"I have what is probably a silly question about freezing milk. My husband says his mother always bought milk and froze it, but the times I've done so (I don't do anything special, just toss it in the freezer) it sours almost immediately when defrosted. What am I doing wrong?"
It is not a silly question at all. My first answer is that you are not doing anything wrong. One problem could be the brand of milk. A major store sells milk that I have never had any luck using after it is frozen. It separates as soon as it thaws and cannot be shaken enough to return to a proper consistency. I know several people who have had the same problem when trying to freeze this brand of milk. Try switching brands if possible. I shop at a different store which sells a different brand of milk and very rarely have problems.
I try to freeze milk for no more than 6-8 weeks. After that it can be more prone to separating.
I do let the milk sit on the counter until it is partially thawed but never completely thawed. Terri mentioned the same thing in her reply. Thawing a gallon of milk is not a quick process and rushing it will not give you the best results.
Freezing milk really is just as simple as putting it in the freezer. It allows you the freedom to stock up while the price is good.