Extrusion behaviour of cohesive potato starch pastes: II. Microstructure-process interactions

The effect of shear imposed during isothermal ram extrusion of a mixture of potato starches on extrudate microstructure was investigated by comparing the microstructure generated in the mixture with the microstructures generated by extruding its components, namely native starch, potato granule and potato flake, individually. Hydration tests indicated that the fractions with pre-gelatinised starch (granule and flake) absorbed free water rapidly, so that the native starch in the 40 wt.% w.b. water mixture was extruded in an almost unhydrated, hard form. The three components exhibited markedly different extrusion behaviours, ranging from visco-elastic effects to chronic dewatering and particle rupture. Electronic and optical microscopy, DSC and WAXS were used to characterise microstructure changes across the length scales: the results indicated that the flake acted as an amorphous binder which allowed the other components to retain their structure during the shear regime generated during extrusion.