The greasy or soapy substance, it could be lipase. Lipase is responsible for breaking down the fat in the milk. If there is an excess of this enzyme then the fat gets broken down to quickly after being expressed thus resulting in the change. The milk is not harmful, and most babies are not bothered by the mild change. But the longer the milk sits the more obvious the taste and smell becomes which then will result in the baby most likely having an aversion to it.
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