Some women find that when they thaw their frozen milk it has a strong, soapy, or even rancid, smell. Assuming the milk was fresh and smelled fine when placed in the freezer, this is most often due to an excess of the enzyme lipase in the milk. Lipase helps to break down the fats in breast milk, making it easier for babies to digest. Women who have a lot of lipase in their milk find that it continues to break down the fats even when the milk is frozen, resulting in a soapy or rancid odor when the milk is thawed. This breakdown of fats is not harmful to your baby, and the milk is safe to feed her. But some babies will balk at the change in smell; other babies seem to hardly notice.
Once the milk has been frozen, there isn't much that can be done about the lipase in it, and it is likely that all of your milk will smell a little odd when you thaw it. But, once you’re aware that your milk seems to contain a higher level of lipase, you can deactivate the lipase before freezing the milk, so that there's change in odor. All you have to do is heat the fresh milk just to the point of boiling, then let it cool again before freezing it. This is not necessary if you will be feeding your milk while it's fresh, only if you plan to freeze it.