My Breast Milk Smells Soapy or Sour

Most mothers say breast milk smells sweet or doesn’t really have a smell. However, sometimes mothers notice that their expressed breast milk smells or tastes soapy or sour—either soon after expressing or after storage in the freezer. Occasionally a baby will refuse expressed breast milk. What should breast milk smell or taste like? Why might a baby refuse pumped breast milk? This article answers frequently asked questions about the smell and taste of expressed breast milk.

What should breast milk smell or taste like?

Breast milk generally has very little odour. Sometimes it may smell or taste like something the mother has eaten recently, or been stored next to in the fridge, such as onion or garlic. Sometimes it is said to smell or taste sweet or soapy, metallic or even a little sour. Bear in mind we normally do not sample the smell or taste of breast milk before babies drink from the breast so in most cases, whatever the unique taste or smell of your freshly expressed breast milk it is likely to be delicious to your baby and quite normal. When stored appropriately, breast milk is very robust and does not easily “go off” or spoil due to bacterial contamination because it is full of protective factors. For much more information about breast milk storage see How Long Does Breast Milk Last?

Occasionally a medication or supplement that the breastfeeding mother is taking might taint breast milk with a new flavour or smell. It is also possible that a strong deodorant, perfume or nipple cream might cause an unpleasant taste.

Why does breast milk smell soapy?

Breakdown of fats

Having a slightly soapy taste or smell is likely to be quite normal. A soapy or sour smell is thought to be connected to the activity of a fat digesting enzyme called lipase. There are two types of lipase in breast milk; lipoprotein lipase and bile salt-stimulated lipase.¹ Lipase in breast milk breaks down fats into fatty acids (a process called lipolysis) and it is these fatty acids that are thought to cause the soapy taste.² In the baby’s tummy, lipase helps to digest fats and in stored milk the lipolysis process may help to protect breast milk from microorganisms as there is an antimicrobial effect.³

Storage of breast milk

Some mothers have found that the soapy or sour smell appears to be more apparent once breast milk has been stored in the refrigerator or freezer. Some describe a “rancid” smell. Theories for stored breast milk tasting or smelling sour or rancid include:

Lipase activity during storage. Instead of going straight to your baby’s stomach, pumped and stored breast milk is subjected to different storage times, temperatures and air contact which could affect lipase activity.⁴ Freezing milk does not stop lipase activity in domestic freezers.⁵ ⁶
Freezing may damage milk fat globule membranes. One study found that the freezing process damaged the milk fat globule membranes so that lipase could break down the lipids in frozen milk.⁷ Bile salt-stimulated lipase is not normally activated until triggered by bile salts from the baby’s gall bladder, however Penn et al reference the idea that freezing may trigger this activity (Penn et al, 2014). Lawrence and Lawrence say that the soapy smell has been linked to the effect of a fridge freezer’s self-defrost cycle (freeze/thaw action) on the lipid structure and that a rancid smell is perhaps associated with true lipid breakdown. However the exact cause of the souring of human milk is still a theory and unconfirmed.⁸
Chemical oxidation. Another theory for particularly rancid smelling milk, despite correct storage, is chemical oxidation of certain constituents in breast milk perhaps due to exposure to the air, contaminants in a mother’s drinking water or due to certain fats a mother has eaten.⁹¹⁰ See below for more information about chemical oxidation.
Contaminated breast milk. Breast milk is very robust and full of protective factors but it is possible that bacterial contamination could be a cause of rancid smelling milk in some cases if it has not been collected under clean conditions.

Too much lipase?

In the past some breastfeeding authors have suggested that breast milk turns sour in mothers who have particularly high lipase levels in their breast milk. However a study by Dr Ruth Lawrence and others did not find higher lipase levels in samples of sour milk.¹¹

Is soapy or sour milk safe to drink?

In most cases a baby will be happy to drink breast milk that has been safely stored including frozen and thawed. The Art of Breastfeeding says strange-tasting milk is safe and most babies will still drink it.¹²

What if my baby refuses my breast milk?

Sometimes a baby appears to dislike stored breast milk that tastes or smells sour or rancid and will refuse it. The time interval before stored milk begins to have a stronger taste varies between mothers but it is not possible to reverse the taste once this has happened—which can lead to wasted breast milk. Some mothers have found that diluting the affected milk in equal parts with freshly expressed breast milk can make it acceptable to the baby. In addition, experts offer a number of options to try to avoid the unacceptable taste whether the cause is enzyme activity (lipase) or oxidation (see below).

How can I prevent lipase activity in stored breast milk?

