Allow raw milk to sit for a while, and it will separate, with a layer of cream forming on the top. This is a result of milk's chemical composition -- the fat within the milk just doesn't mix well with the other elements. However, the mechanical process called homogenization helps "force" the fat to stay mixed in.
Emulsion
Though it may appear a nice, creamy white, raw milk isn't a uniform liquid. Rather, as described by the Department of Dairy Science and Technology at the University of Guelph in Canada, it's an emulsion: a combination of two liquids, neither of which will dissolve in the other to form a true mixture. With milk, the emulsion is globules of milk fat floating in the nearly fat-free liquid known as skim milk.
Distribution
When the milk comes out of the cow, the fat globules are more or less evenly distributed. But as the milk sits, and especially when it cools, the milk begins to separate. This happens for the same reason that an oil-and-water emulsion will separate: The fat is less dense than the surrounding liquid, so it gradually rises to the top and floats there, forming a layer of high-fat cream. The globules vary in size, with the larger ones rising to the top more quickly.
Homogenization
During homogenization, a machine forces milk at high pressure and high speed through a fine screen. The screen breaks the fat globules into much smaller droplets of a uniform size. Not only do these smaller globules take far longer to rise, but the fact that the globules are all of the same size means they all rise at the same rate. That prevents the "clumping" that normally occurs as larger fat globules rise through the milk and "collect" smaller ones on the way up. In addition, the pressure used in homogenization increases the density of the resulting globules, bringing it closer to that of the surrounding liquid and reducing the tendency to rise. Finally, homogenized milk usually undergoes heat pasteurization to kill bacteria; as a side effect, pasteurization reduces the tendency of globules to stick together. At the end of the homogenization process, milk is still an emulsion -- but one in which the fat globules remain all but suspended in the liquid.
Skim Milk
Though marketed as nonfat or fat-free, skim milk still retains a minute amount of suspended milk fat, according to Mike Hutjens, a dairy extension specialist at the University of Illinois. Hutjens says "nonfat" milk is defined as milk with no more than 0.5 percent fat. Even this small amount could form a cream layer, so skim milk also typically undergoes homogenization.
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