Images on a micron scale and the stress-strain behaviour of gel structures during tension were simultaneously recorded in real time using a mini fracture cell under the confocal laser scanning microscope (CLSM). beta-lactoglobulin gels tailor-made to vary in density, connectivity, thickness of strands and size of aggregates and clusters were used as a food model system. Amylopectin and gelatin were used to generate different types of beta-lactoglobulin network microstructures and also as a second continuous phase. Both rheological and structural differences in fragility between beta-lactoglobulin gels were verified according to the density of their aggregated network structure. A dense gel has a more brittle behaviour where the clusters are rigid and the crack propagates smoothly compared to a gel with an open network structure, which has a discontinuous crack growth, via a winding pathway around clusters, and also break-up of the pores far from the crack tip. Differences in the stretchability of the aggregated beta-lactoglobulin structure, induced by addition of amylopectin solution, were proved and related to differences in stress-strain behaviour and crack propagation. Gelatin gels in the pores between the beta-lactoglobulin clusters do not affect the structure of the beta-lactoglobulin network but make the fracture fragile giving a smooth fracture surface, cause continuous crack growth and fracture propagation through,beta-lactoglobulin clusters. This is a consequence of that the mixed gel follows the behaviour of the gelatin gel when the gelatin phase is stronger than the beta-lactoglobulin network. (C) 2004 Kluwer Academic Publishers.