Breast milk provides optimal nutrition for babies and contains antibodies that help protect them from illness. However, breast milk is a bodily fluid that can spoil and transmit bacteria, viruses and other pathogens if not stored and handled properly. Leaving expressed breast milk out at room temperature or warming previously frozen milk can increase the risk of bacterial growth and contamination that could make a baby sick.
How long can breast milk sit out unrefrigerated?
The general guideline is that freshly expressed breast milk is good for:
- Up to 4 hours at room temperature (up to 77°F or 25°C)
- Up to 24 hours in the refrigerator (39°F or 4°C)
After that, the milk should be discarded to prevent bacterial growth that could cause illness. The timeframe depends on factors like the temperature and how much bacteria was present to begin with.
What happens if a baby drinks breast milk that has been left out?
Drinking breast milk that has been left out too long or not properly refrigerated can expose babies to harmful bacteria that multiply rapidly at room temperature. If the milk smells rancid, tastes bad, or has changed consistency, it has likely been contaminated and should not be fed to a baby.
Potential risks include:
- Food poisoning – Staph, strep, salmonella
- Gastrointestinal distress – vomiting, diarrhea, dehydration
- Other infections – eye infections, meningitis, ear infections
Babies have immature immune systems and gulp milk down faster than older children or adults would consume contaminated foods or liquids. So even a small amount of bacteria in breast milk can make them sick.
How to store breast milk safely
To avoid the risks of bacterial contamination, follow safe breast milk handling and storage guidelines:
- Wash hands before pumping or handling bottles and pump parts
- Use clean containers, preferably plastic or glass bottles designed for breast milk storage
- Label milk with date it was pumped
- Seal container tightly and store toward back of refrigerator or freezer where temperature is coldest
- Limit storage time – no more than 4 days in fridge, 6 months in freezer
- Do not re-freeze thawed milk
- Thaw frozen milk gradually in fridge or under warm running water – do not microwave
- Gently swirl milk after thawing to mix separated cream back in
- Discard any leftover heated milk within 1-2 hours after the baby is finished feeding
Can breast milk be reheated after refrigeration?
Previously refrigerated breast milk can be safely reheated once, but it is best to limit the time milk is warmed. The CDC recommends:
- Thaw refrigerated milk before warming it
- Warm milk gradually by holding bottle under warm running water or placing in container of warm water
- Do not let water get into the milk
- Avoid microwaving, especially with microwave-unsafe bottles which can create hot spots that scald milk
- Gently swirl bottle to evenly distribute heat and check temperature before feeding
- Never re-heat or re-freeze milk once it has been warmed
After warming, milk should be consumed or discarded within 1-2 hours. Bacteria multiply rapidly as milk cools down to room temperature, so reheating introduces risks of illness the longer milk sits.
Can thawed breast milk be left out?
Once frozen breast milk is thawed, it should be treated like fresh milk when it comes to storage time at room temperature:
- Thawed milk can sit out at room temperature for up to 4 hours
- Keep thawed milk as cold as possible until ready to use
- Do not return thawed milk to the freezer unless it was kept chilled and not left out more than 4 hours
- Discard any unused thawed milk within 24 hours after it has been warmed for feeding
The safest practice is to thaw frozen milk overnight in the refrigerator, where it can keep for 24 hours if not used right away. Thawing at room temperature introduces more opportunities for bacterial growth as the milk warms.
Tips for using breast milk on-the-go
When taking expressed breast milk on outings:
- Pack milk in insulated cooler bag with ice packs
- Limit time milk sits out by keeping cooler bag close by
- Warm chilled milk by holding bottle under warm running water for a few minutes
- Never re-freeze or return milk to cooler bag after warming to use later
Breastmilk can be transported short distances without cooling as long as it is used within 4 hours. For longer trips, packing milk in a cooler is safest.
Signs of spoiled breast milk
Before feeding expressed milk to baby, check that it:
- Smells fresh, not rancid or sour
- Does not contain clumps, flakes or discoloration
- Has not separated, with cream floating to the top
Swirling gently should redistribute any separated cream. If milk smells bad or tastes off, err on the side of caution and discard it.
Can spoiled breast milk make adults sick?
Adults are at lower risk than infants, but it is still possible to get sick from ingesting spoiled breast milk. Potential risks include:
- Food poisoning if milk contains staph, strep, salmonella, etc.
- GI distress – vomiting, diarrhea, cramps, nausea
- Localized infections – eye infection if milk comes into contact
To avoid getting sick, anyone handling and tasting breastmilk should also follow safe food hygiene practices like washing hands before pumping, and refrigerating milk promptly after collection.
