Characterizing Length Scales that Determine the Mechanical Behavior of gels from Crosslinked Casein Micelles

Mechanical behavior of a protein gel plays a large role in sensory properties. Despite the large amount of research on caseins, the origin of the mechanical behavior is not well understood yet. To determine the length scales that are relevant for the mechanical behavior of casein gels, casein micelles were crosslinked with increasing amount of transglutaminase followed by acidification to form gels. The gel heterogeneity, observed with confocal microscopy, electron microscopy and light scattering showed a gradual decrease on a micrometer length scale with increasing crosslinking. Such gradual change as a function of crosslinking was also observed in the elastic modulus and the Young’s modulus of the gels. Furthermore, particle size both prior to gelation and in the gel decreased with increasing crosslinking. Casein micelle stiffness (determined by atomic force microscopy) showed a maximum and the amount of water entrapped by the gel particles and their aggregates in the gel (determined by neutron scattering) showed a minimum with increasing crosslinking. These extrema coincides with the extrema observed in kinetics of gel formation and in gel breakdown properties. It was concluded that the elasticity of the gel originates on the length scale of the casein micelle (a few hundred nanometer), while fracture properties are determined at a smaller length scale, by the structure within the casein micelle.