Adding warm breastmilk to chilled: We know it’s controversial, but the science is too exciting not to share!

Photo by Mariel and Joey Lifestyle Photography

You may have heard that you can’t combine newly expressed milk to previously chilled milk, right? It’s one of those “breastmilk rules” that many know moms know about, and has caused too many moms to worry about whether they made a scary mistake or are doing “the right thing.” 

But we are living in exciting times!  Although traditional medicine once ignored the power of breastmilk, recent research and established medical sources like the American Academy of Pediatrics, confirm that breastmilk is powerful, antimicrobial, full of living cells and fundamentally different from any other food.  

Not surprisingly, the American Academy of Pediatrics has updated their recommendation regarding adding freshly expressed milk to chilled milk. They have now confirmed what all of the other researchers have found: combining different temperatures of milk does not increase the risk of bacteria in breastmilk. Their latest revision acknowledges that many common recommendations are not evidence-based or practical, stating that,

“Mothers can mix warm milk and cold, or even consider pooling milk from 24 hours together, which may help even out variability in nutrients due to pumping time or breast emptying (which influences fat content of the milk).” 

The AAP has long been a trusted source for breastfeeding families and is committed to ensuring the well-being of all children. This is an excellent example of a growing, formal recognition of the quality and reliability of the research being conducted by renowned pediatricians and biochemists in the field of human milk medicine.  

Moms often want to combine their milk because they're interested in the benefits of the “pitcher method” or are just plain looking to simplify their challenging breastfeeding journey. A growing number of moms have reached out to us with questions about whether they can pool their breastmilk. As a breastfeeding mom, I was lucky to be able to turn to my lactation consultant and pediatrician to get their thoughts. Both told me that they were aware of that recommendation, but said it was fine to combine milk as long as it was over the course of the same day. 

Then I found a lot of mamma blogs and other health websites that said something totally different. I kept searching and found published scientific research that established the composition and live antibacterial qualities of breastmilk. I decided to try combining my milk based on everything I'd learned, and it worked well for me! However, this was only my experience. I know that the conflicting information can be confusing and pretty overwhelming. I understand that what worked for me doesn’t work for every mom and the information isn’t always accessible.  

It's so hard to know what to do when you have different answers to pretty much the same question: Is my milk safe if I pump into the same container all day?

I hope you know me well enough by now to understand that besides being a mom, there's pretty much nothing in the world I'm more committed to than removing barriers for breastfeeding moms. With that goal always in mind, we’ve been studying safe milk storage practices, working with our Science Advisory Board, and reaching out to the American Academy of Pediatrics and Academy of Breastfeeding Medicine to ensure breastfeeding women have the most up-to-date and reliable information. 

A quick Google search will show you dozens of mamma blogs and health organization web pages that recommend parents cool breastmilk before combining it. These sites often reference organizations such as Mayo Clinic or the CDC, which certainly strive to provide parents with important information regarding breastfeeding protocols. However, those recommendations have typically not been updated in at least a couple of years. Many of them don't include the incredible scientific discoveries that have occurred over the last 12-18 months. Researchers and organizations specifically dedicated to breastmilk science have become increasingly aware that the information provided on these sites is not based on human milk data and composition, but rather on a general understanding of how other foods react to temperature change. The recommendation regarding not combining breastmilk never cites to breastmilk research and instead has been acknowledged to be based on raw or prepared food handling practices to decrease bacterial growth in meat or pasteurized dairy products. Not surprisingly, the recommendation that initially advised against adding warm milk to chilled milk hasn't been updated in at least two decades.

Over the last four years, however, research has shown us that breastmilk is unlike any other food in so many truly incredible ways — it has live antibacterial and antimicrobial properties that work to fight harmful bacteria! A research study published in Scientific Reports explained that,

“human milk contains many known antimicrobial and immunomodulatory molecules...the reduced pH caused by these bacteria [in breastmilk], and additionally, by known and unknown components of human breast milk make it more difficult for potentially pathogenic organisms to colonize.”

