Structure-Control of the Rheological Behaviour of Particulate Gels
The focus of this thesis is the relation between the rheological and structural properties of mixed particulate gels, based on β-lactoglobulin, determined under both dynamic and static conditions. Rheological characteristics were recorded during formation of the gels, on set gels and during deformation and fracture of the gels, using viscoelastic measurements in shear and in compression and fracture mechanics in tension. Structure evolution, structure in the set state and structural behaviour during deformation were evaluated with the microstructural methods of light microscopy (LM), confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM) on different length scales. The microstructure was quantified by image analysis. Structure-fracture measurements, a new type of technique, were used, where rheological behaviour and structural alterations during deformation and fracture were measured simultaneously. The model structures used were generated by altering the kinetics of aggregation of β-lactoglobulin using potato amylopectin. The β-lactoglobulin structures formed were particulate networks varying in density of the overall network structure, size of pores and clusters, density of clusters and connectivity of strands and clusters.
The main message from this thesis is that structural behaviour during deformation and fracture is strongly correlated to the way the structure is evolved. It is thus possible to predict what structural characteristic control a certain rheological property if a combination of dynamic and static measurements is used on different length scales. Higher gel strength is related to increased density of clusters as long as the connectivity between clusters in the strands is maintained. Stress at fracture is more sensitive to changes in connectivity than the storage modulus. Strain at fracture is controlled by the ability of clusters and strands to alter in form and to extend during deformation. Unique information on the connectivity and stretchability of the structures was obtained through recording the structure while it was being extended. It was feasible to relate the structural differences: stretching of clusters contra brittle fracture of clusters and merging of small voids into larger cracks contra rapid fracture without pore formation, to differences in stress-strain behaviour and crack propagation.
This work shows that the kinetics of aggregation and gel formation of β-lactoglobulin are strongly influenced by variations in the concentration and molecular weight of amylopectin. Both increased concentration and increased molecular weight induce acceleration of the aggregation mechanism into particle aggregates and clusters and reduction of the gelling temperature. The altered kinetics resulted in different microstructures of the β-lactoglobulin, which on an overall level of structure become more open the higher the concentration and the higher the molecular weight of amylopectin. Open network structures were constructed of large and dense β-lactoglobulin clusters, while dense network structures were formed of smaller and more open clusters. Increased gel strength coincided with a rise in the density of clusters as long as the connectivity of the β-lactoglobulin strands of clusters was unaffected. At a sufficiently high concentration and molecular weight of amylopectin, the continued aggregation to a connected network was prevented during the structure formation due to decreased mobility of the β-lactoglobulin constituents in the highly viscous medium, resulting in poorer connectivity of the finished gel. As a result, fracture of the structure occurred via large sheets of clusters.