Kinetic and mechanistic considerations in the gelation of genipin-crosslinked gelatin

The gelling properties (gel time (t(gel)) and gel strength) of a 10% (w/w) gelatin sol were investigated as a function of genipin (GP) concentration (0-15 mM) and temperature (25-55 degrees C) to discern mechanisms and optimal conditions for fixation. Gel time increased with increasing temperature, reached a maximum, and then declined as temperature was raised further. By contrast, network strength data followed the opposite trend. From the thermal behavior of t(gel) and network strength, it was inferred that gelation in the low-temperature regime was dominated by hydrogen bonding, while in the high-temperature regime it was dominated by covalent crosslinking. At higher temperatures, crosslinking was described by an Arrhenius rate law expression, with activation energies between 63.2 and 67.8 kJ/mol, depending on GP concentration. In the low temperature regime, an Arrhenius plot resulted in negative activation energies of -75.8 and -64.4 kJ/mol in the presence of 10 and 15 mM GP, respectively. With an increase in both GP concentration and temperature, the gelatin network gradually shifted from being dominated by hydrogen bonds (physical crosslinks) to covalent crosslinking (chemical crosslinks).