Particle size of milled barley and sorghum and physico-chemical properties of grain following extrusion

Milled barley and sorghum grains were separated into three size fractions (fine, <0.5 mm; medium, 0.5–1.0 mm; coarse, >1.0 mm) and extruded at two maximum temperatures (100 °C; 140 °C). Mechanical resistance and specific mechanical energy during extrusion was significantly higher for fine fractions, and extrusion at high temperature resulted in higher mechanical resistance. Pressure generated during extrusion was higher for the fine fraction in sorghum but lower in barley. Expansion index was highest for the fine fraction for barley, but did not differ significantly between sorghum fractions or with extrusion temperature. For all samples, extrusion at low temperature resulted in a higher final paste consistency and lower water absorption index, but there was no significant effect on water solubility index (WSI). Fraction size showed a significant effect on WSI in sorghum but not in barley. The results are rationalised in terms of differences in grain composition between sorghum and barley.