We all know that milk contains important nutrients such as calcium and protein that help build bones and muscle. But how much do we really know about these ingredients at the molecular level? Casein—the type of protein accounting for about 80% of the protein in cow’s milk—is bundled into spherical aggregates about 100–150 nm in diameter called “micelles.” Calcium is also found inside the micelles, as colloidal calcium phosphate nanoclusters. Despite the nutritional and economic value of casein micelles (in popular products ranging from protein supplements to cheese), there is still uncertainty over their internal structure. Many small-angle x-ray and neutron studies have been performed to obtain structural information, but interpretation of the data, especially regarding the calcium nanoclusters, is still debated.
Resonant soft x-ray scattering (RSoXS) studies at ALS Beamline 11.0.1 now provide conclusive evidence about scattering features arising specifically from calcium. The technique involves tuning the x-rays to the calcium absorption edge and observing the difference between scans at nearby energies. Surprisingly, a feature that had been attributed to calcium nanocluster size in nearly all small-angle x-ray scattering (SAXS) studies showed no energy dependence near the calcium edge. In contrast, another feature emerged in a position similar to one that had been previously observed in small-angle neutron scattering (SANS). The results support the hypothesis that the SAXS feature is due to protein inhomogeneities rather than calcium nanocluster size, helping to tip debate in favor of a model where micelle structure is stabilized primarily through calcium phosphate nanoclusters and protein hydrophobic bonding.
Resonant soft x-ray scattering (RSoXS) at the calcium L2,3 edge highlights the signal from colloidal calcium phosphate nanoparticles (red/orange dots). An x-ray scattering feature commonly assigned to the particle size is instead due to protein inhomogeneities..
Work performed on ALS Beamline 11.0.1.
B. Ingham, G.D. Erlangga, A. Smialowska, N.M. Kirby, C. Wang, L. Matia-Merino, R.G. Haverkamp, and A.J. Carr, “Solving the mystery of the internal structure of casein micelles,” Soft Matter 10.1039/C5SM00153F (2015). Selected as a Soft Matter “Hot Article” for March 2015.