Rheo-chemistry of gelation in aiyu (fig) jelly

Abstract

We investigated the gelation characteristics of aiyu jelly derived from the polymeric extract obtained by washing the seeds of Ficus Pumila var. Awkeotsang. The main gel component is low methoxyl pectin, which forms crosslinks with divalent ions that bind the exuded polysaccharide chains. Unlike many fruit pectins used in jam and jellies that require heating, additional sugars, and acidic conditions to gel, the aiyu exudate gels at room temperature with just the addition of water. In this study, the time-resolved dynamics of the gelation process and the evolution of the viscoelastic relaxation spectrum with frequency and gel age are obtained via Optimally Windowed Chirp rheometry (OWCh), conventional time-sweep rheometry, and cryo-scanning electron microscope (cryo-SEM) microstructural characterization.During gelation, we observed distinctive frequency-dependent inflection points in both the time-evolving storage and loss moduli which occur nearly simultaneously in time. Close inspection of high resolution cryo-SEM images taken at different times during the gelation process showed that the gels initially form nano-fiber networks with mesh sizes on the order of tens of microns and the stiffer mature gels observed at longer times develop sub-micron mesh sizes. These observations suggest a progressive transformation between microstructures may accompany the progressive growth in the elasticity of the aiyu gels and the local inflection points in the viscoelastic moduli.We also developed a multi-species reaction kinetics model for calcium – pectin binding in aiyu gelation. By including methylesterase enzymatic conversion of pectin binding sites, combined with calcium binding and the slower formation of consecutively-bound junction zones, the distinctive inflection points in the viscoelastic moduli are captured by the kinetic gelation model. By combining rheological measurements, microstructural observations, and reaction kinetics modeling, this study not only characterizes a unique water-soluble natural extract that gels at room temperature with no additives or thermomechanical processing, it also demonstrates that the aiyu polysaccharide system is a safe and controllable model for gelation systems in which binding site activation is concurrently coupled to crosslink formation.