Buttermilk: a by-product of the butter industry
Milk fat

During the churning process, the cream is transformed into butter by the agglomeration of micrograins, then grains. A whitish, liquid, aqueous phase called "buttermilk" flows between the grains. Its chemical composition is similar to that of skimmed milk. However, it differs in certain respects, giving it some interesting functional properties. While it contains less casein than skimmed milk, some of which is bound to the butter grains, it is rich in phospholipids, derived from the membranes of fat globules damaged during churning.