Comparison of the Effects of Extrusion Cooking on Some Cereal Starches

The physicochemical properties of extruded starches from wheat, maize and rice were compared. These samples were extruded with a twin-screw extruder at varying water feed rates of 1.78, 1.25, 0.70, 0.35 and zero kg/h. The expansion of the samples (producing puffed products in all cases) increased with increasing the specific mechanical energy (SME). However maize flour produced the most expanded product, whilst rice samples produced the least expanded one. The cold-water viscosity of the samples at 25.0 degrees C measured by the RVA were different; for wheat samples it decreased with increasing the SME input, for rice samples it showed a reverse relationship with the SME and for maize samples it was rising at lower SME inputs and then falling at higher SMEs. The x-ray diffraction patterns of extruded starches changed from the A-pattern to V- and E- patterns. Determination of the apparent sedimentation coefficients at a constant concentration (s* value) of amylose and amylopectin of the samples showed that degradation of mainly amylopectin could occur during extrusion cooking. Differences in composition and molecular structure of these samples along with the breakdown of amylopectin may be the key factors in explaining the different physicochemical behaviors of these samples.