Hi Jeanette: I found out the hard way a few years ago that even though the mother believed her milk had not thawed, the opening & closing of the freezer door had, over time caused a bit of thawing and refreezing. I refused to believe her complaint that the milk had "gone bad" until I tasted it myself and it was quite sour. I'm now very careful to instruct moms to pack it in ice &/or place the milk in the back of the freezer (unless they are using a deep freeze to begin with, in which case this isn't a problem. Nancy