It is my understanding that the sour smell of frozen milk following  thawing is attributed to changes in the milk fats during the freezing/thawing process, but is harmless to the baby. 

Boiling the milk would probably not be a good idea, since heating milk above 56 degrees centigrade destroys many of the immunological components in the milk.
This subject has been discussed a few times on Lactnet.  Check the archives.

Yael Wyshogrod IBCLC
Rechovot, Israel

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