For those of you who, like me, tend to drink a lot of milk but only like powdered milk for cooking, here is my accumulated wisdom from freezing, thawing and using around 65 gallons since my beginning and biggest find at 50-cents/gallon last Memorial Day weekend.
(And, no, that doesn't include the number of gallons we drank w/o freezing... We go through a lot of milk.)
Be sure the plastic jugs you pick have one or two of those indentations. They'll expand out as things freeze and you don't have to break the seal before you are ready to use it.
It's the ones we had to open (because, in our inexperience, we didn't check for circles) that went bad the quickest.
The floaty spots are fat, best as I've been able to tell. I've never been able to find any consistency in thawed milk's fat separating or not, so I always shake the jug before I pour.
If you use hot water to speed thawing it can give the spots an off-taste, though they might not be fully "bad" yet. I won't drink anything that tastes weird. That said, I have no problem with drinking expired milk.
I find those dates to be highly conservitive and am thrilled when I can find next-day milk for a dollar.
I buy as much as can fit in my cart (sometimes it's too heavy to push) and freeze it all.
When you drink as much milk as we do you can save $20-60 a run. And you don't have to worry about running out of milk.
Milk is pretty consistently good for a week after sell-by (one local market has added a "use-by" under the sell-by date.) and freezing seems to halt the aging, so once thawed we have week (No. problem.) to get through a gallon.
I usually set a new gallon in the sink when I have half a gallon left in the fridge, and that works pretty well.
Milk can be thawed more quickly in water, but that (especially hot water) doesn't work as well IMHO. My own theory is that the hot water speeds up all the aging in the shell of melted milk, making it go bad sooner.
(Oh, and when 1% and whole are both on-sale, I do all this x2. It's worth it to have exactly what you want when you want it! I do the 1% and the rest of the family--DH and two preschoolers-- do the whole.)
More ideas at Rocks in my Dryer.