How will I know if breast milk is spoiled?

Determining if breast milk is spoiled can be tricky for new mothers. Breast milk is a natural food source for babies that contains important nutrients to support their growth and development. However, like any food product, breast milk can spoil under certain conditions. This article provides helpful information on signs of spoilage, proper storage techniques, and guidance on what to do if breast milk goes bad.

What causes breast milk to spoil?

Breast milk can spoil due to bacterial growth or oxidation. Bacteria thrive in warm, moist environments and can multiply rapidly in expressed milk if not stored properly. Oxidation occurs when fats in breast milk are exposed to air and break down, causing rancidity. This gives spoiled breast milk a sour, soapy smell and taste.

Factors that contribute to spoilage include:

  • Improper storage temperatures – Milk stored at room temperature for over 4 hours can spoil.
  • Old milk – Milk that is pumped more than 5 days ago is at higher risk of bacterial growth.
  • Poor hygiene – Unclean pump parts or storage containers can introduce bacteria.
  • Multiple freeze-thaw cycles – Repeatedly freezing and thawing milk breaks down nutrients and allows oxidation.

Following proper handling guidelines helps prevent breast milk from spoiling prematurely.

How can I tell if breast milk is spoiled?

Watch for these signs that indicate breast milk is no longer safe to feed your baby:

  • Change in color – Fresh milk is white or slightly yellow. Yellow, grey, blue, or pink hues indicate spoilage.
  • Change in smell – Milk that smells rancid, soapy, or sour has likely gone bad.
  • Change in taste – Sour, bitter, or metallic flavors are a sign of spoilage.
  • Lumpy texture – Milk separating or containing clumps or curds has spoiled.
  • Mold – Any presence of mold means milk should be discarded.

Keep in mind that thawed, previously frozen milk may smell and taste different due to changes during the freeze-thaw process but is generally still safe if properly stored.

How should I store breast milk to prevent spoilage?

Follow these breast milk storage guidelines to help prevent spoilage:

Room Temperature Storage

  • Store freshly pumped milk at room temperature (up to 77°F) for up to 4 hours.
  • Cover milk containers and avoid direct sunlight which can degrade nutrients.
  • Discard any milk that exceeds room temperature storage time.

Refrigerator Storage

  • Refrigerate milk within 4 hours of pumping. Keep at 39°F or lower.
  • Store milk towards the back where temperature is coldest.
  • Refrigerate for up to 4 days from the time it was pumped.

Freezer Storage

  • For long term storage, freeze milk immediately after pumping.
  • Store frozen milk at 0°F or lower.
  • Milk can typically be frozen for up to 12 months.
  • Limit freeze-thaw cycles and thaw in refrigerator overnight before use.

Proper Handling

  • Wash hands and all pump parts thoroughly before pumping.
  • Transfer milk to clean storage bottles, bags or containers made specifically for breast milk.
  • Label containers with date pumped and amount.
  • Seal containers tightly to prevent leakage and contamination.

What should I do if breast milk smells or tastes sour?

If your breast milk has an off smell or taste, it is safest to discard it. While sour milk does not always indicate spoilage, rancid odors suggest bacterial growth or oxidation that make it unsafe for your baby. Erring on the side of caution can prevent illness.

Take these steps if your milk smells or tastes off:

  1. Smell and taste a small drop first before feeding to baby.
  2. If any odor or taste seems awry, discard the milk.
  3. Clean pump parts and storage containers thoroughly before next use.
  4. Review storage guidelines and ensure pumps parts and containers are fully dry before use.

If milk smells odd after thawing previously frozen milk, it may be from changes during freezing. Taste a small drop to determine if it is truly spoiled before discarding large amounts of pumped milk.

Can I re-freeze breast milk after thawing?

Previously frozen breast milk that has been thawed in the refrigerator can be safely re-frozen once. Follow these steps:

  1. Thaw frozen milk overnight in the refrigerator.
  2. Use thawed milk within 24 hours.
  3. Only re-freeze unused milk that still contains ice crystals and is 40°F or colder.
  4. Store re-frozen milk towards the back of the freezer.
  5. Use re-frozen milk within 24 hours after thawing a second time.

Avoid re-freezing thawed milk more than once, as this can allow bacteria growth and lower nutritional value. Re-frozen milk also tends to separate easily when thawed.

Is it safe to add freshly pumped milk to already refrigerated milk?

It is generally safe to combine freshly pumped milk with chilled milk from a previous pumping session following proper storage guidelines:

  • Make sure previously pumped milk is no more than 4 days old and is stored at 39°F or colder.
  • Quickly chill freshly pumped milk in refrigerator or ice water bath before adding.
  • Gently swirl combined milk to mix together thoroughly.
  • Store combined milk at back of refrigerator.
  • Use within 4 days from the time the first milk was initially pumped.

Avoid adding warm milk directly to already chilled milk, as this can raise the temperature and risk bacterial growth. New milk should be quickly chilled first. Also discard any leftovers when combining milks near the 4 day limit.

