Kinetics of gelation of aqueous gelatin solutions

The kinetics of gelation of gelatin + water solutions have been investigated by several experimental methods, in particular optical rotation, 1 H nuclear magnetic resonance and rheological measurements. During quenching and isothermal annealing of the solutions, the gelatin chains undergo a conformational coil→helix transition which is responsible for the network growth. The amount of helix (derived from optical rotation measurements) shows a time evolution which could be analysed as the combination of two processes, the first obeying an Avrami equation, the second proceeding logarithmically in time. The water 1 H n.m.r. measurements reveal that the spin‐spin relaxation time T 2 is affected by these processes, thus implying that a fraction of water contributes to the network structure. Finally, the sol‐gel transition could be followed with a Weissenberg rheogoniometer in oscillatory shear, for a suitable range of temperatures, when the system changes from a purely viscous liquid (the shear modulus G ′=0), to an almost purely elastic solid (the loss modulus G ″=0).