Skim milk is homogenized to avoid any separation of milk fat from the milk (very small amounts of fat remain in skimmed milk; nonfat milk is defined at less than 0.5% milk fat). In milk processing plants, fat is removed and then added back to meet fat standards of less than 0.5, 0.5, 1.0, 2.0, or 3.25% (whole milk) as milk from farms varies from 3.3 to 5.5% fat when it arrives at the plant.
Mike Hutjens, Extension Dairy Specialist, U of Illinois - Illini DairyNET