Effects of different extrusion temperatures on physicochemical, rheological and digestion properties of rice flour produced in a pilot‐scale extruder

Summary This research was to study the effects of different die temperatures (125, 135, 145, 155, 165 °C) on the physicochemical, rheological, structural properties and in vitro digestion characteristics of extruded rice flour. In this work, the moisture content of feed rice flour was controlled at 18% (w/w, on a dry basis). A twin‐screw with 65 mm diameter was used. Feed rate and screw speed were set at 120 kg h −1 and 120 rad min −1 respectively to control variables. The results indicated that the water solubility index improved but the water absorption index reduced at a higher temperature. The total starch content reduced as the temperature increased, but the amylose content was insusceptible to the temperature. Average particle size significantly decreased after extrusion due to the damage of starch granules. The water dispersion system of the extruded rice flour possessed more liquid characteristics than that of raw rice flour. Steady shear analysis revealed that the flow was in a shear thinning behaviour, and apparent viscosity was improved by extrusion but negatively related to extrusion temperature. Crystal type of starch turned from A to V after extrusion. The interaction among rice flour components was mainly non‐covalent and could be enhanced by extrusion. The ratio of 1047/1022 cm −1 reflected that the ordered structure was destroyed by extrusion, which was further confirmed by the differential scanning calorimeter result. In addition, the digestibility of extruded rice flour was improved. The in vitro digestibility of protein increased from 75.77% to 86.44% and resistant starch content significantly decreased as extrusion temperature increased.