Should I scald breast milk before storage?

Scalding, or heating milk briefly to temperatures around 160°F, is sometimes recommended to kill bacteria before storage. However, this process can affect immunological properties and is not necessary if milk is handled hygienically.

Key takeaways on scalding breast milk:

  • The Holder pasteurization method brings milk to 145°F for 30 minutes.
  • Flash pasteurization quickly heats milk to 162°F for just 15 seconds.
  • Heating also causes loss of vitamin C, B vitamins and antibodies.
  • Scalding is only needed if milk was exposed to unsanitary conditions.
  • Proper refrigeration and freezing restricts bacterial growth without heat treatment.

Discuss scalding with your pediatrician to determine if appropriate for your circumstances.

Does alcohol in breast milk mean I have to pump and dump?

Mothers who ingest alcohol do not necessarily need to pump and dump their milk. Follow these guidelines regarding alcohol consumption while breastfeeding:

  • Aim to minimize alcohol intake. Avoid breastfeeding until alcohol has fully cleared system.
  • For light consumption, wait at least 2 hours before nursing or pumping after one standard alcoholic drink.
  • After moderate or heavy intake, wait 3-5 hours before resuming breastfeeding.
  • Pumping and dumping does not speed up alcohol elimination from milk.
  • Storing a supply of milk before drinking can allow feeding without wasting pumped milk.

The effects of alcohol vary per person. If you feel at all impaired, refrain from breastfeeding. Consult your doctor with any concerns over alcohol and breastfeeding.

Does smoking marijuana affect the safety of breast milk?

Marijuana and THC can accumulate in breast milk with repeated use. Effects on infants are still unclear, but breastfeeding is not recommended when using cannabis.

  • THC is fat soluble and can be detected in breast milk for up to 6 days after use.
  • Potential effects in infants include poor feeding, sleepiness, and delayed motor development.
  • Refrain from breastfeeding for at least 4-6 hours after smoking marijuana or longer with ingestible forms.
  • Pumping and dumping does not remove THC from breast milk.
  • Consult your doctor before using cannabis products while breastfeeding.

Limiting marijuana intake and avoiding breastfeeding when under the influence can help reduce risks to your baby until more research is available.

How long can breast milk sit out if not being fed to baby?

Freshly pumped breast milk can safely sit out at room temperature for:

  • Up to 4 hours if the room is 77°F or colder.
  • Up to 3 hours if the room is warmer than 77°F.

Follow these additional guidelines for milk not being fed:

  • Use clean hands when handling pumped milk.
  • Avoid direct sunlight which can degrade milk.
  • Discard any milk that reaches the above time limits.
  • Refrigerate or freeze milk that will sit out longer.

For milk fed to baby, once baby begins drinking, finish feeding within 2 hours. Bacteria from saliva can contaminate milk. Leftovers should be discarded after feeding.

Can I store freshly pumped milk in the freezer?

Freshly pumped breast milk can be safely stored directly in the freezer following proper guidelines:

  • Quickly chill milk in refrigerator or ice bath before freezing if possible.
  • Pour milk into clean freezer storage bags or containers.
  • Ensure all air is removed and bag or container is properly sealed.
  • Label with date pumped and amount.
  • Freeze immediately at 0°F or lower.
  • Use within 12 months for optimal quality and nutrition.

Freezing promptly helps protect nutrients and antibodies. Thaw frozen milk overnight in refrigerator before feeding to baby.

Can I combine milk from different pumping sessions before refrigerating?

It is best to refrigerate breast milk in separate containers for each pumping session. However, if needed, follow these precautions when combining milk from multiple sessions:

  • Only combine milk pumped on the same day.
  • Fully chill each batch of milk in refrigerator before mixing together.
  • Gently swirl combined milk to incorporate.
  • Store combined milk towards back of refrigerator.
  • Clearly label container with first pumping date for reference.
  • Use combined milk within 4 days from initial pumping time.

Never add warm freshly pumped milk to already chilled milk. Refrigerating milk promptly after each session is ideal for safety.

Does breast milk continue to change as baby ages?

The nutritional composition of breast milk changes over time to adapt to your baby’s needs:

  • Colostrum – thick, yellow first milk produced after birth is high in antibodies.
  • Transitional milk – produced roughly 10-14 days after birth as milk adjusts to your baby.
  • Mature milk – thinner, whiter milk more abundant after 2-6 weeks containing higher fat and lactose.

Key nutrients like proteins, fats, lactose, and immunologic factors adjust throughout each feeding and infant age to support growth and development. Consulting a lactation consultant can help ensure optimal milk composition.


Checking for changes in the look, smell and taste are the best ways to determine if breast milk is still safe and fresh for your baby. Following proper storage techniques helps prevent spoilage and bacterial growth. With some precaution, mothers can feel reassured their milk provides ideal nutrition for their growing infant.

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