Some mothers who are pumping and storing breastmilk for later use find that their milk develops a soapy smell. This may be noticeable after a short time in the freezer or after longer periods of freezing. The likely cause is high levels of lipase, the enzyme that breaks down fat. The breakdown of the fat in breastmilk by lipase is normal but not noticeable when the baby is feeding directly at the breast.
Breastmilk that smells a bit soapy after freezing is considered safe for the baby and most babies will accept it. If the baby refuses the milk the change in smell/taste can be prevented by scalding the milk shortly after it is pumped. Scalding deactivates the lipase. Milk frozen after being scalded will not develop a soapy smell from the action of lipase.
Stored milk that smells or tastes sour or rancid is more likely to be caused by chemical oxidization or bacterial contamination. Chemical oxidization could be caused by high iron or copper levels in the mother’s local drinking water or by high intake of polyunsaturated fats (fish and flax oils).
Detailed information about breastmilk storage and scalding breastmilk can be found on the La Leche League International website
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