Early stages of gelation in gelatin solution detected by dynamic oscillating rheology and nuclear magnetic spectroscopy

This study focuses on the early stages of the gelation of an aqueous type A (pig skin) gelatin solution. The thermo-reversible mono and triple helix formation was observed by rheology and proton NMR relaxation measurements. At high temperatures (T > 330 K), gelatin molecules form flexible random coils of small hydrodynamic radius, the elastic modulus of the solution is relatively low. On decreasing the temperature (330–320 K), mono helix formation begins, connected with an increase of the storage modulus and the hydrodynamic radius. The absence of a significant concentration dependence of this early variation of the modulus indicates the intramolecular nature of this structural change. The simultaneous decrease of the spin–spin relaxation times of the 1H signals of certain aminoacids confirms its effect on the molecular mobility. As this affects especially the signals of arginine and lysine, we conclude that these basic aminoacids play a significant role in forming the intramolecular interactions. The formation of a three-dimensional network occurs at a point at which the viscosity begins to increase rapidly near the gel point (T < 320 K). This process is clearly dominated by intermolecular interactions, as the slope as well as the starting point of the rapid increase significantly depends on the concentration.