That old cliche about cream rising to the top is true. With cow's milk, fresh from the farm, cream does separate from the milk and rise because fat is the main ingredient in cream and fat is lighter than the rest of the milk.
But the full fat milk, or whole milk, you buy in the store doesn't separate like farm milk because whole milk is put through a process that keeps the fat molecules interspersed with the rest of the milk. This process is called homogenization.
During homogenization, milk is forced under high pressure through a small nozzle. This process breaks down the fat globules to about a quarter of their original size. These microscopic fat globules become receptors for milk proteins.
Once the damaged fat globules bond with milk proteins, the fat in milk becomes weighted down by the heavy protein, and is no longer light enough to rise to the top of a container, causing the milk to become a uniform mixture of protein and fat.
Uniform In Texture, But More Vulnerable To Spoilage
In addition to allowing milk to be uniform in its texture and consistency, homogenization creates a blander and whiter product. Also, after milk is homogenized, it becomes more vulnerable to spoilage from exposure to light and heat.
Before milk is homogenized, it must be pasteurized because if fresh milk were homogenized, it would go rancid within minutes.
That's because the milk-spoiling enzymes which are killed during pasteurization work much more rapidly on the smaller fat globules produced by homogenization than on the larger fat globules that float to the top of non-homogenized milk.