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Adding Warm Breastmilk to Chilled: We Know It’s Controversial, but the Science Is Too Exciting Not to Share!

April 13, 2021

Photo by Mariel and Joey Lifestyle Photography

You’ve probably heard that you shouldn’t combine freshly pumped milk with previously chilled milk—right?

It’s one of those "breastmilk rules" that many moms have been told, often leaving them worried about whether they’ve made a scary mistake or if they’re doing the right thing.

But here’s the exciting part: science is catching up to the power of breastmilk!

Breastmilk Science Is Changing—And It’s Amazing

Although traditional medicine once ignored the unique properties of breastmilk, recent scientific research confirms what many moms have always known:

✔ Breastmilk is antimicrobial
✔ Breastmilk is full of living cells
✔ Breastmilk is fundamentally different from any other food

The American Academy of Pediatrics (AAP) previously stated:

"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).”

However, in September 2021, the AAP updated its recommendations, linking instead to the Academy of Breastfeeding Medicine’s guidelines and the CDC’s storage protocols—which are based on the same scientific research. You can find those links and a helpful article cited by the AAP here: Healthy Children Article Tips for Freezing and Refrigerating Breastmilk, CDC Proper Storage and Preparation of Breastmilk, and the AMB Clinical Protocol Human Milk Storage Information for Home Use For Full-Term Infants.

So, what does this mean for pumping moms?

Does Mixing Warm & Chilled Breastmilk Pose Any Risks?

Many moms ask: What happens if I combine milk from different pumping sessions without cooling it first?

The CDC and the Academy of Breastfeeding Medicine state that combining milk "can rewarm the older stored milk."

And that’s it.

No other risks are identified, and no research suggests there’s a safety concern.

So why do so many mom blogs and parenting sites still recommend cooling breastmilk before combining it?

Why Are Old Breastmilk Storage Rules Still Circulating?

Many health websites still reference the Mayo Clinic, CDC, and other organizations, stating that breastmilk should be cooled before combining.

But here’s what’s important to know:

Most of these guidelines haven’t been updated in years.

The “don’t mix temperatures” rule is not based on breastmilk research.

It was originally adapted from food safety guidelines for handling raw meat & dairy.

That’s right—breastmilk was once treated like a perishable food, despite its unique antibacterial properties!

Over the past four years, however, groundbreaking research has confirmed that breastmilk actually fights bacteria—making it unlike any other food.

The Science Behind Pooling Breastmilk

Published scientific research found 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.”

In simple terms? Breastmilk actively fights bacteria and helps prevent contamination.

Similar studies conducted by Vanderbilt University, National Jewish Health, and pediatric researchers confirm the same findings:

✔ Pooling breastmilk does not increase bacterial contamination
✔ Breastmilk’s composition stays stable even when adding warm milk to chilled milk
✔ Pooling can create more consistent calorie and nutrient content

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!

So, Should You Mix Warm & Chilled Breastmilk?

The research says yes—but ultimately, the decision is yours.

If you feel comfortable following the old rule, you can:
✔ Cool your milk before combining
✔ Use separate containers for each session
✔ Store milk in multiple chillers

But if you’re looking for an easier, research-backed option, pooling your breastmilk safely can be a huge time-saver.

Pro Tip: Many moms use Ceres Chill Breastmilk Chillers to safely store pooled breastmilk for up to 20 hours—without the need for ice packs or extra bottles!

The Future of Breastmilk Storage Guidelines

We recognize that not all parents will feel comfortable changing how they store breastmilk. That’s why it’s important to know that:

✔ The recommendation about combining milk is being reviewed for revision in 2022
✔ The Academy of Breastfeeding Medicine will consider all new research
✔ Moms deserve updated, science-backed guidelines—not outdated food safety rules

While we wait for official guidelines to catch up, our mission remains the same: empowering moms with the best, most up-to-date breastfeeding information.