Casein (/ˈkeɪsiːɪn/ KAY-seen, from Latin caseus "cheese") is a family of related phosphoproteins (αS1, αS2, β, κ). These proteins are commonly found in mammalian milk, comprising c. 80% of the proteins in cow's milk and between 20% and 45% of the proteins in human milk. Sheep and buffalo milk have a higher casein content than other types of milk with human milk having a particularly low casein content.
Casein has a wide variety of uses, from being a major component of cheese, to use as a food additive. The most common form of casein is sodium caseinate. In milk, casein undergoes phase separation to form colloidal casein micelles, a type of secreted biomolecular condensate.
As a food source, casein supplies amino acids, carbohydrates, and two essential elements, calcium and phosphorus.
Casein preparation in an old etching operation in Müllheim
Casein paint is a fast-drying, water-soluble medium used by artists. Casein paint has been used since ancient Egyptian times as a form of tempera paint, and was widely used by commercial illustrators as the material of choice until the late 1960s when, with the advent of acrylic paint, casein became less popular. It is still widely used by scene painters, although acrylic has made inroads in that field as well.
Casein-based glues, formulated from casein, water, hydrated lime and sodium hydroxide were popular for woodworking, including for aircraft, as late as the de Havilland Albatross airliner. Casein glue is also used in transformer manufacturing (specifically transformer board) due to its oil permeability. While largely replaced with synthetic resins, casein-based glues still have a use in certain niche applications, such as laminating fireproof doors and the labeling of bottles. The popular Elmer's School Glue was originally made from casein because it was non-toxic and would wash out of clothing.