How Many Times Can You Safely Reheat Breast Milk? Updated Guidelines

By Alison Heyerdahl and Vanessa Jupe

As a breastfeeding mother, you know how challenging it can be to express and correctly store your breast milk. It takes enormous effort to follow safe storage rules when half asleep after a 3 am feed or when you're running late for a 10 am meeting, and so you really don't want to see that precious liquid gold wasted.  What happens if you've heated a bottle of expressed milk that was stored in the fridge and your baby doesn't finish it?  The answer varies and will depend on a few factors.  Read on to learn more.

Can You Reheat Breast Milk?

The short answer is yes, it is safe to reheat breast milk, but you can only do so once. Reheating destroys good bacteria and nutrients found in breast milk.  Furthermore, it is best to reheat that same milk within four hours because bacteria from your baby's mouth could contaminate it. 

When is it safe to reheat breast milk?

The storage of freshly expressed milk is a different issue from leftover milk from a feeding session because bacteria from the baby's mouth enter the milk once she starts feeding.  Many mothers reheat excess milk that has been saved for longer than two hours (sometimes up to 48 hours) with no problems, but this depends on several factors:

• Freshly expressed breast milk has more bacteria-fighting power than previously refrigerated or frozen breast milk.  Also, milk that has been frozen for a shorter period will have more anti-bacterial than milk that has been frozen for longer. 

• Bacteria grow more slowly in any breast milk than it does in formula, so it is safer to reheat breast milk than to reheat formula. 

• The cleanliness of your breast milk-expressing equipment, including your breast pump and storage containers, and whether you have washed your hands adequately.  This will also affect the amount of bacteria in the milk.  

• If your baby is sick, she may be less able to handle "normal" amounts of bacteria.  In this case, you may want to follow stricter guidelines.

Can you put breast milk back in the fridge after warming?

Some mothers put the milk back in the refrigerator after warming it, while others leave it at room temperature.  No studies have been done on the safety of either method, so it is unclear which method is preferable.  However, we do know that refrigeration slows the growth of bacteria.  Many mothers rewarm the milk without issues, but it is best to smell it before doing so. 

When shouldn't I reheat breast milk?

There are times when it is inadvisable to reheat breastmilk:

• Do not reheat the breastmilk if it has been standing out for too long and you suspect it may be spoilt.  You will know if it is off because it will smell foul.

• Do not reheat breast milk if your baby is sick.  Breast milk loses its anti-bacterial properties with heating, so it's best to feed your baby a fresh bottle of breast milk that has not been previously heated and cooled down. 

• Do not reheat milk previously frozen, as it will have minimal anti-bacterial properties and spoil much easier. 

How should milk be stored for heating or reheating?

As mentioned previously, storing your milk properly will ensure that you can reheat it with no issues:

• Before you express your milk into storage containers, make sure you wash your hands and clean all your equipment.

• Always label the container so you know how long you can store the milk.  Breast milk lasts for:

  o   4 – 6 hours at room temperature.

  o   8 days in the fridge.

  o   6 months in a freezer.

• Always thaw your milk before heating it up.

How should milk be heated?

Never warm your breast milk in a microwave – it can overheat the milk and can cause degradation of the milk proteins and nutrients.  Warm some water on the stove and place the milk container in the hot (but not boiling) water.