Changes in rheological properties of rice flour during extrusion cooking

Flour from two rice cultivars, one (IR-64) with a high (28.6%) and the other (Agonibora)
with a low (5.0%) amylose content, having 20% moisture, was extruded at
different barrel temperatures (80–120C), through a twin-screw extruder at a constant
feed rate (15 kg/h) and screw speed (400 rpm).The effect of barrel temperature
and amylose content of rice extrudates on pasting and rheological properties were
examined using rapid visco-analyzer and a coaxial cylinder system, respectively.
Extrudates from low-amylose rice exhibited lower paste viscosity than from highamylose
rice. Different rheological models (Power law, Herschel–Bulkley, Bingham
and Casson) were examined for the best description of the shear rate/shear stress
data. The Herschel–Bulkley model fitted best (r 0.948–0.989, P 0.01) for the
high-amylose rice, whereas the Power law model showed best fit (r 0.998–0.999,
P 0.01) for the low-amylose rice extrudate.