The T₂ relaxation time of the protons of water adsorbed on the surface of colloidal particles in aqueous colloidal dispersions measured by nuclear magnetic resonance (NMR) is significantly shorter than that of bulk water. The normalized relaxation rate R_2sp derived from T₂ is proportional to the total surface area of the colloidal particles. Since the surface area decreases when the colloidal particles aggregate, R_2sp is used to evaluate the aggregation behavior of colloidal particles. We attempted to observe milk clotting behavior using NMR. We calculated R_2sp from the T₂ of milk to which rennet was added at a concentration of 10-20 ppm, and observed its time development. R_2sp decreased rapidly after a certain induction time after rennet was added. The time at which R_2sp decreasing was shortened depending on the concentration of rennet added. On the other hand, the time at which viscosity begins to diverge was determined from the time development of the complex viscosity of the raw milk to which rennet was added, and this was found to be consistent with the time at which R_2sp decreasing. From these results, it was clear that the time of milk clotting could be determined from the time development curve of R_2sp by NMR.

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