Mine does that too! Thankfully my 15 month old isn't really taking bottles anymore and I am home with him so it hasn't been a huge issue, but we were wondering why even stuff that I had *just* frozen the day before tasted like that.
Per kellymom.com:
To scald milk:
* Heat milk to about 180 F (82 C), or until you see little bubbles around the edge of the pan (not to a full, rolling boil).
* Quickly cool and store the milk.
Scalding the milk will destroy some of the antiinfective properties of the milk and may lower some nutrient levels, but this is not likely to be an issue unless all of the milk that baby is receiving has been heat-treated.
Per Lawrence & Lawrence, bile salt-stimulated lipase can also be destroyed by heating the milk at 144.5 F (62.5 C) for one minute (p. 205), or at 163 F (72 C) for up to 15 seconds (p. 771).
http://www.kellymom.com/bf/pumping/lipase-expressedmilk.html