Casein (/ ˈ k eɪ s iː n / KAY-seen, from Latin caseus "cheese") is a family of related phosphoproteins (αS1, aS2, β, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of the proteins in human milk. [1] Sheep and cow milk have a higher casein content than other types of milk with human milk having a particularly low ...