On the inverse quenching technique applied to gelatin solutions

Gelatin gels are known to be nonequilibrium systems, because of the continuous growth and rearrangements of physical junctions, even in the solidlike state. Establishing a relationship between the relative degree of cross-linking and macroscopic elasticity would be crucial in understanding, modeling, and predicting the transformation processes of gelatin solutions. Performing rheological experiments on a distinct gel structure, with a definite number of cross-links, is, however, a challenging task. In isothermal conditions, indeed, the density of physical cross-links changes indefinitely, and network evolution cannot be arrested. Inspired by the inverse quenching technique applied in the past to semicrystalline polymers, we here apply an unusual thermal history to an aqueous solution of gelatin in the semiconcentrated regime (6.67 % w pig-skin gelatin), in order to freeze the system in a metastable condition for a time sufficiently long to perform a rheological characterization. The solution, initially kept in the sol state at 60 °C, is rapidly cooled below gelation temperature, and isothermal gelation is started at 10 °C. After soaking at this low temperature for a given time, the sample is rapidly heated (inverse quenching) up to a value in the range 24-29 °C, where kinetics is monitored. If the waiting time at low temperature and the inverse quenching temperature are suitably chosen, sample elasticity will remain stationary for a relatively large time window, and rheological experiments can then be reliably performed.