When milk is fresh from the cow, it contains variable amounts of fat. Cows produce different amounts of milk fat at different times of year, and the amount of fat in milk varies with the cow. To make sure that all milk that goes into the production of dairy products (ice cream and cheese, for example) is consistent for every production batch, excess milk fat (which becomes cream) is removed, and the milk is then homogenized. For milk to be homogenized, it first has to be pasteurized so enzymes that would otherwise oxidize the fat and make it rancid are deactivated.
Cool the milk. When cows are milked, the milk temperature is a little lower than the body temperature of the cow. To prevent it from spoiling and to keep any bacteria in the milk inactive, the milk is cooled to 4 degrees Celsius (49.2 degrees Fahrenheit).
Remove excess fat. This is done in a separation machine, where the heavier cream floats to the top. Usually, all fat is removed and then the desired amount (which depends on the type of milk -- skim milk has much less fat than whole milk) is added back.
Pasteurize the milk. This is done using special equipment in which milk is heated to at least 69 degrees Celsius (156.2 degrees Fahrenheit) for an extended period of time. Depending on local legislation, this can be up to 30 minutes.
Pass the skimmed and pasteurized milk through a valve at high pressure. This forces the fat drops in the milk to split into a uniform size. After this step, the milk is considered homogenized.