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

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, R_g = 110 nm, R_hydr = 96 nm (at 90° scattering). Often, use is made of dynamic light scattering (DLS), which gives an R_hydr = 〈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.