Extrusion behaviour of cohesive potato starch pastes: I. Rheological characterisation

The extrusion behaviour of a cohesive mixture of hydrated potato starch solids was characterised using capillary rheometry. Data were collected from isothermal ram extrusion tests and were modelled using (i) the modified plasticity approach described by Benbow and Bridgwater, and (ii) standard fluid mechanics approaches for power law and Herschel-Bulkley fluids. Neither approach proved to be entirely satisfactory, with particle size effects strongly manifested in the Benbow-Bridgwater parameters, and problems with the Mooney analysis for describing the materials as a fluid. Shear-thinning behaviour, as reported elsewhere other starch doughs/mixtures, was evident, while both wall slip and extensional shear contributions were important. The utility of the Benbow-Bridgwater approach was demonstrated by its application to modelling of annular dies, where it provided reasonably accurate predictions once particle size effects were accounted for.