NMR study of the polyethyleneglycols diffusion in casein suspensions and gels

PFG-NMR spectroscopy was used to study the diffusion of molecular probes (polyethyleneglycols) in casein suspensions and gels in terms of the effects of probe molecular size (molecular mass between 1080 and 634000 g/mol), casein concentrations (from 3.24 to 16.22 g/100 g) and effects of rennet coagulation. A strong dependency of diffusion on probe size was observed, both in casein suspensions and in gels: as the PEG size increased, the diffusion was reduced. This effect was more pronounced for higher casein concentrations. Changes in casein structure after addition of rennet increased the diffusion coefficient for 82250 and 634000 g/mol PEG. Assuming high internal micelle porosity, a diffusion model with two diffusion pathways, one outside and one inside the micelles was used to explain the PEG diffusion in casein solutions and gels. The results were discussed in the context of variations in casein micelle voluminosity after renneting.