Lipase is an enzyme that breaks down the fats to help babies digest human milk. When lipase occurs in higher levels, expressed breastmilk can smell and/or taste soapy. Milk with excess lipase is safe to drink, but some babies dislike the taste and refuse it.
People who plan to pump and freeze their milk can freeze some test batches and thaw it out after a week to be sure it doesn't smell or taste soapy. If the test milk smells off, future milk can be treated by heating expressed milk to a scald immediately after collecting it and then quickly cooling and freezing it. Scalding inactivates the lipase. Once the milk has acquired the rancid smell, however, treating the milk will not help.
You can find more information on this subject on the KellyMom website.
Have you ever found that your breastmilk "turned" after refrigerating or freezing? If so, did you throw it out or did your baby drink it anyways? I never encountered this personally, however I would love to hear your stories! Please drop me a comment below.