Rheo-Impedance Behavior during the Gelation Process of a Gelatin Solution

Since a gelatin solution easily undergoes a reversible sol-gel phase transition by heating and cooling, it has been the subject of research using various approaches for a long time. However, there have been few studies that have dynamically and in real time evaluated the series of processes that transform a sol to a gel (or from a gel to a sol) via the sol-gel transition. We therefore decided to use the rheological impedance technique, which can be measured in real time, to evaluate the dynamic changes in the internal structure of the gelatin during the transition process from the sol to the gel during cooling. The point at which the elastic moduli (<i>G</i>' and <i>G</i>″) of the gelatin rapidly increased shifted to a lower temperature and time with the increasing cooling rate. The <i>G</i>' at the point where the gelation converged was higher for the slower cooling rates. The impedance |<i>Z</i>| during the gelation process increased with decreasing temperature. The resistance value, <i>R</i> ₁, which is thought to be due to the internal structure of the gelatin, exponentially increases until near the sol-gel transition. Thus, it seems that <i>R</i> ₁ indicates the resistance of electrolytes and ions moving inside the cross-linked collagen molecules. Since the apparent plateau modulus <i>G</i>'_p is in a proportional relationship with <i>R</i> ₁, we concluded that <i>R</i> ₁ corresponds to the increase of molecular weight accompanying the progress of the cross-linking. Therefore, we found that the rheo-impedance measurement is a very effective method for examining the dynamic and real-time changes in the internal structure of the gelatin during the entire sol-gel transition process.