Casein micelle structure and flow properties

Structure of Casein Micelle Suspensions characterized by SAXS, USAXS, and Cryo-Electron Microscopy

In the past, numerous studies have proposed two different models for the internal structure of casein micelles. The first model describes the interior of the micelle as a cluster of sub-micelles approximately ten nanometers in size (Walstra, 1999). The second model describes the interior of the micelles as a uniform protein matrix (De Kruif and Holt, 2003).

SAXS and USAXS measurements have allowed us to observe, for the first time, the structure of casein micelles over a broad range of length scales from 2 nm to 1000 nm.

All the results obtained on the dispersions of model skimmed milk (low heat, PPCN) or fresh skimmed milk, show the same profiles of the scattered intensity I(q) as a function of the scattering wave vector q (Fig. 1), with an inflection point at large scattering wave vectors and a shoulder at smaller scattering wave vectors.

This curve reveals an organization of casein micelles at several hierarchical levels of length scales, modeled by the unified Beaucage function. The analysis of these scattering curves has provided strong evidence in favor of an organization of caseins according to the model of statistical random coil chains leading to a uniform protein matrix and demonstrates the absence of "submicellar" structures (Pignon et al. 2004).

These results, obtained by X-ray scattering, were confirmed by cryo-transmission electron microscopy measurements, which revealed the internal structure of the micelles. These micelles consist of a complex, heterogeneous network of caseins within which nanometric calcium phosphate particles are distributed (Fig. 2) (Marchin et al. 2007).

This breakthrough resolves a debate that has persisted for several years regarding the internal organization of casein micelles, which is central to the aggregation and gelation mechanisms used on a large scale in industrial dairy processing.

Références 

• Pignon F., Belina G., Narayanan T., Paubel X., Magnin A. and Gésan-Guiziou G.,"Structure and rheological behavior of casein micelle suspensions during ultrafiltration process", The Journal of Chemical Physics, 121(16), 8138-8146 (2004). https://doi.org/10.1063/1.1800931

• Marchin S., Putaux J.L., Pignon F. and Léonil J., "Effects of the environmental factors on the casein micelle structure studied by Cryo-TEM and SAXS/USAXS", The Journal of Chemical Physics, 126, 045101 (2007). https://doi.org/10.1063/1.2409933

• Wastra P., Int. Dairy J., 9, 189-192 (1999).

• De Kruif C. G., Holt C., Advances dairy Chemistry I, Part A. Proteins, 233-276 (2003).

• G. Beaucage, J. Appl. Cryst., 28, 717 (1995).