Shear and elongational viscosities of a complex starchy formulation for extrusion cooking

The viscous behavior of a ready-to-eat (RTE) blend with different moisture contents (17–25%, wet basis) was determined at temperatures below (100–120 °C) and above (140–160 °C) their melting points, by using a capillary rheometer with pre-shearing (Rheoplast®). The elongational behavior of the molten RTE blend was also investigated. In all cases a moderate pre-shearing treatment, consisted on 20 s at 200 rpm, was applied prior to rheological measurements. Product modifications were determined by differential scanning calorimetry (DSC) and rapid visco-analyzer (RVA) measurements and compared to a commercial product. A pseudoplastic behavior, well described by the power law was found above melting point, with power law indices decreasing from 0.8 above melting point, close to 0 below. The significance of elongational flow was evidenced by the large values of Trouton numbers (250–2200). For non molten material (T < 120 °C), the consistency index K could be related to the degree of transformation (< 65%), using the dependence of the viscosity of a suspension on its solid volume fraction.