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.
Keywords: Cellular structure; Elongational viscosity; Expansion; Pre-shearing rheometer; Starch; Zein.
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