Yes, you can freeze milk for cooking and baking later, and it does tend to separate a little bit as it thaws. The fat separates from the liquid, making it look a bit watery or grainy when you first pour it out. To fix this, you just need to give it a really good shake once it is completely thawed, or whisk it well before adding it to your recipe. It works wonderfully for casseroles, pancakes, and muffins where texture does not matter as much as taste. Just make sure to use a container with enough room at the top because milk expands when frozen, and try to use it within a month or two for the best flavor.