Milk can be frozen, but it will not be the same after thawing: the fat and water separate, giving the surface a grainy appearance. Vigorous shaking largely brings it back together; even so, thawed milk is better used in cooking, batter, and hot drinks than as a cold beverage. Do not fill the bottle to the brim, because milk expands as it freezes.
Milk is an emulsion: droplets of fat are dispersed in water, surrounded by protein membranes. This mixture is stable but can break, and freezing is one cause. As the water freezes, ice crystals grow, squeeze the fat droplets between them, and concentrate the proteins. During thawing, some of those droplets come together. As a result, the milk no longer looks smooth; fine grains appear on the surface, along with a yellowish layer around the edges. This is not spoilage; the structure has reorganized.
The appearance can often be improved. Shake thawed milk thoroughly in its closed bottle or whisk it in a container; the fat redistributes, and the milk largely returns to its former state. Low-fat milk handles this better because it contains less fat to separate; whole milk becomes noticeably grainier. Ultra-pasteurized and homogenized milk also separates less than raw or regular pasteurized milk because its fat droplets have already been broken into smaller particles, making them less likely to come together during freezing.
The texture does not fully return, and that determines how to use the milk. You can notice the difference when drinking thawed milk from a glass or using it in a cold dessert. The difference disappears when it is cooked: in custard, béchamel, soup, rice pudding, cake and pastry dough, or hot coffee and tea, there is practically no difference between thawed and fresh milk. Freezing milk is therefore a way to keep a supply for cooking, not for drinking.
How much you fill the container matters. Milk expands as it freezes; if a full bottle freezes, it can lift the cap or warp the plastic, while a glass bottle may crack. Leave about a glassful of space at the top; if using a carton, pour some of the milk into a glass first. Dividing the milk into portions also helps: instead of freezing 1 liter, divide it among small containers or an ice cube tray so you can take out the amount a recipe calls for and thaw it much faster. Frozen milk cubes also provide a ready portion for coffee or tea.
Thaw the milk in the refrigerator; depending on the amount, it may take overnight or longer. Do not thaw it on the counter. Shake thawed milk before taking it out of the refrigerator, and use it within a few days; do not refreeze milk that has thawed and warmed up. It also matters when you freeze the milk: do not freeze milk that is close to its date or has been open for days, because freezing stops spoilage but does not reverse it. Freeze milk while it is still fresh. This applies to all milk products: before freezing, ask yourself whether you would drink it today.