FOOD. Science. Technology. Quality

Food. SCIENCE. Technology. Quality

Food. Science. TECHNOLOGY. Quality

Food. Science. Technology. QUALITY




Rheology and structure of cross-linked starch dispersions


The rheological properties of chemically cross-linked waxy maize starch (CWMS) dispersed in water has been studied in relation to its swelling behaviour. The main parameters that were considered were starch concentration (2-4%), pasting temperature (96°C-136°C) and shear conditions (~105 s-1). The swelling behaviour was assessed by means of swelling experiments and by a measure of the size distribution of the swollen particles. The rheological study was performed by means of steady shear measurements (viscometry) and in oscillatory shear (viscoelasticity). In all conditions, starch dispersions exhibited the behaviour of suspensions of swollen particles as assessed from viscosity measurements. The flow behaviour of the dispersions was shear-thinning and a yield stress was clearly evidenced when the concentration was high enough. The viscoelastic behaviour became measurable as soon as the volume fraction of starch swollen particles was high enough for them to fill a large part of the available volume. This was typical of a gel-like system with G’ > G” and a flat frequency dependence of G’. All these properties strongly depended upon the pasting temperature with an optimum determined by the degree of crosslinking of starch granules. Sensitivity of starch granules to shear also was strongly dependent upon the pasting temperature. When starch granules were undercooked, their swelling properties, and hence their rheological properties, were reinforced by high shearing. In contrast, when the starch granules were overcooked the rheological properties were depressed by shearing as a result of their high fragility. These overall results allow ways to evaluate the swelling behaviour of crosslinked starch in the formulation of starchy products according to processing conditions.