“Shaken, Not Stirred”: How Does Your Preemie Take Their Milk?

Jae Kim, MD, PhD / March 2019


Virtually every baby I have seen prefers mother’s own milk to infant formula. As a card-carrying gastroenterologist, it was imperative in training to taste all the infant formulations we prescribed to our patients, including all the elemental formulas with broken down ingredients. This was entertaining for others to watch, and sometimes nauseating for us as the tasters.

Many parents of infants who need specialty formulas have tried the same thing, and come to the same realization I did: a bewilderment that most infants, preterm and term, can take the worst, and I mean worst, tasting formulas from the get go without so much as a wince. It is quite rare to find a baby cringe at the taste of these elemental feeds if started very early in life. It is a good thing, though, that their taste buds are so pliable to start. Later in life is a whole different picture where, if there is a need for these awful tasting formulas, a more selective taste preference is often at hand.

Mothers’ milk variety

We find great variation in composition with mothers’ milk and this, as a consequence, may alter its taste. Interestingly the molecules that pass on flavors, particularly from a mother’s diet, can find its way into her milk. Recently I had an outing with my family to our favorite Korean BBQ place. With these meals, I always request and consume the added taste of raw garlic slices that ideally spice up the meal with only the annoying adverse effect of having the pungency of garlic seep out of every pore of my body for the next day. If I were capable of lactating I would guarantee the production of a high quality garlic flavored breast milk after such a meal. Studies have shown exactly that, where maternal diet and its flavors can be transferred to their milk.1 It is still poorly understood, but many mothers can describe unique changes to their milk based on their recent diet.

Like all organic foods, natural decay is the order of things with mothers’ milk. Exposure to microbes, light, oxygen, and varying temperatures will alter a complex liquid tissue like mothers’ milk. We are only just beginning to understand how important the microbes in mothers’ milk are, how they get there, and what determines their mix. No two mothers are the same. We know too that the milk sugars that mothers make act as prebiotics and are genetically determined – but we don’t know why there is such diversity and variation between individuals.

When you let mothers’ milk sit at room temperature, its innate properties actually prevent the growth of bacteria. However, sooner or later microbes win the day and start doubling over and over to contaminate the milk. Refrigerated mothers’ milk is quite safe for 96 hours in the refrigerator due to these bioactive factors in the milk.2

We did a study several years ago and found that some mothers had tremendous colonies of bacteria in their pumped milk samples.3 That turned out not to be from heavy growth of bacteria from mothers’ breasts, but rather contamination and overgrowth of microbes in the pump equipment that spiked the pumped milk with unwanted bacteria. Milk is a rich growth medium, especially at warmer temperatures, and poor cleaning of equipment can be an important and currently unmonitored safety risk. Guiding mothers to properly clean their breast pump equipment is helpful to prevent such issues.

Breast Milk Temperature

So, what temperature should milk be fed at? Scant data shows which temperature matters. In some cases, cold milk in fact provides a strong sensory stimuli that may be beneficial to infants. Feeding aversion is a tricky problem, with many of our preemie graduates being seen later in feeding clinics. Our occupational therapists have found some success with a few infants who have a history of aversion by encouraging them to take the feed and coordinate the feed better. I certainly have strong memories enjoying lots of cold refreshing milk growing up, but maybe that was just conditioning. On the other end, drinking warm milk is an age-old elixir for helping people fall asleep. Could this be recreating a deep memory of breastfeeding?

Mother’s milk is not a smooth, slick liquid, but more colloidal than a simple solution. The fat globules are delicate packages of fat. These tend to rupture and then release their contents and subject to digestion by the lipase found in mother’s milk. These lipases release fatty acids that have many biologic properties. Freezing particularly tends to rupture these globules, exposing more fat to digestion. The lipases in mother’s milk is a hardy enzyme and can work at all ranges of temperatures, hot and cold.

Freezing and long term storage of milk tends to cause a vexing problem of distaste.  Mothers who have been lactating for a few months seem more susceptible to this. Most blame the lipase and certainly the home remedy of scalding the milk seems to support a protein such as lipase that gets denatured and inactive. This controversial problem results in a rancid smell and taste that is particularly bad as I have seen term and preterm infants simply starve to avoid drinking this altered milk. It has to be really bad if they can usually tolerate elemental formula at that stage. Whatever its cause, we need to be aware how adulterated some milk can become with time.

Beneficial Fat in Breast Milk

Mothers’ milk is intentionally creamy, with a large chunk of the milk as fat. In fact a good half of the calories in mother’s milk comes from fat. This is critical to meet the rapid growth needs of newborns. Infants start with a thin watery foremilk and end up with a likely delicious creamy hindmilk ending their feeding.4 So what happens when we pump milk into a container, and pour and transfer the milk from a minimum of 3-4 containers? The biggest adherent component that gets left behind is the fat in the milk. Many times we may be unknowingly creating a reduced fat diet based on our handling of milk. This is like switching to 2% instead of your usual double cream in your morning coffee. Ugh!

Pasteurization processes

Pasteurization has a powerfully negative effect on milk that we trade off to have safe milk. We are all now comfortable with providing donor milk, that when provided, safely needs to be pasteurized to high temperatures. The most accepted approach is Holder pasteurization, a treatment of 62.5C for 30 min. This ends up being longer than a half hour, since the milk is exposed to heat on the way up and way down to and from the holding temperature.

Some newer commercial strategies involve pushing the temperature even higher for shorter times or with high pressure, but these methods cause similar deterioration of the product. If you are a fan of slow cooking, you will know that food is altered by the time and temperature we expose it to. Mothers’ milk is no different. But we need to deal with the practical aspects of storage and donor milk pasteurization needs. Do we have alternatives to heat treating our milk to preserve its most salient properties? So far, we have not seen any clear non-pasteurization method emerge, however, pulsed electric field or UV light exposure approaches have been described in the literature.

Closing the gap on mother’s milk

So how do our infants really want their milk? It is apparent that for taste, and most assuredly for quality, the best mothers’ milk is fresh breast milk. It all still points to straight from the source, no bottle, no handling, no treatment, just unadulterated direct from mothers’ own breast. Let’s all keep with this goal for our preemies, and only use donor milk as a bridge to mothers’ milk, and get more breastfeeding at the end.

I am encouraged by how many are looking to find more creative ways to shorten the distance between mothers and their preemies. Maybe the right answer to the original question is “Suckled, not stirred!”



  1. Forestell CA. Flavor Perception and Preference Development in Human Infants. Ann Nutr Metab. 2017;70 Suppl 3:17-25. doi: 10.1159/000478759. Epub 2017 Sep 14. Review. PubMed PMID: 28903110.
  2. Slutzah M, Codipilly CN, Potak D, Clark RM, Schanler RJ. Refrigerator storage of expressed human milk in the neonatal intensive care unit. J Pediatr. 2010 Jan;156(1):26-8. doi: 10.1016/j.jpeds.2009.07.023. PubMed PMID: 19783003.
  3. Stellwagen LM, Vaucher YE, Chan CS, Montminy TD, Kim JH. Pooling expressed breastmilk to provide a consistent feeding composition for premature infants. Breastfeed Med. 2013 Apr;8:205-9. doi: 10.1089/bfm.2012.0007. Epub 2012 Oct 5. PubMed PMID: 23039396.
  4. McDaniel MR, Barker E, Lederer CL. Sensory characterization of human milk. J Dairy Sci. 1989 May;72(5):1149-58. PubMed PMID: 2745823.


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, spinprogram.ucsd.edu). He is the co-author of the book, Best Medicine: Human Milk in the NICU. Dr. Kim is a clinical consultant with Medela LLC.