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Effect of Calcium Concentration on the Structure of Casein Micelles in Thin Films

P. Müller-Buschbaum ^*, R. Gebhardt , S. V. Roth , E. Metwalli ^* and W. Doster ^*

^* Physik-Department, Technische Universität München, Garching, Germany; European Syncrotron Radiation Facility, Grenoble, France; and HASYLAB at DESY, Hamburg, Germany

Correspondence: Address reprint requests to P. Müller-Buschbaum, Tel.: 49-89-2891-2451; E-mail: muellerb{at}ph.tum.de.

The structure of thin casein films prepared with spin-coating is investigated as a function of the calcium concentration. Grazing incidence small-angle x-ray scattering and atomic force microscopy are used to probe the micelle structure. For comparison, the corresponding casein solutions are investigated with dynamic light-scattering experiments. In the thin films with added calcium three types of casein structures, aggregates, micelles, and mini-micelles, are observed in coexistence with atomic force microscopy and grazing incidence small-angle x-ray scattering. With increasing calcium concentration, the size of the aggregates strongly increases, while the size of micelles slightly decreases and the size of the mini-micelles increases. This effect is explained in the framework of the particle-stabilizing properties of the hairy layer of -casein surrounding the casein micelles.