Cryo-scanning electron microscopy (cryo-SEM) as a tool for studying the ultrastructure during bead formation by ionotropic gelation of calcium pectinate

Understanding the gel ultrastructure is of great importance for process and product development having great effects on the product characteristics. The samples containing high amount of water could not be directly observed using scanning electron microscope (SEM) without removing of water. However, cryo-SEM can be used to study the ultrastructure of hydrated samples. In this study, ultrastructural information of internal structure was obtained by imaging the cryo-fractured beads in a cryo-SEM. This technique was found to be excellent for studying the detailed morphology of structural development and showed better images than normal SEM procedures using freeze-drying for sample preparation. Also, the studies illustrated a morphological change, e.g. from net-like structure to membranous structure caused by syneresis, accompanied by a significant increase in mechanical properties, when the beads are formed by ionotropic gelation. The gelation time of 20 min was found to be the minimum for a complete bead formation, based on the mechanical and SEM data. The results demonstrate the advantageous of cryo-SEM for examining the ultrastructure during bead formation of calcium pectinate.