κ‐Casein micelles: structure, interaction and gelling studied by small‐angle neutron scattering

Abstract

Small-angle neutron scattering (SANS) measurements on dilute and concentrated dispersions of ti-casein micelles in a buffer at pH = 6.7 were made using the D11 diffractometer in Grenoble. Results indicate that the micelles have a dense core with a fluffy outer layer. This outer layer appears to give rise to a steeply repulsive interaction on contact. In fact, the hard-sphere model best fits the measured scattering intensities. Adding chymosin to the dispersion initiated a fractal flocculation of the micelles and consecutively a coalescence of the micelles. This unexpected second process resembled that of spinodal demixing. The dispersion phase thus separates into a water and a protein phase on a time scale of hours. The observed phenomona contribute to the understanding of the cheese-making process.Fresh bovine milk contains 2.5% (by mass) casein which is associated in micelles. The main caseins components are found in a molar ratio of CI,~:C(,~ ( p + z ) :~= 4 : 1 : 3 : 1 . 3 . KCasein plays a crucial role in the stabilization of the micelles. The number of K-casein molecules/surface area is roughly constant although the micelle size varies, mainly between 100 -250 nm in diameter. Fairly detailed pictures of these micelles exist, but not all details are fully understood. The basic model developed by Schmidt [1] was derived from electron microscopy studies. It was suggested that cow milk casein micelles 'consisted of nearly spherical subunits with a diameter of 10-15 nm' [I]. Later Schmidt and Bucheim [2] made a more extensive electron microscopical study and reported size distributions for the different types of (sub)micelles between 5 -20 nm. The picture was refined by Holt [3] and Walstra [4] who proposed a 'hairy' micelle model. The hairs provide steric stabilization of the micelles and are identified as the K-casein. The models of Schmidt, Holt and Walstra are essentially the same. Differences are mainly topographical. Their common feature is that the casein micelles show a highly regular conglomerate of highly uniform submicelles. In neutron and Xray scattering experiments such a conglomerate would give a very pronounced peak in the scattering intensities as a function of wave vector. The small-angle neutron scattering (SANS) results of Stothart [5, 61 show only a very weak maximum in the structure factor. Identifying the position of this very broad peak to a correlation length seems justifiable, but it need not necessarily be the size of a submicelle. It could well be the average size of condensed protein structures as is depicted by Griffin [7]. The analysis of the small-angle X-ray data of Pessen et al. [S] is not correct since they add intensities instead of field strengths. It thus seems that the picture of Griffin [7] is more realistic. BA EDE, The NetherlandsIn all descriptions K-casein is situated at the surface of the micelle thus providing the 'mainly steric' Stabilization. The phenomenon of steric stabilization is well understood (see e. g. Napper [9]) and plays an important role in other...

Characterization of Enzymatically Induced Aggregation of Casein Micelles in Natural Concentration byin SituStatic Light Scattering and Ultra Low Shear Viscosimetry

“…For particles of arbitrary shape and polarizibility, ⌬␣ ជ Ј(r), the pair distance distribution function is given by p͑r͒ ϭ r 2 ⌬␣ Ј͑r͒, [5] where ⌬␣ Ј(r) is the convolution square of ⌬␣Ј(r ជ) averaged over all directions in space. The pair distance distribution function p(r) is represented by a series of cubic B-splines.…”

Dissociation from the Oligomeric State Is the Rate-limiting Step in Fibril Formation by κ-Casein

“…Second, it is more likely that the dissociated species would enable the ordered aggregation required for fibril formation to proceed, since it would expose the hydrophobic "legs" that we have identified make up the protease-resistant core of the fibrils (see below). In contrast, in the oligomeric form of the protein, the hydrophobic regions are thought to be internally buried (23,40), which would limit their ability to associate to form fibrils.…”

Particle sizes of purified κ-casein: Metal effect and correspondence with predicted three-dimensional molecular models

“…Complete reduction of K-casein yields a selfassociating system which has been characterized by Vreeman et al (1986) as a polymer of n = 31 in equilibrium with its component monomer. The latter model has been the subject of some debate and two alternative polymeric models have been suggested (Thurn et al, 1987, deKruif andMay, 1991). This paper investigates the overall polymer size distribution of purified K-casein as determined by electron microscopy and correlates these observations with predictive molecular models for K-casein (Kumosinski et al, 1993).…”