Structure and mechanical behaviour of corn flour and starch–zein based materials in the glassy state

Abstract

Corn flour and starch–zein based samples were prepared by extrusion and thermomoulding and then analysed at a moisture content of 12.0% (wb). Starch–zein blends (5–50% zein, db) were used to study the influence of starch–zein ratio on material properties. Glass transition temperatures were determined by differential scanning calorimetry and molecular relaxations by dynamic mechanical thermal analysis. Behaviour at large deformations was examined by the three-point bending test. The behaviour of materials made from glassy corn flour and starch–zein blends was compared to the behaviour of their components. Amorphous starch was ductile whereas blends and corn flour samples were brittle. This difference could not be explained by molecular mobility. Blend morphology observed by confocal scanning light microscopy (CLSM) showed that proteins undergo aggregation during thermomechanical processing, which largely conditioned their mechanical properties. The consequences of processing on the morphology of the protein phase in the corn flour were also discussed.