Multiple particle tracking investigations of acid milk gels using tracer particles with designed surface chemistries and comparison with diffusing wave spectroscopy studies.

Multiple particle tracking (MPT) has been used in an attempt to probe the heterogeneity of acid milk gels, made with and without added pectin, by following the distribution of the displacements of added tracer beads during and after gelation using the Van Hove distribution. Furthermore, the surface chemistry of the latex probe particles was modified in an attempt to control their location in the system and probe the microrheological properties of the protein network and aqueous-phase voids independently. In addition, the mean square displacement (MSD) of the casein micelles/casein aggregates themselves, obtained by diffusing wave spectroscopy (DWS), has been compared to the ensemble-averaged MSD calculated from the data obtained by tracking the movement of the added tracers, with and without a kappa-casein coating. For the kappa-casein-coated tracer particles, upon acidification and subsequent gel formation, the MSDs obtained by MPT superimpose remarkably well with the MSDs obtained by DWS, despite the fact that one is obtained by tracking the movement of the particle network elements themselves and the other is obtained from directly tracking added tracers. This result has important implications: (i) it demonstrates that, although the DWS measurement is intrinsically ensemble-averaged, it really gives insight into the dynamics of the colloidal gel network; (ii) it confirms that the kappa-casein-coated probes used in this MPT experiment are well incorporated into the gel network; and hence (iii) that at least in gelled systems kappa-casein-coated latex probes are interesting probes that reveal the dynamics of the casein network.