Casein. Computer artwork of a molecule of the milk protein casein. The atoms in the molecule are represented as spheres and are colour-coded: oxygen (green), carbon (yellow), nitrogen (orange), phosphorus (red), magnesium (grey) and sulphur (large dark yellow, upper left). Casein is precipitated out of milk in acid conditions or by the action of rennin. It is the principal protein in cheese. Pure casein is easily prepared and can be used as a protein supplement, particularly in cases of malnutrition. Casein can also be used commercially in adhesives, binders, protective coatings and other products.