The casein micelle: Historical aspects, current concepts and significance

The caseins, a group of unique milk-specific acid-insoluble phosphoproteins, represent ≈80% of the total protein in the milk of cattle and other commercial dairy species. Owing to their commercial importance, the caseins have been studied very extensively and are probably the best characterized food protein system. It has been recognized since the work of Schübler in 1818 that the caseins exist in milk as large particles suspended in the aqueous phase (milk serum). Initially, the casein particles were usually referred to as “calcium caseinate–calcium phosphate particles”. The term “casein micelle” was introduced in 1921 and the two terms were used interchangeably for several years but since about 1960, the latter term has been used exclusively. It has been suggested that the calcium caseinate–phosphate particles are not true micelles. The term “micelle” was introduced by Nägeli and Schwendener [Nägeli, C.W., & Schwendener, W. (1877). Das Mikroskop: Theorie und Anwendung Desselben (2nd ed.). Leipzig: W. Engelmann] for microparticles of cellulose in plant cells visible in the light microscope; later, it was used for various other types of aggregates. Owing to the importance of the casein micelles for many of the physico-chemical properties of milk and dairy products, their structure and properties and the effects of compositional and processing factors thereon have been studied extensively. Since the discovery of the micelle-stabilizing protein, κ-casein, in 1956, several models of the casein micelle have been proposed and refined. This review will focus on the following aspects: introduction and use of the term “micelle”, early views on the stability of casein in milk, introduction of the term “casein micelle” for the calcium phosphate–calcium caseinate particles in milk, and the structure and stability of casein micelles.