Effect of filtration forces on the structure of casein micelles

Pressure-driven membrane filtration is a widely used method to separate casein micelles (CM) from smaller components in milk. The structure of CM attached on the membrane has been investigated because in such a deposited state they reduce the performance of the filtration process. Scattering experiments with nano-and micrometre sized X-ray beams and a filtration setup with silicon micro-sieves as membranes were used. Grazing-incidence small-angle X-ray scattering (GISAXS) experiments above porous regions of the micro-sieves show that spherical CM become stretched in the direction of the filtration flow. The one-dimensional scattering functions extracted from the two-dimensional GISAXS patterns were analyzed by a single ellipsoidal form factor fit. According to the model, CM assume a prolate ellipsoidal shape at a transmembrane pressure of Delta p = 400 mbar (1 mbar = 100 Pa). With increasing transmembrane pressure, the shape of the CM undergoes a transition towards an oblate structure between 400 and 600 mbar. Small-angle X-ray scattering experiments with a 200 nm beam allow for transmission experiments on CM in a single pore of the micro-sieve. Typical characteristics of the internal structure could not be identified in the scattering functions of CM subjected to filtration forces.