Flow and foam properties of extruded maize flour and its biopolymer blends expanded by microwave.

Maize flour and blends from starch and zein biopolymers were processed as dense materials by extrusion (120°C, 300J·g-1) and press-molding (140°C, 10min) at a constant moisture content (26%wb), and then foamed by microwave heating. The mechanical properties of foams, determined by a 3-point bending test, were governed by density, in agreement with an open solid foam model. The density and 3D cellular structure of the foams were determined by X-ray tomography. In the same interval of density [0.15, 0.3g·cm-3], foams from microwaved materials had a finer cellular structure than directly expanded materials at extruder outlet. The study of melt rheological behavior with Rheoplast® (100-160°C, SME≤200J·g-1) showed that protein content (0-15%) did not affect shear viscosity but increased elongational viscosity. This trend, similar to the one reported for the storage modulus in a rubbery state, could be attributed to dissipative effects in a starch/protein interphase, explaining the difference of expansion between starch, blends and flour.