Structure of casein micelles in milk protein concentrate powders viasmall angle X-ray scattering

Abstract

The stability of milk protein concentrate (MPC) powders and their solubility in water is presumed to depend on the interfacial and internal structure of the casein micelle. This study reports the internal micellar structure for MPC powder for the first time. We have investigated the nanostructure of MPC powders and of dilute solutions thereof using small-angle X-ray scattering (SAXS). In addition, we have measured the scattering from the primary components of MPC in their powder and solution state to enable an assessment of their contribution to the overall scattering from the whole system. The interfacial and internal structural detail of the casein micelle is still an area of debate and we have considered the two popular models for casein micelles, namely the submicelle and nanocluster models; we find that the latter adequately describes the observed experimental results. The scattering curve for the powders may be described by three characteristic regions. The first region up to 0.03 Å⁻¹ is interpreted to correspond to the scattering from a sharp and smooth interface. The second inflexion point, observed at ∼0.045 Å⁻¹, may be attributed to a mean intercluster correlation length of colloidal calcium phosphate (CCP) nanoclusters distributed within casein micelles. This is the first time such a feature has been observed in the powder state but its presence is reminiscent of a similar feature observed previously with solvent contrast variation small-angle neutron scattering. We interpret that its presence here is due to an enhancement of the scattering contrast by virtue of the removal of water from the system (and replaced by air) combined with an increase in particle density due to drying. The third inflexion at 0.1 Å⁻¹ may be interpreted, in common with other authors, as a signature of colloidal calcium phosphate nanoparticles.