Composition of Milk
Milk contains globules of butterfat that are larger and lighter than milk molecules. When you allow unhomogenized milk to sit, the cream naturally rises to the top. Most of the milk you purchase at the grocery store is pasteurized and homogenized. During the homogenization process, the milk passes through a series of filters that break the butterfat into smaller and smaller globules, until they're too small to separate from the rest of the milk.
Cream Separator Function
A cream separator works via centrifugal force. The machine spins raw milk in a tub or basin. During this process, the lighter butterfat globules are flung to the outside of the container, where they can be siphoned off. The machine separates the cream much faster than the gravity method, and also separates more of the cream from the milk.
A Swedish engineer, Dr. Carl Gustaf De Laval, began experimenting with a mechanical cream separator in 1859, using a barrel to spin the milk, then skimming the cream from the top after the barrel stopped. Next, he built a separator that utilized a series of buckets to separate 35 gallons of milk at a time. In 1878 he introduced a continuous cream separator that could process 300 pounds of milk an hour.
A cream separator won't separate cream from homogenized milk. Smaller machines designed for home use separate milk and cream a few gallons at a time, but large commercial separators allow for continuous separation of large amounts of milk and cream. Skimmed milk flows from one outlet of the machine and cream from another.