Simply put, breast milk has the power to fight bacteria and inhibit the growth of bacterial organisms. Studies completed by Vanderbilt University, National Jewish Health, pediatricians and dozens of credible scientific sources all come to the same conclusion.

Unfortunately, breastmilk research lagged behind other medical research for years. Katie Hinde, a biologist who specializes in breast milk and evolutionary biology, pointed out that fact in this entertaining and eye-opening Ted Talk. Additionally, The Conversation, a website that cites various breastfeeding studies, stated, “remarkably, we still don’t fully understand the composition of breast milk, or the biological basis for its many health effects. In fact, more scientific papers have been published on headaches than breastfeeding.”

In recent years, there has been more of an effort to change that, specifically regarding the topic of combining milk temperatures. Studies published in the Breastfeeding Medicine Journal and Semantic Scholar note that pooling breastmilk over the course of a day and adding warm milk to chilled milk does not increase bacterial counts, and actually offers a more consistent caloric product. There have been far more articles supporting this conclusion, some of them published by and 

“While it was originally thought that a mother had to cool her fresh breast milk before combining it with previously expressed cooled milk, the latest research now shows that you may pump directly into already refrigerated or cooled milk as long as the milk is added within 24 hours of the first milk expressed.” —

We've also reached out to Dr. Trillitye Paullin, a Molecular Biologist and Founder of Free to Feed, to provide us with guidance on milk storage. She referenced several studies, most notably the study done by Breastfeeding Medicine Journal, and stated, “as discussed above, temperature fluctuations in pooled milk does not lead to increased bacteria counts or nutritional breakdown. In fact, the study found that the average nutritional content of 24-hours’ worth of individually stored milk was nearly identical to that of the pooled samples and that pooled milk did not have increased bacterial contamination.” You can find this post in our Instagram Bio under The Ceres Series!

We understand that not all parents will be comfortable with changing the ways that they have been storing their breastmilk. Still, we want to give you all of the research that has come out in recent years and let you know that the recommendation about combining breastmilk is up for revision in 2022 and the board will be considering all of the recent research (these recommendations are reviewed every five years — and a lot has happened in the last five years!). 

It’s important to do what you feel is best for you and your baby. Moms that are concerned about combining breastmilk stick with coolers and multiple bottles or purchase two chillers (one for chilling and one for storing). Please know that we are not advocating for feeding human milk that: (1) is several days old mixed with milk of various temperatures; (2) has been thoroughly and repeatedly reheated; (3) has been left out for several hours without refrigeration (increasing the temperature above the recommended 59 degrees Fahrenheit) or (4) otherwise feeding milk potentially contaminated by unclean pump parts/bottles. 

We'll continue to stay on top of all the recent research to give you the information you need to make the right decision for you and your baby. The information and research that is now coming to light about the power of breastmilk is so incredible. You can find videos and links to the more information on the articles we discussed here

Also, be sure to sign up for our Ceres Club (in the footer of this page) if you'd like to receive tips, tricks and more information on the magic of breastmilk.  


  • Where on the aap website does it says this? I clicked the link you provided which takes me to an faq page, I cannot find the text stating you can mix warm and cold milk.

  • I read the AAP updated guidelines recommending pooling milk; however, I did not see anything on the AAP website that says combining warm and cold milk is recommended. Could you publish a link to this information/ research? Thanks

  • I read the AAP update when it first came out, but it has since been removed from the FAQ page. Have you seen it posted elsewhere by the AAP?

  • I’m so glad I looked this up as I have literally just made the mistake of combining my freshly pumped milk to milk I have previously pumped and been in the fridge 4 hours roughly i quickly thought noooo iv just wasted 5oz of milk that’s taken me a while to get and while feeding my little one I thought I had to chuck it away but after reading this I will try it out thanks
    Cassie from the UK


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