Smell of Stored Human Milk

Is it safe to give it to my baby? Breastmilk is food for your baby and also contains hundreds of components necessary for digestion and the protection of the baby’s health. When breastfeeding directly, human milk passes from the mother’s breast to the baby’s mouth and digestive tract. Changes in the milk can occur anywhere along this chain if it is disturbed. Digestive enzymes and other components of breastmilk remain active even when milk is expressed or pumped. Expressed human milk may undergo odor changes when stored. Parents sometimes describe expressed or defrosted milk as smelling odd – metallic, soapy, or even rancid.  The reasons are not well understood.  One theory is that lipases, which are enzymes that naturally occur in human milk, continue working even when milk is frozen, breaking down fats and releasing fatty acids that can smell soapy or metallic. Similarly, milk that leaves the breast and is later stored is generally exposed to air for extended periods of time, leading to the oxidation of unsaturated fatty acids in the milk. This may also alter the smell of the milk. Certain components of frozen expressed milk may change when the milk moves from liquid to frozen and then is thawed again. There is no evidence that milk that has undergone these changes is unsafe and most babies will drink it. Many human fluids have a smell and milk is no exception. Breasts are thought to have evolved from sweat glands so a smell is not surprising. Read more in this article which reminds us “Humans are smelly animals.” (Mitchell, 2022) It is well known that family foods flavor milk, for example garlic or curry. The presence of flavors in human milk that are typical in the family diet may assist with the transition to solid foods later on. I’ve heard of “high lipase” causing a smell or taste change.  Does this mean I have “high lipase”? Lipase is an important enzyme that is in all human milk. Researcher Justin Silpe tested freshly-expressed human milk from multiple donors in the lab of the Princeton Molecular Biology Department. He found no clear pattern between the extent to which a donor self-reported their own milk changing in smell, taste, or color during storage and the levels of lipase measured from that donor’s milk in the laboratory. Dr. Silpe notes that while he is interested in defining the biological and chemical consequences of storage on human milk, more research and larger sample sizes are needed. (Silpe and Moore, unpublished) Another research study done in 2019 confirmed that the rancid smell was not due to lipase. Frozen milk previously refused by the mother’s own infant was collected from 16 mothers at five different time points and tested. None of the samples contained high levels of lipase. (Pitano et al., 2019) If your baby will accept breastmilk that you have stored following recommended storage guidelines, it is generally considered safe. See storage guidelines here. One research study found that bacteria in breastmilk were not higher in milk refused by the baby; in other words, the reason that babies were refusing the milk was not that it was contaminated. (Pitano et al., 2019) Reluctance to take expressed milk may not be related to smell. We don’t have a scientific explanation for why babies would reject milk based on odor or taste. The Academy of Breastfeeding Medicine points out in their 2017 Milk Storage protocol, “Many foods that humans eat, such as eggs, cheese, and fish, have an unpleasant odor that does not affect taste.” One possibility is that your baby may be rejecting the bottle, rather than the contents (see ideas for bottle refusal here) or reacting to a change in the environment, such as a new caregiver. Look for ways to ease your little one into these new feeding routines. What if, despite trying ideas above, I feel it is the smell or taste that is interfering with my baby accepting my defrosted milk? Here are some ideas worth considering: Some babies readily accept freshly expressed milk but not previously frozen milk. If this describes your baby, try a mix of the two. Start with adding small amounts of previously frozen milk to avoid discarding if your baby is reluctant to take the combination. Gradually add more defrosted milk to the mix. The longer milk is frozen, the greater the potential for changes in smell or taste. Aim to keep the milk in the freezer for the shortest time possible. Many nursing parents who are separated from their children after returning to work or school express and leave milk for the next day only. They may have a small freezer backup or stash for emergencies only. Consider how much milk you need in storage. This will vary depending on your own circumstances.  While it can be reassuring to have some amount as backup, consider making just what your baby needs in your situation.See this article: Why I Avoid the Freezer Stash. Keeping the focus on nursing when you are at home can decrease the amount of bottles needed while you are apart. If you are apart from your baby during the day, plan to breastfeed just before separating and as soon as you are reunited. Breastfed babies may be able to obtain a large volume of milk when nursing in the evenings and nights. If you find that you aren’t expressing enough milk to provide for all of your baby’s nutritional needs each day, this could point to a need to adjust your pumping or expressing strategy and the approach to feeding your baby from the bottle in your absence. Check in with a La Leche League Leader about ways to make pumping and bottle feeding more effective. You may want to try methods to increase your milk production. Consider your storage temperatures. Colder temperatures may slow changes in expressed breastmilk. Always store breastmilk in the coldest part of your freezer. If you will not be using your expressed milk within 24-48 hours, aim to move it to the freezer as soon as possible. (Expressed breastmilk can safely be stored in a refrigerator for four days and sometimes longer; moving the milk to the freezer sooner may be worth considering if your baby is refusing the milk.) When possible, defrost milk gradually either by placing it in the refrigerator overnight or by placing the closed storage bag or bottle in a cup of warm tap water. For more general guidelines, see also Storing Human Milk. Scalding milk is believed to slow lipase’s action in the milk. Research shows that lipase is not responsible for the changes in smell. Removing lipase removes an important enzyme that helps the baby digest fats. The Academy of Breastfeeding Medicine and other health organizations recommend against scalding due to the possibility of destroying other immunological factors in milk. There are also no clear guidelines on how to scald milk, which leaves room for error. However, if you have tried this process previously and find that it works for your family, you may decide it is worthwhile. Your scalded milk still offers more protection than commercial infant formula. You may hear the suggestion to add alcohol-free vanilla extract or other flavors to the milk. This has not been researched. According to Dr. Ruth Lawrence: “A few mothers have noted improvement when they lowered the pressure and speed of the pump”. This is anecdotal and has also not been researched. (Lawrence, 2021) Some mothers donate unwanted milk to their local hospital’s milk bank. This can be another option to consider. (Pitano et al., 2019)  See also Milk Donation. Discovering that your stored milk smells odd can be discouraging and you may worry that your milk is lacking in some quality. Know that this is not true. Regardless of smell or taste, properly stored human milk is an excellent and health promoting food for your baby. Sometimes the process to gently encourage a baby to accept a bottle is lengthy, frustrating, and requires alternatives. Reluctance to accept this milk may be more about your baby’s physical and emotional development, and thus resolve with the strategies above and time. Mitchell, K. (2022, May 28). “High Lipase” Breastmilk – Physician Guide to breastfeeding. Physician Guide to Breastfeeding. https://physicianguidetobreastfeeding.org/mythbusters/high-lipase-milk/Pitino, M. A., Stone, D., O’Connor, D. L., & Unger, S. (2019). Is Frozen Human Milk That Is Refused by Mother’s Own Infant Suitable for Human Milk Bank Donation? Breastfeeding Medicine, 14(4), 271–275. https://doi.org/10.1089/bfm.2018.0193ABM (2017) Clinical Protocol #8: Human Milk Storage Information for Home Use. https://www.bfmed.org/assets/DOCUMENTS/PROTOCOLS/8-human-milk-storage-protocol-english.pdfLawrence, R. A., & Lawrence, R. M. (2021). Breastfeeding: A Guide for the Medical Professional (9th edition). Elsevier Health Sciences (pg 691). Justin Silpe, PhD and Maggie Moore, MPH, CLC, unpublished research