Casein is a protein found in milk. It can be separated by the action of acid, the enzyme rennin, or bacteria (souring); it is also the main protein in cheese. Casein is used as a protein supplement in the treatment of malnutrition and is used commercially in cosmetics, glues, pigments and as a sizing for coating paper. Casein is insoluble in water, but is freely soluble in alkaline solutions. Research Casein