Does reheating breast milk kill bacteria?
Simply reheating breast milk is not enough to make contaminated milk safe again. According to the Academy of Breastfeeding Medicine:
- Holding milk at room temperature allows bacteria to rapidly multiply to dangerous levels. Most kitchen gadgets do not reliably sanitize.
- Heating milk just warm enough for drinking does not kill most bacteria or viruses that can cause foodborne illness.
- Breast milk would need to be boiled at 212°F (100°C) for 5-10 minutes to neutralize most pathogens.
- Boiling can impact some protective immunological properties of milk, so is not routinely recommended.
For food safety, it is always best to froze and thaw milk properly so bacteria levels stay low and follow guidelines on time allowed unrefrigerated.
Can leaving breast milk out affect its nutrients?
Letting pumped breast milk sit out too long can affect its nutritional quality as well as safety:
- Lipase enzymes may start breaking down milk fats
- Vitamin C and other nutrients can degrade over time, especially at warmer temperatures
- Immunological components may start to break down after 96 hours refrigerated
- Freezing stops this breakdown which is why frozen milk keeps better
While some decline in nutrients may occur before visible spoilage, following storage guidelines optimizes breast milk’s nutritional value.
Pasteurizing breast milk at home
The only way to reliably kill pathogens in expressed breast milk is through pasteurization. This involves heating the milk hot enough and long enough to kill bacteria and viruses, then rapidly cooling it.
The safest approach recommended by health authorities is the Holder pasteurization method:
- Heat milk to 145°F (62.5°C) and hold for 30 minutes
- Then quickly cool to 39°F (4°C) on ice or in fridge
Heating to that temperature for that duration neutralizes most pathogens while preserving more nutrients than boiling or sterilizing. Home pasteurization requires carefully monitoring with a food thermometer to maintain proper temperatures throughout.
Pasteurization pros and cons
Potential pros:
- Kills pathogens from unsanitary collection or storage
- Allows pooling milk from multiple pump sessions
- Preserves more IgA antibodies than sterilization
Potential cons:
- Destroys some white blood cells and anti-infective properties
- May alter taste/smell
- Equipment can be expensive, process tedious
- Errors could fail to kill bacteria or overheat milk
For most healthy babies, following safe storage guidelines is enough. Pasteurizing is more often done for premature or immunocompromised babies in hospitals.
When to avoid feeding breast milk
It is best not to feed a baby breast milk that:
- Is past recommended storage times, especially if left out over 4 hours
- Smells bad, looks abnormal, or was accidentally contaminated
- Is from a mother undergoing radiation or chemotherapy
- Contains certain prescription medicines, illegal drugs, etc.
Pumping and dumping milk can allow lactation to continue while temporarily avoiding feeding the expressed milk. In some cases, donated banked milk may be an alternative.
Food safety tips for pumping moms
To avoid contaminating expressed breast milk:
- Always wash hands thoroughly before handling pump and bottles
- Clean pump parts properly after each use
- Use clean containers made for milk collection and storage
- Don’t store milk in door of refrigerator where temperature fluctuates
- Label milk with date so oldest can be used first
- Don’t combine fresh milk with previous collections
Following lactation room best practices at work can also reduce contamination risks.
What if a baby drinks spoiled milk?
If a baby accidentally drinks milk that was left out too long or otherwise appears contaminated, watch them closely for any signs of illness like:
- Fever
- Vomiting
- Diarrhea
- Dehydration – no wet diapers, sunken soft spot on head
- Lethargy/irritability
Make sure the baby stays hydrated and contact a pediatrician if concerning symptoms last more than 24 hours.
Bottle feeding pumped milk
To safely bottle feed pumped breast milk:
- Select the bottle nipple size recommended based on baby’s age
- Never warm bottles in microwave which can create hot spots
- Always check temperature before feeding
- Throw away any milk left in bottle after feeding
- Only reheat refrigerated milk once
- Discard milk within 1-2 hours after heating to room temperature
Following guidelines on bottle sanitizing, milk storage times, and warming methods reduces safety risks.
Conclusion
Leaving breast milk unrefrigerated can allow bacteria to multiply to dangerous levels that could make a baby sick. Expressed milk should be promptly chilled, stored within recommended time limits, and handled hygienically to avoid contamination. Reheating or repeated warming of refrigerated milk also introduces risks. Following safe pumping, storage, and feeding practices optimizes the safety and nutritional value of breast milk for infants.
