Structure of polysaccharide-starch composite gels by rheology and confocal laser scanning microscopy: Effect of the composition and of the preparation procedure

The aim of this study was to describe the structure of fruit preparations taken as models of polysaccharide-starch composite gels, according to the composition and the preparation procedure. For that purpose, model matrices have been prepared, the gelling agent being either pectin or carrageenan in combination with cross-linked waxy corn starch. The microstructure of these composite gels was studied by rheology and confocal laser scanning microscopy (CLSM) at different steps of the preparation procedure. The viscoelastic behaviour of these mixed systems clearly revealed the contribution of close-packed swollen starch granules in the dispersed phase and of the gelled network arising from the polysaccharide, located in the continuous phase. By CLSM, a heterogeneous microstructure with swollen starch granules unevenly distributed in the medium was shown. Pectin or carrageenan was mainly found in the continuous phase forming gelled fragments; part of the polysaccharide lay inside starch granules. These structural features are shown to be the direct consequence of the stirring applied during the cooling step of the preparation procedure while gelling of the polysaccharide took place. The resulting composite system was therefore more heterogeneous than expected.