Hi Marie, 

This was discussed at great length back in December-January. 
While some people state adamantly that this should never happen, others state just as emphatically that they see this frequently, and it has something to do with the breakdown of the fats when frozen.  But the milk is still good,
just not good to our taste, most babies do drink it, and do not have any ill effects.

Annecdotally speaking -- I had to express and save milk for my premie, way back when, 24 yrs. ago.   So nature-mother me, at one point, thought "I wonder how icecream would work, made of breast milk?"  I defrosted about a half-gallon of breast milk, tasted it and found "that taste" and immediately tossed several gallons of milk that had been accumulating in my freezer.  When I went to the NICU, in tears because all of my milk had gone bad, the nurses cried with me, but also couldn't restrain their giggles, because it happens to so many people, and they do the same thing before they consult with the nursery.  So?  Now I remember to tell moms that this might happen, and so far as I know, nobody dumps seas of BM needlessly.

Chanita, San Francisco