Why Can’t You Add Warm Breast Milk to Cold?

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As a nursing mother, you often have to express milk at different times of the day. You can wonder: “can I mix warm breast milk with cold milk?” The answer is no! You should not. 

Today, we’ll help you answer the question of why mixing cold and warm breast milk is a no-no and provide you with other helpful tips for storing breast milk. Now, let’s find the answer!

Why Can’t You Mix Warm And Cold Breast Milk?

It’s not wise to mix milk at different temperatures, as it will allow bacteria to enter the cold breast milk. Specifically, adding warm breast milk to cold one will increase your milk’s temperature, creating favorable bacteria growth conditions.

Immediately after pumping, you will receive milk with a temperature of about 98.6°F, roughly the same as your body.

Meanwhile, the milk in your refrigerator will be cooled to around 38 to 40°F, allowing it to be stored properly. When you combine two different milk temperatures, the temperature of your cold milk rises above 40°F. So, you’ve inadvertently created the ideal environment for bacteria to grow, which could make your baby sick.

As we all know, babies can quickly get sick from bacteria because they don’t have perfect immune systems like older children or adults. As a result, it is essential to avoid the combination of freshly pumped milk and refrigerated milk to help your child avoid being exposed to the growth of bacteria, which causes illnesses.

mixing cold and warm breast milk

How to Combine Fresh Breast Milk with Refrigerated

If you need a way to combine breast milk at two different temperatures, the following section is for you.

1. Bring Them to the Same Temperature

There are two ways to solve this problem, including cooling warm milk or heating cold milk. These ways allow you to bring the two liquids to the same temperature.

If you go with reheating cold breast milk, you will need to use a bottle warmer to warm it up. Alternatively, you can also place it in hot water (about a few minutes).

Conversely, if you choose the latter way, all you need is to put the fresh milk in your refrigerator until it’s cold enough.

After that, you can add new refrigerated milk to the bottle of cold milk. This process is called layering.

2. You Can Separately Use Them

If there’s a good reason you can’t balance the temperatures of the two kinds of milk, another solution is to feed the baby without combining them.

It is possible to let your child consume cold milk before drinking warm milk, as long as your little one has no problem with this.


Can You Mix Fresh Breast Milk With Room Temperature One?

It is possible to mix freshly pumped milk with room temp one. But you need to make sure that the temperature milk you have collected is still within its expiry date.

You can leave fresh milk at room temperature for about four hours. After that period, it is not safe to use.

If a few pumpings are going on during that time, you can combine your fresh milk with the one you pumped before. Then, you can use your breast milk for feeding or store it for later use.

Can You Add Fresh Breast Milk to Defrosted One?

If you’ve defrosted the frozen breast milk and make sure it is the same temperature as the fresh one, it is possible to mix them in one bottle. Then, you can feed this mix to your baby.

Yet, remember that freshly expressed breastmilk is always more beneficial than previously frozen one. Hence, it is better to feed your baby the fresh one.

We recommend feeding your baby with fresh breastmilk first. Then, finish by feeding your little one defrosted milk.

Thanks to that, you won’t waste your fresh one. If there is any leftover, it should be your defrosted milk, and you can still do some things fun with it.

We do not recommend mixing freshly expressed milk with defrosted one for storage purposes. The reason is that after thawing, the frozen breast milk is not feasible to refreeze again.  Hence, you have to utilize it within twenty-four hours.

how to organize breast milk freezer stash

Can You Pump Into the Same Bottle All Day?

Yes, people often call this the Pitcher method. However, when you add milk from different sessions to the same bottle at room temperature, it is necessary to consider all the milk in the bottle to be as old as the oldest pumped milk.

So, suppose you mix new milk that was just pumped with the milk you pumped three hours ago and left out at room temperature. In this case, you should consider the entire bottle, including the fresh milk, to have been out of room temperature three hours ago.

Can You Combine Breast Milk From Different Days?

It is best to mix pumped breast milk on the same day.  But if you have to mix milk from two different dates, we recommend labeling them with the earlier date.

Also, it is essential to obey storage recommendations based on the earlier date.

Can You Mix Breast Milk From the Left and Right Breast?

If you pump both breasts during the same pumping session, combining milk from both your breasts into one bottle is possible.

The reason is that this nutritional liquid in both bottles is at the same temperature when your milk is freshly pumped.

Tips to Keep Track of Pumped Breast Milk

After you store breast milk in the freezer or refrigerator, it is essential to monitor your pumped liquid. Here are some of the most popular ways.

  • You can use a erase marker to note the date on your containers. It should be easy to wipe off when washing them.
  • Stick notes onto your containers. It is okay to secure these pieces of paper with a rubber band if their stickiness is not strong enough to secure them onto your containers.
  • It is wise to utilize bottles or lids with different colors for each day.
  • We also recommend placing your freshest milk in the back of your refrigerator and letting the oldest in the front. So, when you want to feed the baby, utilize bottles from the front.
  • You can also use the app to keep track of pumped breastmilk. We will suggest you some apps for the rest of the article.

Best Apps for Track Your Milk

The following pumping apps can be a fantastic tool to track how much you are pumping, how much your little one is eating, etc.

1. Pump Log


  • Its alarm setting functionality is flexible.
  • Its countdown calculator is perfect for those who want to pump and build a freezer stash.
  • It is possible to log sessions after pumping.
  • Its graphing feature allows you to see your stats work well.
  • No advertising.


  • This app only helps you track pumping. Hence, it won’t be for you if your goal is to track everything baby-related.

2. Huckleberry


  • It is convenient for you to log pumping sessions.
  • It allows you to track your baby’s feeding & sleep, diaper changes, medication, solids, etc.


  • It does not help you track your stored breastmilk.
  • Not feature statistics or graphs to see trends in output over time.

3. Milk Maid


  • Allow you to see how much breastmilk you have at any given location.
  • Calculate how many days of breastmilk you have collected based on the baby’s intake and how much you have collected.
  • Show how much you pump on each side, how long you pump, how much you feed your baby, etc.


  • It can be more complex than expected if you don’t have the stash and typically feed your baby right after pumping.
  • No option to try free before purchasing

4. Milk Stash


  • It will show you that your pumping output fluctuates.
  • It is possible to access this app through the web.
  • It is free.
  • No ads.


  • The breastmilk storage guidelines are overly conservative.

The Bottom Line

Now you know why you shouldn’t add warm to cold breast milk and other valuable answers.

Mixing breast milk from different pumping sessions is convenient. It allows you to transport and store breast milk more effortlessly. Moreover, it saves you many containers and space in your freezer or refrigerator.

You can safely mix breastmilk at different temperatures by following our instructions. There’s no need to worry about your liquid gold going bad.

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