Physicochemical and structural properties and digestibility of rice flour with different moisture extrusion cooking

Extruded rice flour with different feed moisture content (FMC, 14%, 16%, 18%, 20%, 22%) were obtained by a twin-screw with two co-rotating screws of 65 mm diameter at 120 kg h(-1) feed rate and 120 rad min(-1) screw speed. The results showed that the higher the FMC, the smoother the extrudate surface, the lighter the colour, and the lower the radial expansion; the water absorption index increased whereas the water solubility index was in opposite as FMC increased. The bound water was the major fraction in extruded rice flour and with FMC increasing, a shift to a long relaxation time of the peak of T-21 could be found accompanied by the signal intensity enhancement. Extrusion did not alter the chemical structure of starch but turned crystal type from A to V. The rice flour possessed better digestibility after extrusion, but higher FMC mitigated the digestion by improving the branching of the starch structure and weakening the degradation of the protein.