed and those that were not exposed to the baby’s mouth for 5 out of 6 participants. All milk samples had colony counts in the acceptable range of < 105 colony-forming units per milliliter (CFU/ml). Although this project provides evidence that it may be safe to re-feed a child a bottle of breastmilk, due to the small sample size, further tests should be performed. ↩︎
6. Lawrence, Ruth. Breastfeeding, A Guide for the Medical Profession. 6th Edition. 2005 p. 438. Print. ↩︎
7. USDA – Freezing and Food Safety ↩︎
8. Anne Eglash, et al. 2017. Breastfeeding Medicine Vol. 12, Number 7. ABM Clinical Protocol #8: Human Milk Storage Information for Home Use for Full-Term Infants ↩︎
9. K. Israel-Ballard, et al. 2006. Acceptability of Heat Treating Breast Milk to Prevent Mother-to-Child Transmission of Human Immunodeficiency Virus in Zimbabwe ↩︎
10. This can be the case if a donor heat-treats the milk because of high lipase or after a recipient heat treats the milk and there is milk leftover after a feeding for instance. ↩︎