The rennet coagulation mechanisms of a concentrated casein suspension as observed by PFG-NMR diffusion measurements

Abstract

Pulsed field gradient-nuclear magnetic resonance (PFG-NMR) was used to monitor the diffusion of caseins throughout the rennet coagulation of a highly concentrated casein suspension. Two types of casein species were distinguished according to their diffusing properties and attributed to the dissolved casein fraction, i.e. dissociated caseins and caseinomacropeptides (CMP), and casein particles, respectively. The NMR signal intensity coming from the dissolved casein fraction, which is related to the number of protons it contains, increased all along the experiment whereas the opposite tendency was observed for casein particles. This was explained by the increasing amount of CMP in the whey and the reciprocal loss of protons contained by casein particles, and used to quantitatively estimate the kinetics of the chymosin action. The diffusion of the dissolved casein fraction remained nearly constant during the coagulation process whereas, as revealed by rheometry measurements, the diffusion of casein particles was very sensitive to the structural reorganization of the sample. It decreased by about 35% during the sol–gel transition and increased in similar proportions during the succeeding rise in gel porosity. Our results also provide different types of information on the respective behaviors of dissociated caseins and casein particles during the course of the process, the most remarkable one being that all the casein particles did not aggregate during the sol–gel transition of our sample. This strongly suggests that the rennet gelation of a concentrated dairy solution may be better visualized by the formation of a network backbone during the sol–gel transition which is thereafter reinforced upon further incorporation of casein particles.