Probing the internal and external micelle structures of differently sized casein micelles from individual cows milk by dynamic light and small-angle X-ray scattering

Together with calcium and other minerals caseins, a family of specialised phosphoproteins, form micelles. These micelles vary in size both within and between individual cows depending on the ratios of specific caseins and levels of glycosylation. In this work, milks were obtained from individual cows with known genetic variants of κ- and β-casein, and β-lactoglobulin, and a range of native micelle diameters. Large and small micelle fractions were also prepared to narrow the distribution of micelles at either extreme of the natural range. When examined by SAXS, both native micelles with different sizes, and fractionated micelles were found to have identical scattering patterns in the high q region, implying that the internal structures of casein micelles are the same regardless of their overall size. When the micelles exposed to EDTA at above 2 mM, the shoulder inflexion at q ~ 0.035 Å-1 in the SAXS curves were diminished indicating disruption of the colloidal calcium phosphate (CCP) nanoclusters within the micelles. Adding ethanol to milk caused changes to the micelle diameter which is attributed to changes in the micelle internal structure and to the collapse of the ‘hairy layer’ at the micelle surface. This is evident by the appearance of the shoulder inflexion at ~0.08 Å-1 and changes in the SAXS curves at the low q region. Despite the large natural variation in casein micelle size, the internal organisation of the micelles is not influenced by overall diameter when probed by a range of different solvent environments.