Structural heterogeneity of milk casein micelles: a SANS contrast variation study

We examine the internal structure of milk casein micelles using the contrast variation method in Small-Angle Neutron Scattering (SANS). Experiments were performed with casein dispersions of differentorigins (i.e., milk powder or fresh milk) and extended to very low q-values ( 9 10 4 °A 1), thus makingit possible to precisely determine the apparent gyration radius Rg at each contrast. From the variation ofI(q / 0) with contrast, we determine the distribution of composition of all the particles in thedispersions. As expected, most of these particles are micelles, made of casein and calcium phosphate,with a narrow distribution in compositions. These micelles always coexist with a very small fraction of fatdroplets, with sizes in the range of 20–400 nm. For the dispersions prepared from fresh milk, whichwere purified under particularly stringent conditions, the number ratio of fat droplets to casein micellesis as low as 1 to 106. In that case, we are able to subtract from the total intensity the contribution of thefat droplets and in this way obtain the contribution of the micelles only. We then analyze the variation ofthis contribution with contrast using the approach pioneered by H. B. Stuhrmann. We model the caseinmicelle as a core–shell spherical object, in which the local scattering length density is determined by theratio of calcium phosphate nanoclusters to proteins. We find that models in which the shell has a lowerconcentration of calcium phosphate than the core give a better agreement than models in which theshell has a higher density than the core