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Casein is the predominant phosphoprotein found in fresh milk. When coagulated with rennet, casein is sometimes called paracasein. British terminology, on the other hand, uses the term caseinogen for the uncoagulated protein and casein for the coagulated protein. As it exists in milk, it is a salt of calcium.

Casein is not coagulated by heat. It is precipitated by acids and by rennin, a proteolytic enzyme obtained from the stomachs of calves. The enzyme trypsin can hydrolyze off a phosphate-containing peptone.

In addition to being consumed in milk, casein is used in adhesives, binders, protective coatings, and other products such as knitting needles. The purified protein is water insoluble. While it is also insoluble in neutral salt solutions, it is readily dispersable in dilute alkalis and in salt solutions such as sodium oxalate and sodium acetate.

Some people with autism and Asperger's disorder are sensitive to casein and gluten. See more at gluten-free, casein-free diet.