Casein is the main protein component of milk and is of remarkable colloidal stability. Under the influence of milk clotting enzymes casein shows the striking behaviour of coagulation. This clotting process has already been studied by other groups, neglecting the fact that casein is not a homogeneous protein. The purpose of the present study is focused, in this first stage, on the determination of the structure of the various casein components. In cooperation with other laboratories we have been able to obtain the well separated individual proteins. Studies have been performed so far with/~-and x-casein. For detailed structural information we carried out small angle neutron scattering and combined static and dynamic light scattering measurements and determined the molecular weight, Mw, the radius of gyration, ($2), the hydrodynamic radius, Ru, the 0-value and the particle scattering factor, Pz(q). The two caseins show a strikingly different behaviour. For the/~-casein we found a star-like structure, i. e. an aggregation pattern that is expected for a common micelle. The micelle consists of about 38 monomer chains. The aggregates of x-casein appear to be composed of star-like submicelles, where each submicelle contains nine ~e-casein chains and the total degree of aggregation is about 140.

Abstract:The dynamic light scattering behavior of poly(butylmethacrylate) PBMA microgels and of kappa-casein micelles is compared with that from hard sphere latex particles. The latex particles and the kappa-casein micelles exhibited a single exponential decay of the time correlation function (TCF). For the microgels, progessively stronger deviations from a single exponential were observed as the scattering angle was made larger. These deviations are interpreted as being the result of internal modes of motion. From measurement of the first cumulant of the TCF, extrapolated towards zero angle, the translational diffusion coefficients D were determined, and the hydrodynamically effective radii were calculated via the Stokes-Einstein relationship. The ratio of the radius of gyration to the hydrodynamic radius was found to be.Q = 0.775+0.012 for the latex particles, in good agreement with theory. The microgels, however, exhibit much lower Oparameters of 0.49 to 0.58, while the kappa-casein micelles showed the opposite behavior with values between 1.1 and 2.5. The results are interpreted on the basis of the DebyeBueche and Deutsch-Felderhof theory for porous spheres.