Milk contains hundreds of types of protein, most of them in very small amounts. The proteins can be classified in various ways according to their chemical or physical properties and their biological functions. The olden way of grouping milk proteins into casein, albumin and globulin has given way to a more adequate classification system. The table below shows an abridged list of milk proteins according to a modern system. Minor protein groups have been excluded for the sake of simplicity.

Whey protein is a term often used as a synonym for milk-serum proteins, but it should be reserved for the proteins in whey from the cheesemaking process. In addition to milk-serum proteins, whey protein also contains fragments of casein molecules. Some of the milk-serum proteins are also present in lower concentrations than in the original milk. This is due to heat denaturation during pasteurization of the milk prior to cheesemaking. The three main groups of proteins in milk are distinguished by their widely different behaviour and form of existence. The caseins are easily precipitated from milk in a variety of ways, while the serum proteins usually remain in solution. The fat-globule membrane proteins adhere, as the name implies, to the surface of the fat globules and are only released by mechanical action, e.g. by churning cream into butter.

Concentration of proteins in cow's milk: