Investigation of particle dynamics in gels involving casein micelles: A diffusing wave spectroscopy and rheology approach

The different physical properties observed by rheology and light scattering have been studied in detail for three different gelling systems, namely acidified heated skim milk, heated skim milk plus κ-carrageenan and heated skim milk plus agar. The use of diffusing wave spectroscopy (DWS) permitted the study of undiluted mixtures of skim milk and polysaccharides. The results of this investigation show that, for the particular sample conditions used in this study, rheological measurements find the G′ value of the agar plus milk system to be the largest while placing the acidified milk system as the weakest gel. DWS, in contrast, shows a fluid system for the agar plus milk case but greatly restricted dynamics in the acidified milk system. This work is not concerned with the strengths of the gels per se, but rather with the relation between the gel macrorheology and the dynamics of the particles in the gel. These results demonstrate the differences in the basic physical principles between rheology, which follows the viscoelastic response of system to an applied stress, and DWS which follows the short-term dynamics of a scattering particle. The results obtained here highlight the importance of a multi-technique approach when studying complex systems.