When milk comes from the farm, it contains about 3.5 percent butterfat, says Marcia McGlochlin, the director of food safety and regulatory affairs for Clover Stornetta Farms. The fat is separated from the rest using a big centrifuge.

“It spins, [and] the fat is lighter than the skim milk portion, so the cream elevates to the top of the centrifuge and the skim goes out the bottom via pipes,” she explains.

Kimberlee J. Burrington, dairy ingredients applications program coordinator at the Wisconsin Center for Dairy Research, offers a more scientific description: “Inside the machine is a series of discs with vertically aligned holes stacked together. The discs spin at high speed. The fat moves inward in the separator and the skim milk moves outward. It is a physical separation based on the difference between the density of the fat versus [that of] the skim milk.”

Depending on the dairy, the amount of fat left in the milk is controlled in different ways. At Clover Stornetta, McGlochlin says, metering pumps determine the amounts of skim milk, cream, and condensed skim milk that are recombined and pumped into the pasteurizer. (California state law requires dairies to add “nonfat milk solids” back into 1 percent and 2 percent milk to make them more nutritious.) Other dairies have sophisticated computerized setups that separate out only a certain amount of cream to begin with.

CHOW’s Nagging Question column appears every Friday.

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