Casein is a group of phosphoproteins that is found in milk and milk-based products. When a coagulant is added to milk, it separates into curds (the casein) and whey. Casein is most often supplemented in the form of a dry powder. The most common supplement forms of casein are micellar casein and casein hydrolysate. Micellar casein is most commonly used, and this form is digested very slowly. Casein hydrolysate is a predigested form of casein and will be absorbed more quickly.