Casein micelle structure: location of κ-casein

Mammals, Milk, Molecules, and Micelles

After a brief description of my family background and school days, my professional career as a dairy scientist is described under three headings: research, teaching, and writing. My research activities fall into four areas: biochemistry of cheese, fractionation and characterization of milk proteins, heat stability of milk, and dairy enzymology. Finally, I offer some advice to young scientists.

Effect of milk concentration on the rennet coagulation time

Measurements have been made of the coagulation times (CT) of concentrated and diluted milks when treated with chymosin. After dilution, milks showed a large increase in CT: conversely, concentrated milks showed a similar CT to unconcentrated milk. The results are discussed in terms of a mechanism which reduces to a form similar to that proposed by Holter (1932).

Review: Casein micelle structure; an examination of models

The casein micelle system of bovine milk is unique in that protein aggregates of similar spherical shape but extreme variability of size are formed by the self-assembly of three major nonidentical subunits. The monomeric subunits appear to be approximately the same size and shape with similar amphiphilic natures, the chief difference in properties being in the carbohydrate-containing kappa-casein which acts to stabilize the system against precipitation by calcium ion. Micelle models with kappa-casein exclusively in the interior lack a stabilization mechanism and can be eliminated. Statistical considerations of a chain polymer model also lead to its rejection. Electron microscopy reveals spherical submicellar aggregates which at present can be accounted for by only three models. Of these three, the experimental data are predicted only by one in which, alphas 1-, beta-, and kappa-casein subunits are associated into spherical soap micelle-like particles with the kappa-casein segregated into one portion, giving these submicelles an amphiphilic nature. The alphas 1- and beta-caseins are hydrophobic while the kappa-casein portion of the submicelle surface is hydrophilic. Of particular interest is the ability of this micelle model to explain the formation of a minimum micelle which is larger than a submicellar particle.

Casein micelle structure: susceptibility of various casein systems to proteolysis

The susceptibility of the components of various caseinate systems (skim-milk, β-casein-depleted milks, colloidal phosphate-free (CPF) milk, sodium caseinate and isolated β-casein) to proteolysis was investigated. Isolated αs1- and β-caseins were quite susceptible to proteolysis, but their susceptibility decreased in heterogeneous soluble systems and even more so in heterogeneous aggregated systems. In skim-milk and β-casein-depleted milks only about 50% of both αs1- and β-casein was hydrolysable by high levels of rennin, and in CPF milk all αs1- and 70% of the β-casein was hydrolysable. It is suggested that about 50% of micellar β-casein is firmly fixed within the micelle and is unavailable for proteolysis, while the remainder can dissociate from the micelle on cooling and is then readily hydrolysable.

The compatibility of the data with the various published models of the casein micelle is discussed, and a modification of Rose's (1969) model is proposed.