| Summary: | [Departement_IRSTEA]Ecotechnologies [TR1_IRSTEA]SPEE International audience By studying the diffusion of probe molecules of various sizes, information can be obtained on the microstructure of a sample at different length scales. The diffusion of poly(ethylene glycol)s (PEGs) measured by pulsed field gradient (PFG)-Nuclear Magnetic Resonance is probably the most widely used method to perform these investigations. PFG-NMR is a very powerful and nondestructive technique to determine self-diffusion coefficients, and PEG molecules selected as probes offer several advantages. They are water-soluble and available in a wide range of molecular weights with low polydispersity indices, and their NMR signal is a sharp band. Moreover, PEGs present very weak interactions with proteins. PEG diffusion determined by PFG-NMR techniques makes possible the observation of obstruction effects in real biological matrices. In the present study, we illustrated the potentiality of the PFG-NMR technique to investigate structural changes in dairy protein gels, and the sensitivity of probe diffusion to reveal dynamic information on evolving systems at different length scales. The NMR experiments were performed on highly concentrated casein system and the three different coagulation processes were studied: a chymosin coagulation, a coagulation induced by acidification alone and with the concomitant action of chymosin. The self diffusion of a small and a large PEG were investigated by PFG-NMR throughout each type of coagulation in order to probe the microstructure at different length scale. Probe diffusion in casein suspensions and gels is greatly dependent on both the volume fraction occupied by casein particles and the probe size. The reduction in diffusion coefficient for a given volume fraction of casein particles is smaller for smaller probes. This phenomenon was explained by assuming a model with two diffusion pathways, one around and one through the casein micelles. According to the two site model, variations in the diffusion rate ...
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