In-line determination of plasticized wheat starch viscoelastic behavior: impact of processing

In-line slit and capillary die viscometers were specially designed to study the viscoelastic behavior of plasticized wheat starch under low-hydrated conditions. A wide range of shear rates could be obtained by the use of different die geometries. The effects of temperature (110–150 °C), glycerol (10–35%) and moisture contents (0–20%) were studied. The influence of the specific mechanical energy (SME) (100–650 kWh/t) applied to the products was also accounted for. In the range of extrusion conditions, the viscosity data of starch melts followed the classical power-law behavior. A general rheological model was derived from experimental results. Discrepancies of viscosity values found between in-line viscometry results and those of a pre-shearing rheometer, the Rhéoplast®, could be explained by carefully taking into account the temperature and SME conditions. The impact of processing in regards to the starch transformation was investigated through intrinsic viscosity measurements and thermal analysis. The elastic properties of starch melts were also assessed through the analysis of entrance and exit pressures data.