The structure of casein micelles: a review of small-angle scattering data

Abstract

Casein micelles are association colloids found in mammalian milk. Small‐angle scattering data on casein micelles have been collected and are reviewed, including contrast variation. The scattering spectra are quite consistent at medium and high scattering wavevectors [Q = 4πnsin(θ/2)/λ, where n is the refractive index, λ is the wavelength and θ is the scattering angle]. Differences are noted, especially at low Q, which may be attributed to sample preparation, particularly the presence of residual fat globules. Scattering spectra are calculated using a generalized scattering function and a composite particle model, and it is possible to give a self‐consistent calculation of the spectra using one set of parameters for all contrasts in both small‐angle X‐ray scattering and small‐angle neutron scattering. The data and calculations show that a casein micelle is a homogeneous particle. The polydispersity in size is about 35% and therefore experimental data on particle size depend very much on the method used. A `reference set' of numbers is proposed for casein micelles from pooled cows' milk, which may be given as follows: β = 0.35, R10 = 60 nm, Rg = 110 nm, Rhydr = 96 nm (at 90° scattering). Often, use is made of dynamic light scattering (DLS), which gives an Rhydr = 〈R6〉/〈R5〉 of 80–100 nm at 90° scattering. Values will be considerably higher at low(er) angles, and lower at backscattering angles, which are currently used in many DLS setups. Larger values are probably due to clusters of casein micelles or residual fat. The structure of a casein micelle can best be described as a protein matrix in which calcium phosphate clusters (2 nm radius) are dispersed. The protein matrix has density variations on a similar length scale. The casein micelle–submicelle model and models with large voids and channels are highly improbable.

Proteostasis and the Regulation of Intra- and Extracellular Protein Aggregation by ATP-Independent Molecular Chaperones: Lens α-Crystallins and Milk Caseins

“…49 In a typical casein micelle of 100 nm radius, there are about 800 nanoclusters that give rise to the characteristic substructure of casein micelles on a scale of 18 nm. [50][51][52]…”

Casein‐Based Powders: Characteristics and Rehydration Properties

“…However, the exact structure of the casein micelles has not yet been fully elucidated. Several models have been proposed to describe the structure of casein micelles and their interactions with other components (Dalgleish , ; Farrell and others ; Dalgleish and Corredig ; De Kruif and others ; De Kruif ). Still, some concepts are generally accepted: Most κ‐casein is present on the micelle exterior, but the κ‐casein is not homogeneously distributed on the surface Hydrophilic segments of κ‐casein extend into the serum The interior of the micelle can be considered as a network of α s ‐ and β‐caseins connected to nanoclusters of calcium phosphate Both the content of colloidal calcium phosphate and hydrophobic interactions between casein molecules are necessary for micellar integrity. …”

Elemental fingerprinting of mineral species in iron-fortified milk: anomalous small-angle X-ray scattering and resonant soft X-ray scattering studies

“…The discrepancy with the SAXS data can be ascribed to a misinterpretation by other authors as to the identity of the main feature at higher q. This feature is commonly attributed to the CCP form factor; however, it is much more likely to be scattering due to local density fluctuations in the protein structure (De Kruif et al, 2012;De Kruif, 2014;Ingham et al, 2016). Here we show the application of RSoXS to iron-fortified cow milk (with 10 or 20 mM added Fe 2+ ) at both the Ca and Fe L 2,3 -edges and compare the results with ASAXS at the Fe K-edge, to obtain structural information about both Ca and Fe in iron-fortified milk and to demonstrate the applicability and limitations of the two techniques.…”

“…In small-angle neutron scattering (SANS), contrast variation has been used for many years to investigate the structure of casein micelles in milk (Stothart & Cebula, 1982;Hansen et al, 1996;Holt et al, 2003;De Kruif et al, 2012;De Kruif, 2014;Bouchoux et al, 2015). In this technique, the milk is reconstituted in a mixed H 2 O/D 2 O solvent, where the H 2 O/D 2 O ratio is chosen to have a neutron scattering length that matches that of one part of the system, e.g.…”

“…Since the scattering intensity from each component is proportional to the square of the scattering length contrast, that component is rendered invisible to neutrons. Measurements performed at the protein match point (corresponding to approximately 40% D 2 O) show a feature presumed to correspond to CCP (Holt et al, 2003;De Kruif et al, 2012;De Kruif, 2014;Bouchoux et al, 2015). However, this assignment can only be inferred; the measurement is not specific to calcium per se.…”