Testing expressed milk at various storage intervals, by smelling or tasting the thawed milk and offering it to your baby to find the point at which the smell changes or your baby refuses that milk. Use stored milk before this time interval is reached.
Scalding (heating) freshly expressed breast milk prior to cooling and freezing. Scalding milk can prevent the lipase activity during storage and so prevent the strong soapy taste or smell developing. See the section below on heating breast milk to deactivate lipase. Note that this does not work for all mothers (Lawrence and Lawrence, 2016).
Lowering the pressure and speed of the breast pump is a possibility mentioned by Lawrence and Lawrence.¹³
Freezing below -70ºC. Not a practical solution for the home freezer but Hsiao-Ying et al discuss that storage at this temperature prevents lipolysis.¹⁴

How can I deactivate lipase by heating?

If diet and water supply are not affecting the taste of breast milk but a baby refuses to drink previously stored milk, lipase activity can be deactivated by heating breast milk prior to storage. The best temperature to inactivate lipase but preserve the other living components of breast milk is not clear.

Scalding milk. Kelly Bonyata refers to scalding breast milk to deactivate lipase by heating breast milk to 82ºC (180ºF) or until the milk starts to bubble around the edges of the saucepan (but not a full boil) and then cooling quickly.¹⁵
Flash heating. Eats on Feets say that heating breast milk to 72ºC for 15 seconds (flash heating or flash pasteurising) has a less damaging effect on some of the beneficial components of breast milk and will reduce lipase activity by 97%.¹⁶ A method for flash heating is described on the Eats on Feets website see Flash Heating.
Pasteurisation. Tully et al state that lipoprotein lipase and and bile salt‐activated lipase in human milk are completely inactivated by pasteurisation.¹⁷ Holder pasteurisation used in milk banks involves heating breast milk to 62.5ºC for 30 minutes. Lawrence and Lawrence state that bile salt-stimulated lipase can be destroyed at 62.5ºC for one minute.¹⁸

Scalding breast milk will not remove the soapy or rancid smell if the milk has already become tainted.¹⁹

Will heating breast milk damage it?

The Academy of Breastfeeding Medicine recommends against heating breast milk above 40ºC as this can destroy many protective factors in the milk.²⁰ However, donated milk is frequently heat treated in milk banks (Holder pasteurisation: 62.5ºC for 30 minutes) but is still considered valuable for premature babies for normal growth and development of the immune system.²¹ Kelly Bonyata explains that although some protection will be lost or some nutrient levels lowered this is unlikely to cause issues unless a baby is exclusively receiving heat treated milk in their diet (Bonyata, 2023).

Research is ongoing into how storage techniques might affect breast milk.²²²³

Chemical oxidation, an explanation for sour or rancid breast milk?

One explanation for sour or rancid smelling stored milk is that a chemical reaction has occurred in the milk such as lipid oxidation or proteolysis (breakdown of proteins) (Hung et al, 2018). Oxidation is a chemical reaction involving exchange of electrons between substances. Some authors have suggested this may be associated with something present in a mother’s drinking water such as free copper or iron ions (positively or negatively charged atoms or molecules) or certain polyunsaturated fats in her diet (Mohrbacher, 2016).

How can I prevent chemical oxidation in stored breast milk?

Nancy Mohrbacher, breastfeeding author and IBCLC, mentions some specific changes to the mother’s diet may help preserve breast milk in this situation:

Avoid drinking your local tap water; switch to bottled water for a while.

Stop taking any fish-oil or flaxseed supplements.

Avoid any foods like anchovies that contain rancid fats.

Avoid using local tap water while handling your milk and its containers.

Increase your intake of antioxidants by taking beta carotene and vitamin E supplements.

Eats on Feets, a community breast milk sharing site, say that some mothers found that using distilled water and phosphate-free soap was helpful to prevent breast milk changing taste. However other parents found it only extended the storage time rather than solving the issue. They add that some mothers found no change by altering their diet or water source.²⁴

Storage tips to avoid oxidation

Vangnai et al²⁵ looked at oxidation of breast milk during freezing and made the following recommendations to avoid rancidity of breast milk:

Store frozen breast milk for the shortest time possible, always use the oldest milk first
Thaw frozen breast milk in a refrigerator at 4ºC, warmth can increase oxidation
Store breast milk out of direct sunlight

Nipple preference?

Some babies may refuse stored breast milk because it is offered in a bottle and it is the bottle that they do not like as they have a preference for the breast.

Milk storage guidelines

For information on breast milk storage see How Long Does Breast Milk Last?

Summary

Breast milk generally doesn’t have much of a smell. Breast milk is sometimes described as tasting soapy or a little sour. It can smell or taste of something the mother has eaten recently as some foods can flavour breast milk. Sometimes stored breast milk is described as tasting very sour or rancid after storage in the fridge but particularly after freezing and thawing. Although generally still thought safe to drink, some babies will refuse previously stored milk that now has a strong sour taste or smell. There are several theories for the change in taste. Research has highlighted the action of a fat digesting enzyme in breast milk as a likely cause. The enzyme can be deactivated prior to storage by scalding the milk. Another theory suggests that chemical oxidation during storage may affect taste. More research is needed.