Microstructure investigation of concentrated dairy gels by real-time NMR diffusion experiments

By studying the diffusion of probe molecules of various sizes, information can be obtained on the microstructure of a sample at different length scales. Such experiments have therefore been applied to a great variety of matrices, including polymers and proteins in both liquid and gel states. The diffusion of poly(ethylene glycol)s (PEGs) measured by Pulsed Field Gradient (PFG)-NMR is probably the most widely used method to perform these investigations. Previous pulsed field NMR studies have shown that PEG diffusion in casein suspensions and gels is greatly dependent on both the volume fraction occupied by casein particles and the probe size. For a given volume fraction of casein particles, the reduction in the diffusion coefficient induced by the obstacles is smaller for smaller probes. Recently, it has been demonstrated that the relative change of the self-diffusion coefficient depends on the final gel microstructure. This could be explained by considering that the diffusion of larger probes are more sensitive to the extent of the casein aggregate compaction and thus to the extent of certain rearrangement processes. However, as in all the studies quoted above, the diffusion of a molecule was measured at equilibrium, before and after the perturbation of the system. This method cannot provide any dynamic information on the evolution of the sample microstructure although the modifications of the matrix in reaction to the perturbation can be very progressive. This is precisely the case when casein suspensions are coagulated by addition of chymosin and/or acidifying agent. In the present study, we designed an original method to investigate how and when probe diffusion rates vary during the coagulation induced by chymosin action and during acid coagulation. Self-diffusion measurements were repeated throughout the coagulation processes by means of pulsed field gradient (PFG)-NMR techniques. This method enabled us to reveal dynamic information on the sample microstructure modifications during sol-gel transition. This study therefore constitutes a new illustration of the potential of probe diffusion measurements to reveal structural changes in complex and evolving matrices.