Changes in the taste and smell of breastmilk can be attributed to Lipase, oxidation, diet, and/or water and cleaning supplies.¹

Breastmilk is naturally meant to be consumed within the closed bio-system of parent-child. Exposing breastmilk to the different elements outside of this closed bio-system (temperature, air) by expressing and storing the milk affects the activity of lipase, an enzyme that aids in the digestion of fats. Lipase activity is stable at a pH level of 3.5 at 37°C/98.6ºF for one hour, which is just long enough for effective fat digestion at the level of the infant’s small intestine.² Exposure can also cause chemical oxidation of the milk.

Lipase
Some parents find that their breastmilk develops a soapy smell and a metallic taste after it has been frozen and thawed. This change in smell has been attributed to changes in the milk fats related to storage in self-defrosting refrigerators-freezers; it has not been found to be harmful to the baby.³ This suggests that the freeze-thaw cycles of such freezers may change the fat structure of the breastmilk.

How long it takes for lipase to alter the smell and taste of breastmilk depends on the lipase level and varies from person to person. Depending upon the level of lipase, some caretakers notice this rancid smell after milk has cooled in the fridge; others notice it only after the milk has been frozen for a while.^{4 5} Some parents may, therefore, be able to keep their milk refrigerated or frozen for a short time before the soapy taste and metallic smell develop. Others find that their milk begins to smell soapy soon after it has been expressed. Most babies will drink this milk without a problem and the milk is generally considered safe to consume.⁶

Chemical oxidation
Another reason for this change in taste to occur is possibly related to diet and/or metal ions in the water used to clean the supplies. Nancy Mohrbacher writes that sour or rancid-smelling milk is probably unrelated to milk lipase levels. According to some milk storage experts, the most likely cause is chemical oxidation, rather than lipase-caused digestion of milk fat or bacterial contamination.⁷

Possible contributing factors are the intake of polyunsaturated fats or free copper or iron ions in their water. Heating the milk could in this case actually speed up the oxidation, making the problem worse. When a lactating parent is expressing and storing milk for their baby, it is suggested that anyone whose expressed milk smells rancid or sour temporarily avoid their usual drinking water and any fish oil or flaxseed supplements, as well as any foods like anchovies that contain rancid fats. While handling their milk, it is suggested they also avoid exposing it to their local water. It may also help to increase antioxidant intake by taking beta carotene and vitamin E.

Some parents have indeed found that using distilled water (and phosphate-free soap) to clean supplies addressed this issue and that the expressed milk could be cold stored or frozen without problems. However, others found that they were only able to store their milk for a longer period of time than before until changes in taste or smell occurred.

Still, other parents found no change when altering their diet and/or water exposure. If in doubt about whether milk is high in lipase or not, make sure that distilled water and phosphate-free soap are used to clean supplies, and taste the breastmilk several hours after expressing it to see if the milk has acquired this off-smell and -taste.

Heat-treating
When a baby does not like the taste of expressed breastmilk, and cleaning methods and/or diet have been ruled out as the potential cause of the change in smell and taste, lipase can be deactivated by heat-treating the milk.

Heat-treating milk that is high in lipase is done as soon as possible after expressing the milk, or before the milk changes in taste and/or smell. As mentioned above, depending on lipase levels, some parents can keep the milk in the refrigerator for a period of time before heat-treating, sometimes over 24 hours. In most cases, the donor would however be responsible for heat-treating their milk prior to freezing and/or donating. Heat treating human milk to deactivate lipase needs to be done before storing it because once the milk has acquired the rancid smell and taste, treating the milk will not help.⁸

Most references refer to ‘scalding’ as the method used to deactivate lipase. Scalding milk means bringing the milk up to the boiling point, which is 82°C/180°F, and then immediately removing it from the heat source. The high heat of this process combined with the direct heat source potentially damages important components of breastmilk, like lactoferrin and immunoglobulin. Online references say however that this is not likely to be an issue unless all of the milk that a baby is receiving has been heat-treated.

