Small-angle scattering (SAXS) is a well-established characterization method to investigate the nanostructure of proteins in food [1]. SAXS in combination with microbeams and sub-micrometer raster-scans allows probing the microstructure of food. [2] We use microbeam grazing incidence small-angle X-ray scattering (µGISAXS) to explore the structure of proteins from milk and egg-yolk on surfaces and interfaces. In contrast to atomic force microscopy (AFM), GISAXS provides structural information from buried interfaces and can be used as an analytical in-situ technique during food processing. Casein micelles (CM) for instance agglomerate on top of micro-sieves during the microfiltration of milk and undergo a structural transition. Here, we demonstrate that structural changes in the shape and the ensemble structure of CM depend on filtration forces, the content on whey proteins and the micro-architecture of the used micro-sieve. Structural data from the GISAXS experiments allow us to explain deposit resistances from filtration experiments on a molecular basis. We used a similar approach to probe the influence of pH on the molecular structure of HDL-granules in egg yolk. Under acid conditions at pH 4.0, egg yolk is used as an emulsifier for the production of mayonnaises. Decreasing pH from 6.5 to 4.0 causes a higher surface-bound fraction of LDL on the HDL-granules which makes an investigation of HDL-granules by AFM impossible. In contrast, X-ray experiments are sensitive to both the subunit structure of the HDL-granules and the repeating distance of phospholipids in the LDL-vesicles and shed new light on the interaction between LDL vesicles and HDL-granules.
[1] C. Holt, C. G. de Kruif, R. Tuinier, P. A. Timmins, Colloids and Surfaces A, 2003, 213, 275-284
[2] C. Riekel, M. Burghammer, R. Davies, R. Gebhardt and D. Popov, Lect. Notes Phys., 2009, 776, 91-104.
[3] R. Gebhardt, T. Steinhauer, P. Meyer, J. Sterr, J. Perlich, U. Kulozik, Faraday Discuss., 2012, 158, 77-88.
Dr. rer. nat. Ronald Gebhardt ronald.gebhardt@tum.de
