when ミルク is processed into butter, the バター cream separates from the watery liquid (which is actually sold as skim milk). the バター cream contains all the fat which has a high concentration of vitamin A. Vitamin A has a natural yellowish tint. When churned, it is oxidized and aerated, and turns even yellower and another separation occurs when the fat lumps and the バター ミルク (sold as, あなた guessed it "buttermilk) is squeezed out. how do I know this? I grew up in a dairy farm in Wisconsin:)
