The NICU Barista's Guide to Warming Mother's Milk

Jae Kim, MD, PhD / May 2016


Drinking warm milk is quite common these days. Most mornings I start with a smooth cappuccino filled with hot milk and espresso coffee. Steaming milk brings the milk to a temperature of 150-160°F that impacts the chemical structure of cow’s milk to alter its texture, smell and flavor. Foaming milk is a process of high agitation from the steam jet that changes the structure of cow’s milk proteins and sugars to generate that familiar fluffy structured foam above the drink.

As a culture, then, we have become very accustomed to drinking warmed milk, alone or in some other food vehicle with some degree of sophistication. The chemistry with heat and milk does not always work so well. For instance, have you ever added soy milk to hot coffee? I notice that it often precipitates in the coffee, making the drink cloudy and, at least visually, less palatable. I suspect that the proteins in soy milk coagulate much quicker and come out of solution.

We also shake milk all the time. We drink milk shakes, we agitate milk with steam for coffee to create a milk foam, and we add milk to baking mixtures and whisk or blend to create different textures. The cow’s milk we typically drink from the grocery is already homogenized and pasteurized. Homogenization shreds the fat globules down to much smaller sized globules and pasteurization heats milk to high temperatures to sterilize it against bad microbes.

Every day in the NICU, too, we warm up human milk at each bedside, to order, for each of our babies. Human milk is a complex liquid tissue, not unlike blood, which we are very familiar with as a quintessential liquid tissue. Full of cells, bioactive compounds and rich nutrients, both these liquids are chemically very complex and therefore fraught with potential to alterations in makeup with any degree of processing. Heating and mixing human milk is not a trivial process, rather it is an art.

So how can we bring cold or frozen human milk to the best temperature to feed from? Human milk warming has become very sophisticated now. I am so glad that in my NICU we have standardized the practice of milk warming. This has created a much more efficient workflow where milk can be warmed up using a very consistent algorithm. We also have the luxury of dedicated milk technicians prepping our milk, but our bedside nurses still participate by taking refrigerated prepped milk and warming it up to feed their babies. I am a bit ashamed to say how we used to warm milk in the past. We used to take a big styrofoam cup that we would fill with warm water by feel (targeted for body temperature) so it was very inconsistent. Frequently milk was being exposed to much hotter water, especially in situations where one was trying to speed up the process. Furthermore the open exposure to free flowing water raised the risk of contaminating the milk.

The main technologies used now for warming human milk are water-based or waterless ones. Both reliably bring the temperature of cold milk up to feeding temperature. The funny thing is that once a syringe of milk has reached body temperature, it rapidly cools once it is out of the warmer and then when it is loaded on a pump and infused through a nasogastric tube. The final temperature arriving to the baby is much closer to room temperature. Not so warm after all. Reminds me of drinking a cup of coffee I left sitting on my desk for half an hour. Unfortunately we don’t have a clear answer to what is the best temperature to consume human milk but can assume that actual breastfeeding is the gold standard and that delivers milk much closer to body temperature (about 37C or 98.6F), no steaming necessary.

We also need to consider the large fat globules in human milk that are responsible for rapid separation of milk into lipid and aqueous fractions. These fat globules are complex structures that cover large droplets of triglycerides in their core. These can also be seen in fresh cow’s milk but these are lost with homogenization in consumer dairy products. Infant formula is very different structurally from human milk. As the name suggests, infant formula is a formulation. The three major macronutrient components, protein, fat and carbohydrates, are re-composed together from independent sources. The protein and fat are separate components originating from cow’s milk while the fat in infant formula is actually plant based. Fat droplets are much smaller and emulsified in infant formula. Comparatively, human milk is not as stable and prone to separation. It needs careful agitation before use. Shaking human milk too hard can actually damage some of the bioactive components such as immunoglobulins found in the milk.

It is clear that the science of human milk feeding still has many questions to answer. To all the human milk baristas (moms, nurses, milk technicians) working our infant lattes day in day out, thanks for being part of such a sophisticated craft! I hope no one takes this as a hint to start a trend making fancy patterns in mother’s milk.:)

Take home points

  1. It is best to provide a stable, consistent method of milk preparation and warming to preserve the quality of human milk.
  2. Warming milk is a science and requires quality controlled equipment to produce the best results to preserve the quality of the milk.
  3. Shaking human milk hard can be damaging to some of the bioactive proteins such as antibodies found in milk. Think agitated, not shaken.


Looking for more information on human milk from Dr. Kim?
Read his previous blog post,
Achieving Break NEC Speed in the NICU: Is Zero NEC Coming?

Is your NICU considering waterless warming solutions?
Learn more about the Medela Waterless Milk Warmer
and the Guardian Warmer with inline warming technology.

About the Author

Jae Kim is an academic neonatologist and pediatric gastroenterologist and nutritionist at UC San Diego Medical Center and Rady Children’s Hospital of San Diego. He has been practicing medicine for over 23 years both in Canada and the USA. He has published numerous journal articles, book chapters, and speaks nationally on a variety of neonatal topics. He is the Director for the Neonatal-Perinatal Medicine Fellowship Program at UC San Diego and the Nutrition Director of an innovative multidisciplinary program to advance premature infant nutrition called SPIN (Supporting Premature Infant Nutrition, He is the co-author of the book, Best Medicine: Human Milk in the NICU. Dr. Kim is a clinical consultant with Medela, Inc.