Scientist Mom Shows That Breast Milk Is Not Sterile…and That Pasteurization is Amazing

Áine Blanchard Quimby, scientist, breastfeeding mom, and woo fighter extraordinaire, decided to do some science for fun and to prove a point about raw milk and pasteurization.

Caveat: While she does have experience in sterility labs and culturing bacteria, food science is not her area of specialization.


I conducted this experiment with raw versus pasteurized breast milk. It shows why pasteurization is important, and why people-to-people raw breast milk sharing could potentially be dangerous.


I used sterile collection techniques with alcohol wipes to clean my nipple after letdown, then gathered milk by manual expression with gloved hands into a sterile microcentrifuge tube.

sterile collection process

I separated and heated 50 microliters of milk at 65 degrees Celsius for 30 minutes, a classic Pasteurization method. The other was left alone. I then plated them onto LB culture plates (I streaked the first area with 30 ul of breastmilk, then did the remaining segments with ten fold dilutions), and left them to grow overnight at 37 degrees Celsius.

equipment used

Results and Findings:

After overnight growth at 37 degrees Celsius, the raw milk sample without pasteurization had significant bacteria growth – approximately 200 colonies. The pasteurized sample had no visible bacteria growth.

Raw milk sample: approximately 200 bacteria colonies. Pasteurized sample: no visible colonies.


I was not able to categorize the bacteria, because that’s not possible in my lab. Consequently, the bacteria found in my milk could potentially be harmless. We may never know. One doesn’t know what bacteria or viruses they have in their body at a given time, never mind what another person might have. I can almost guarantee that someone donating breast milk isn’t using sterile collection techniques in a lab. I wanted to show that even using sterile techniques bacteria can survive to be passed along. It’s possible that the milk may have been contaminated through the collection process, but however the bacteria got into the sample, pasteurization fixed it.

Conclusions and Recommendations:

Raw breast milk is not sterile. You don’t know what might be living in someone else’s breast milk. Or in raw cow’s milk. Pasteurization kills bacteria. Peer-to-peer raw milk sharing is not safe and should not be recommended over milk bank donor milk or formula, if a mother is unable to breast feed or needs to supplement.

Images: Áine Blanchard Quimby, all rights reserved.


Steph is a mom, stepmom, freelance writer, and advocate. When she's not busy writing, chasing kids around, cleaning up messes, and trying to change the world, Steph enjoys snuggling, making pies, politics, reading paranormal fiction, yoga, and fitness. A fully recovered natural parent, Steph now trusts science, evidence, and common sense to lead the way. She has been actively involved in the reproductive and women's rights movements for more than 20 years and is a passionate pro-choice feminist. Her writing can be found on Grounded Parents, Romper, The Cut, and other print and online publications

Related Articles


  1. One could argue still healthier than cows milk, formula or juice.

    This was a 1 person case study. Try a 100 plus in each group and you may have a better case.

    1. This was a science project for fun, not a double blind control study. Also, unscreened breast milk from a stranger is dangerous. Not healthier than formula. Try again.

  2. Right, but breast milk is not meant to be sterile. It has it’s own microbiome, which contributes positively to the health of the baby’s own developing gut microbiome.

    Nevertheless, I wouldn’t recommend unscreened breast milk from a stranger either, not because of the fact that breast milk contains (shock, horror) bacteria, but because there is a small chance that something nasty is passed on to the baby, in the worst case HIV, or perhaps an antibiotic resistant strain of E coli. (The latter was reporting in a hospital setting, where the chance of encountering such a thing is much higher.)

    So in a way I am agreeing with the conclusion, but I don’t like the all-out fear of germs that is given as the explanation. Pasteurisation seems to be a clumsy tool for ensuring that the wrong germs aren’t passed along, because it blitzes all the good microbes as well as the bad. The evidence starting to come in suggests that the absence of ‘friendly’ microbes could turn out be quite harmful in its own right.

    1. True, but when it’s not your own breast milk, it’s way better to be safe than sorry, which is why milk banks pasteurize donor milk. The immune benefits of good bacteria and antibodies are minimal when considering the risk of contaminated milk.

  3. yes, donated milk should be screened, many breast pumps serious source of contamination,  my own using a poorly designed pump to feed my premature child was >3 million/mL. ; raw grade A bovine milk, as is, legal limit is 100,000/mL. Continue to test milk of all sorts 40+ years in certified dairy laboratory,  have tested only few human milk samples of human milk, but quite a different component profile from bovine or goat.  My own, when hand expressed, was <1000 SPC/ mL , <1/mL coliform, <1/mL staph/ species.

Leave a Reply