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What is buttermilk? Where does buttermilk come from?

Natural buttermilk is a product of the butter-making process. After a cow is milked, the cream is removed from the milk by either allowing it to rise to the top and then pouring it off, or using a mechanical device like a cream centrifuge to remove it. The cream is then churned or mixed vigorously so that the butterfat molecules gather together in bigger and bigger lumps. These lumps are butter. When the lumps of butter are finally removed, the liquid that remains is buttermilk.