Heat is known to deactivate lipase in breastmilk and thus flash-heating is another method that can be used. Flash pasteurizing milk at 72°C/161.5°F for 15 seconds deactivates the lipase by 97% (which is presumably enough to stop the activity of the lipase).⁹ The lower temperature used with these methods, as well as the indirect heat source, does not harm the milk as much as scalding does. Please see Flash heating explanation for information on how to do this.

Some parents have used bottle warmers and a kitchen thermometer to heat-treat the milk in order to deactivate the lipase. Per Lawrence & Lawrence, bile salt-stimulated lipase can also be destroyed by heating the milk at 62.5ºC/144.5ºF for one minute, or at 72ºC/163ºF for up to 15 seconds.¹⁰

Recipients should be informed that the milk was heat-treated for the deactivation of lipase. In circumstances where a long-term milksharing arrangement needs to be made, finding a donor who is willing to wet-nurse or whose milk does not require heat treatment is preferable.¹¹

The above information assumes that the baby who will receive the breastmilk is healthy and full-term, and that, considering the anti-infective qualities and superior nutrition of raw milk, heat-treated milk is not the sole source of nutrients for the baby. Please see What about premature babies? for more information on heat-treating and its effect on some important anti-infective properties of breastmilk.

Please consult with a lactation specialist with further questions.¹²

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1. Please know that elevated lipase blood levels can be an indicator of inflammation of the gall bladder or pancreas, high amounts of triglycerides, and other health problems. Elevated lipase levels can be asymptomatic for some time before health problems are obvious. Please consult a healthcare provider if you think there may be a medical reason for excess lipase in your milk. (Free Medical Dictionary ↩︎
2. Garza C, Schanler RJ, Butte NF, Motil KJ. Special properties of human milk. Clin Perinatol. 1987;14(1):11-32.3. ↩︎
3. Lawrence R. A., and Lawrence R. Breastfeeding: A Guide for the Medical Profession, 6th ed. Philadelphia, Pennsylvania: Mosby, 2005: p.696. Print. ↩︎
4. The terms soapy smell and metallic taste on one side, and rancid or sour smell on the other, seem to be used interchangeably when reading about the experiences of parents as well as the recommendations of the experts. ↩︎
5. Nancy Mohrbacher, IBCLC, and Julie Stock, BA, IBCLC. La Leche League’s The Breastfeeding Answer Book, 3rd edition (1997): p 229. Print. ↩︎
6. Per Gaskin, Ina May. “Soapy-, Metallic- or Rancid-tasting Milk that You Have Thawed.” Ina May’s Guide to Breastfeeding. New York: Bantam, 2009. p.165. Print. ↩︎
7. Mohrbacher, Nancy, IBCLC. Breastfeeding Answers Made Simple: A Guide for Helping Mothers. 2010. pg. 461. ↩︎
8. Nancy Mohrbacher, IBCLC, and Julie Stock, BA, IBCLC. La Leche League’s The Breastfeeding Answer Book, 3rd edition (1997): p 229. Print. ↩︎
9. Dr. John May, et al. 2005. La Trobe Tables ↩︎
10. Lawrence R. A., and Lawrence R. Breastfeeding: A Guide for the Medical Profession. 6th ed. Philadelphia, Pennsylvania: Mosby, 2005: p. 205, 771. ↩︎
11. Eats on Feets believes that raw and fresh (not frozen) milk is better than heat-treated milk when possible. ↩︎
12. There can be vast differences in training and expertise. Not every lactation consultant is an IBCLC. IBCLCs are International Board Certified Lactation Consultants who work in lactation clinics attached to hospitals and/or in private practice. They are healthcare professionals who specialize in the clinical management of breastfeeding. IBCLCs are certified and regulated by the International Board of Lactation Consultants Examiners. Find/verify an IBCLC. ↩︎