Physicochemical study of κ- and β-casein dispersions and the effect of cross-linking by transglutaminase

The milk proteins β- and κ-casein are in a monomeric state below their critical micelle concentration (cmc) and spontaneously self-associate above the cmc, at or above room temperature. Small-angle neutron scattering (SANS) measurements at 5 g L^-1 confirmed the high specific volume of these micelles, and ¹H-NMR and calorimetric measurements showed that the polypeptide chains remain open and their side chains largely flexible in the associated state. The experimental values of parameters used to describe both types of micelle were radius of gyration of 8 nm, scattering radius (in ²H₂O) of 11 nm, and interaction radius close to 15 nm. The scattering radius is obtained by assuming a homogeneous sphere, and the interaction radius by assuming hard-sphere-like behavior. The micelle size, structure, and interaction radius were independent of concentration. The interparticle structure factor of the micelles was determined using a polydisperse hard-sphere model, which showed that the wave vector position of the peak of the interparticle structure factor was independent of concentration over the range 5-20 g L^-1, consistent with effective hard-sphere behavior. The SANS and ¹H-NMR experiments indicated that the β-casein micelles were less compact and more dynamic than the κ-casein micelles. The micelle dispersions were treated with the cross-linking enzyme transglutaminase. Scattering results showed that the cross-linking hardly influenced the structure or interaction of the micelles, indicating that there was little or no interparticle cross-linking. Addition of 6 M urea did not change the structure of the cross-linked micelles, whereas addition of the urea to the non-cross-linked micelles caused extensive dissociation.