Experts define the composition of milk as dynamic. It is often called liquid gold and as such, you do not want to waste even a drop. Most moms, however, will lament that they often found yourself pumping, feeding, and falling asleep without having had stored it.

Human milk has properties (namely, antibodies) that protect it from bad bacteria and contamination, allowing it to be left out longer than thought. Even if has gone bad, it will taste badly and lose its potency of nutrients but not harm your baby. Nevertheless, how long it can be kept depends on its storage method. According to experts, the rules for storing a mother’s milk can be summarized in the following four main categories

Breast Milk Storage (And last for)

Room temperature

You can keep freshly expressed milk at room temperature for up to 6 hours. There are clearly caveats to each rule since weather temperatures have a significant influence. For example, whilst I never left my recently pumped liquid gold sitting on the kitchen counter during our hot Shanghai summers, I did leave it sitting on the terrace during our winter skiing holiday in Switzerland.

Cooler that is insulated

Experts assure us that we can keep it in a cooler filled with ice-packs for 24 hours.


Experts claim that you can keep it in the coldest part of the refrigerator for up to 5 days. As with our first category (room temperature), this timeframe is based on certain temperatures and, according to Medela, the temperature of the refrigerator should be 4C or 39F.


It can be kept in a deep freezer for up to 12 months but ideally, it should be used within six months. In a normal freezer, since the door is opened with more frequency, it should not exceed 6 months.

Thawing and Heating

For those new mothers who want to heat breast milk, please don’t use the microwave or a high temperature on the stovetop.

Babies are extremely sensitive to heat and what seems – to you and I – like lightly warmed liquid, is uncomfortably hot to them. Not only will it not heat evenly, “some research suggests that rapid heating can affect the milk’s antibodies.”As a result, when you need to defrost frozen breast milk, the optimal way is to leave the bottle or special nursing storage bag in the refrigerator to defrost naturally.

Of course, life is not always ideal and sometimes, we find ourselves holding a screaming baby at 3 am – in this case, place the frozen milk bottle or bag in a bowl of hot water, cross your fingers and hope for a quick thaw! If you find yourself with a thawed bottle and a baby that falls asleep after only a few sucks, please the bottle back into the refrigerator for storage for no more than 24 hours.

Now, if you cannot express, choose not to express, or are transitioning from breast milk to formula, the rules for storing formula are not exactly the same, whereas formula is more sensitive to storage.

Formula Storage


Experts recommend that you make a new bottle for each feed because, even if you use sterile water to mix it, the milk powder itself is not sterile. As a result, bacteria can build up over just a few hours in the refrigerator. If you have to prepare a feed in advance (I think of friends with twins, in this instance), then Baby Center recommends you put the mix into the “fridge or a cool bag as soon as you’ve made it. Use it within four hours if it’s been stored in a cool bag, or within 12 hours if it’s been in the fridge.”


The powdered formula cannot be frozen because it changes its composition and you will not be able to readily mix it with water at a later time. The prepared or liquid formula cannot be frozen since it creates an oily layer when thawed that cannot be mixed with the rest of the formula.

How to store breast milk

Lastly, a few general tips I wish someone had shared with me when I was a new mum:

  • Wash your hands before pumping
  • Never mix old and newly pumped milk
  • Label the storage containers you freeze and defrost the oldest first
  • To avoid wastage, store milk in small portion sizes (2-4 ounces is recommended)
  • Leave some extra space in the storage containers since milk expands when frozen
  • Never mix formula and your expressed milk
  • If it smells badly or sour, it should be discarded immediately
  • If it is thawed, it should not be re-frozen
  • Breast milk is composed of three different types “quencher, foremilk, and hindmilk” and they may separate into layers when stored. This does not mean that it is bad – simply swirl the layers to mix together.

Breast milk and formula are not the same and as such, should be cared for according to its unique qualities. Interestingly enough, a significant number of adults drink breast milk, believing that it can improve athletic performance or cure chronic diseases. In this light, a male friend once asked me if it could be used for baking and I found the question quite amusing. For a giggle, enjoy the following article.

Enjoy these moments of feeding your baby. Before you know it, your baby will be crawling, walking, talking and no longer in need of your special feeling moments.