Presence of casein in milk

Milk contains 3.3% total protein. There are two major categories of milk protein that are broadly defined by their chemical composition and physical properties. The casein family contains phosphorus and will coagulate or precipitate at pH 4.6. The serum [whey] proteins do not contain phosphorus, and these proteins remain in solution in milk at pH 4.6. The principle of coagulation, or curd formation, at reduced pH is the basis for cheese curd formation. In cow s milk, approximately 82% of milk protein is casein and the remaining 18% is serum, or whey protein. 

The casein family of protein consists of several types of caseins [a-sl, a-s2, and/3-6]. Each of them has its own amino acid composition, genetic variations, and functional properties. The caseins are suspended in milk in a complex called a micelle. 

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